m4
m4
Next: Preliminaries, Up: (dir) [Contents][Index]
This manual (28 May 2021) is for GNU M4 (version 1.4.19), a package containing an implementation of the m4 macro language.
Copyright © 1989–1994, 2004–2014, 2016–2017, 2020–2021 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License.”
GNU m4
is an implementation of the traditional UNIX macro
processor. It is mostly SVR4 compatible, although it has some
extensions (for example, handling more than 9 positional parameters
to macros). m4
also has builtin functions for including
files, running shell commands, doing arithmetic, etc. Autoconf needs
GNU m4
for generating configure scripts, but not for
running them.
GNU m4
was originally written by René Seindal, with
subsequent changes by François Pinard and other volunteers
on the Internet. All names and email addresses can be found in the
files m4-1.4.19/AUTHORS and
m4-1.4.19/THANKS from the GNU M4
distribution.
This is release 1.4.19. It is now considered stable: future releases in the 1.4.x series are only meant to fix bugs, increase speed, or improve documentation. However…
An experimental feature, which would improve m4
usefulness,
allows for changing the syntax for what is a word in m4
.
You should use:
./configure --enable-changeword
if you want this feature compiled in. The current implementation
slows down m4
considerably and is hardly acceptable. In the
future, m4
2.0 will come with a different set of new features
that provide similar capabilities, but without the inefficiencies, so
changeword will go away and you should not count on it.
• Preliminaries | Introduction and preliminaries | |
• Invoking m4 | Invoking m4
| |
• Syntax | Lexical and syntactic conventions | |
• Macros | How to invoke macros | |
• Definitions | How to define new macros | |
• Conditionals | Conditionals, loops, and recursion | |
• Debugging | How to debug macros and input | |
• Input Control | Input control | |
• File Inclusion | File inclusion | |
• Diversions | Diverting and undiverting output | |
• Text handling | Macros for text handling | |
• Arithmetic | Macros for doing arithmetic | |
• Shell commands | Macros for running shell commands | |
• Miscellaneous | Miscellaneous builtin macros | |
• Frozen files | Fast loading of frozen state | |
• Compatibility | Compatibility with other versions of m4
| |
• Answers | Correct version of some examples | |
• Copying This Package | How to make copies of the overall M4 package | |
• Copying This Manual | How to make copies of this manual | |
• Indices | Indices of concepts and macros | |
— The Detailed Node Listing — Introduction and preliminaries | ||
• Intro | Introduction to m4
| |
• History | Historical references | |
• Bugs | Problems and bugs | |
• Manual | Using this manual | |
Invoking | ||
• Operation modes | Command line options for operation modes | |
• Preprocessor features | Command line options for preprocessor features | |
• Limits control | Command line options for limits control | |
• Frozen state | Command line options for frozen state | |
• Debugging options | Command line options for debugging | |
• Command line files | Specifying input files on the command line | |
Lexical and syntactic conventions | ||
• Names | Macro names | |
• Quoted strings | Quoting input to m4
| |
• Comments | Comments in m4 input
| |
• Other tokens | Other kinds of input tokens | |
• Input processing | How m4 copies input to output
| |
How to invoke macros | ||
• Invocation | Macro invocation | |
• Inhibiting Invocation | Preventing macro invocation | |
• Macro Arguments | Macro arguments | |
• Quoting Arguments | On Quoting Arguments to macros | |
• Macro expansion | Expanding macros | |
How to define new macros | ||
• Define | Defining a new macro | |
• Arguments | Arguments to macros | |
• Pseudo Arguments | Special arguments to macros | |
• Undefine | Deleting a macro | |
• Defn | Renaming macros | |
• Pushdef | Temporarily redefining macros | |
• Indir | Indirect call of macros | |
• Builtin | Indirect call of builtins | |
Conditionals, loops, and recursion | ||
• Ifdef | Testing if a macro is defined | |
• Ifelse | If-else construct, or multibranch | |
• Shift | Recursion in m4
| |
• Forloop | Iteration by counting | |
• Foreach | Iteration by list contents | |
• Stacks | Working with definition stacks | |
• Composition | Building macros with macros | |
How to debug macros and input | ||
• Dumpdef | Displaying macro definitions | |
• Trace | Tracing macro calls | |
• Debug Levels | Controlling debugging output | |
• Debug Output | Saving debugging output | |
Input control | ||
• Dnl | Deleting whitespace in input | |
• Changequote | Changing the quote characters | |
• Changecom | Changing the comment delimiters | |
• Changeword | Changing the lexical structure of words | |
• M4wrap | Saving text until end of input | |
File inclusion | ||
• Include | Including named files | |
• Search Path | Searching for include files | |
Diverting and undiverting output | ||
• Divert | Diverting output | |
• Undivert | Undiverting output | |
• Divnum | Diversion numbers | |
• Cleardivert | Discarding diverted text | |
Macros for text handling | ||
• Len | Calculating length of strings | |
• Index macro | Searching for substrings | |
• Regexp | Searching for regular expressions | |
• Substr | Extracting substrings | |
• Translit | Translating characters | |
• Patsubst | Substituting text by regular expression | |
• Format | Formatting strings (printf-like) | |
Macros for doing arithmetic | ||
• Incr | Decrement and increment operators | |
• Eval | Evaluating integer expressions | |
Macros for running shell commands | ||
• Platform macros | Determining the platform | |
• Syscmd | Executing simple commands | |
• Esyscmd | Reading the output of commands | |
• Sysval | Exit status | |
• Mkstemp | Making temporary files | |
Miscellaneous builtin macros | ||
• Errprint | Printing error messages | |
• Location | Printing current location | |
• M4exit | Exiting from m4
| |
Fast loading of frozen state | ||
• Using frozen files | Using frozen files | |
• Frozen file format | Frozen file format | |
Compatibility with other versions of | ||
• Extensions | Extensions in GNU M4 | |
• Incompatibilities | Facilities in System V m4 not in GNU M4 | |
• Other Incompatibilities | Other incompatibilities | |
Correct version of some examples | ||
• Improved exch | Solution for exch
| |
• Improved forloop | Solution for forloop
| |
• Improved foreach | Solution for foreach
| |
• Improved copy | Solution for copy
| |
• Improved m4wrap | Solution for m4wrap
| |
• Improved cleardivert | Solution for cleardivert
| |
• Improved capitalize | Solution for capitalize
| |
• Improved fatal_error | Solution for fatal_error
| |
How to make copies of the overall M4 package | ||
• GNU General Public License | License for copying the M4 package | |
How to make copies of this manual | ||
• GNU Free Documentation License | License for copying this manual | |
Indices of concepts and macros | ||
• Macro index | Index for all m4 macros
| |
• Concept index | Index for many concepts | |
Next: Invoking m4, Previous: Top, Up: Top [Contents][Index]
This first chapter explains what GNU m4
is, where m4
comes from, how to read and use this documentation, how to call the
m4
program, and how to report bugs about it. It concludes by
giving tips for reading the remainder of the manual.
The following chapters then detail all the features of the m4
language.
• Intro | Introduction to m4
| |
• History | Historical references | |
• Bugs | Problems and bugs | |
• Manual | Using this manual |
Next: History, Up: Preliminaries [Contents][Index]
m4
m4
is a macro processor, in the sense that it copies its
input to the output, expanding macros as it goes. Macros are either
builtin or user-defined, and can take any number of arguments.
Besides just doing macro expansion, m4
has builtin functions
for including named files, running shell commands, doing integer
arithmetic, manipulating text in various ways, performing recursion,
etc.… m4
can be used either as a front-end to a compiler,
or as a macro processor in its own right.
The m4
macro processor is widely available on all UNIXes, and has
been standardized by POSIX.
Usually, only a small percentage of users are aware of its existence.
However, those who find it often become committed users. The
popularity of GNU Autoconf, which requires GNU
m4
for generating configure scripts, is an incentive
for many to install it, while these people will not themselves
program in m4
. GNU m4
is mostly compatible with the
System V, Release 4 version, except for some minor differences.
See Compatibility, for more details.
Some people find m4
to be fairly addictive. They first use
m4
for simple problems, then take bigger and bigger challenges,
learning how to write complex sets of m4
macros along the way.
Once really addicted, users pursue writing of sophisticated m4
applications even to solve simple problems, devoting more time
debugging their m4
scripts than doing real work. Beware that
m4
may be dangerous for the health of compulsive programmers.
Next: Bugs, Previous: Intro, Up: Preliminaries [Contents][Index]
Macro languages were invented early in the history of computing. In the 1950s Alan Perlis suggested that the macro language be independent of the language being processed. Techniques such as conditional and recursive macros, and using macros to define other macros, were described by Doug McIlroy of Bell Labs in “Macro Instruction Extensions of Compiler Languages”, Communications of the ACM 3, 4 (1960), 214–20, https://dl.acm.org/doi/10.1145/367177.367223.
An important precursor of m4
was GPM; see C. Strachey,
“A general purpose macrogenerator”, Computer Journal 8, 3
(1965), 225–41,
https://academic.oup.com/comjnl/article/8/3/225/336044. GPM is
also succinctly described in David Gries’s book Compiler
Construction for Digital Computers, Wiley (1971). Strachey was a
brilliant programmer: GPM fit into 250 machine instructions!
Inspired by GPM while visiting Strachey’s Lab in 1968, McIlroy wrote a
model preprocessor in that fit into a page of Snobol 3 code, and McIlroy
and Robert Morris developed a series of further models at Bell Labs.
Andrew D. Hall followed up with M6, a general purpose macro processor
used to port the Fortran source code of the Altran computer algebra
system; see Hall’s “The M6 Macro Processor”, Computing Science
Technical Report #2, Bell Labs (1972),
http://cm.bell-labs.com/cm/cs/cstr/2.pdf. M6’s source code
consisted of about 600 Fortran statements. Its name was the first of
the m4
line.
The Brian Kernighan and P.J. Plauger book Software Tools,
Addison-Wesley (1976), describes and implements a Unix
macro-processor language, which inspired Dennis Ritchie to write
m3
, a macro processor for the AP-3 minicomputer.
Kernighan and Ritchie then joined forces to develop the original
m4
, described in “The M4 Macro Processor”, Bell Laboratories
(1977), https://wolfram.schneider.org/bsd/7thEdManVol2/m4/m4.pdf.
It had only 21 builtin macros.
While GPM
was more pure, m4
is meant to deal with
the true intricacies of real life: macros can be recognized without
being pre-announced, skipping whitespace or end-of-lines is easier,
more constructs are builtin instead of derived, etc.
Originally, the Kernighan and Plauger macro-processor, and then
m3
, formed the engine for the Rational FORTRAN preprocessor,
that is, the Ratfor
equivalent of cpp
. Later, m4
was used as a front-end for Ratfor
, C
and Cobol
.
René Seindal released his implementation of m4
, GNU
m4
,
in 1990, with the aim of removing the artificial limitations in many
of the traditional m4
implementations, such as maximum line
length, macro size, or number of macros.
The late Professor A. Dain Samples described and implemented a further
evolution in the form of M5
: “User’s Guide to the M5 Macro
Language: 2nd edition”, Electronic Announcement on comp.compilers
newsgroup (1992).
François Pinard took over maintenance of GNU m4
in
1992, until 1994 when he released GNU m4
1.4, which was
the stable release for 10 years. It was at this time that GNU
Autoconf decided to require GNU m4
as its underlying
engine, since all other implementations of m4
had too many
limitations.
More recently, in 2004, Paul Eggert released 1.4.1 and 1.4.2 which
addressed some long standing bugs in the venerable 1.4 release. Then in
2005, Gary V. Vaughan collected together the many patches to
GNU m4
1.4 that were floating around the net and
released 1.4.3 and 1.4.4. And in 2006, Eric Blake joined the team and
prepared patches for the release of 1.4.5, with subsequent releases
through intervening years, as recent as 1.4.18 in 2016.
Meanwhile, development has continued on new features for m4
, such
as dynamic module loading and additional builtins. When complete,
GNU m4
2.0 will start a new series of releases.
Next: Manual, Previous: History, Up: Preliminaries [Contents][Index]
If you have problems with GNU M4 or think you’ve found a bug, please report it. Before reporting a bug, make sure you’ve actually found a real bug. Carefully reread the documentation and see if it really says you can do what you’re trying to do. If it’s not clear whether you should be able to do something or not, report that too; it’s a bug in the documentation!
Before reporting a bug or trying to fix it yourself, try to isolate it
to the smallest possible input file that reproduces the problem. Then
send us the input file and the exact results m4
gave you. Also
say what you expected to occur; this will help us decide whether the
problem was really in the documentation.
Once you’ve got a precise problem, send e-mail to
bug-m4@gnu.org. Please include the version number of m4
you are using. You can get this information with the command
m4 --version. Also provide details about the platform you are
executing on.
Non-bug suggestions are always welcome as well. If you have questions about things that are unclear in the documentation or are just obscure features, please report them too.
Previous: Bugs, Up: Preliminaries [Contents][Index]
This manual contains a number of examples of m4
input and output,
and a simple notation is used to distinguish input, output and error
messages from m4
. Examples are set out from the normal text, and
shown in a fixed width font, like this
This is an example of an example!
To distinguish input from output, all output from m4
is prefixed
by the string ‘⇒’, and all error messages by the string
‘error→’. When showing how command line options affect matters,
the command line is shown with a prompt ‘$ like this’,
otherwise, you can assume that a simple m4 invocation will work.
Thus:
$ command line to invoke m4 Example of input line ⇒Output line from m4 error→and an error message
The sequence ‘^D’ in an example indicates the end of the input file. The sequence ‘NL’ refers to the newline character. The majority of these examples are self-contained, and you can run them with similar results by invoking m4 -d. In fact, the testsuite that is bundled in the GNU M4 package consists of the examples in this document! Some of the examples assume that your current directory is located where you unpacked the installation, so if you plan on following along, you may find it helpful to do this now:
$ cd m4-1.4.19
As each of the predefined macros in m4
is described, a prototype
call of the macro will be shown, giving descriptive names to the
arguments, e.g.,
This is a sample prototype. There is not really a macro named
example
, but this documents that if there were, it would be a
Composite macro, rather than a Builtin. It requires at least one
argument, string. Remember that in m4
, there must not be a
space between the macro name and the opening parenthesis, unless it was
intended to call the macro without any arguments. The brackets around
count and argument show that these arguments are optional.
If count is omitted, the macro behaves as if count were ‘1’,
whereas if argument is omitted, the macro behaves as if it were
the empty string. A blank argument is not the same as an omitted
argument. For example, ‘example(`a')’, ‘example(`a',`1')’,
and ‘example(`a',`1',)’ would behave identically with count
set to ‘1’; while ‘example(`a',)’ and ‘example(`a',`')’
would explicitly pass the empty string for count. The ellipses
(‘…’) show that the macro processes additional arguments
after argument, rather than ignoring them.
All macro arguments in m4
are strings, but some are given
special interpretation, e.g., as numbers, file names, regular
expressions, etc. The documentation for each macro will state how the
parameters are interpreted, and what happens if the argument cannot be
parsed according to the desired interpretation. Unless specified
otherwise, a parameter specified to be a number is parsed as a decimal,
even if the argument has leading zeros; and parsing the empty string as
a number results in 0 rather than an error, although a warning will be
issued.
This document consistently writes and uses builtin, without a
hyphen, as if it were an English word. This is how the builtin
primitive is spelled within m4
.
Next: Syntax, Previous: Preliminaries, Up: Top [Contents][Index]
m4
The format of the m4
command is:
m4
[option…] [file…]
All options begin with ‘-’, or if long option names are used, with
‘--’. A long option name need not be written completely, any
unambiguous prefix is sufficient. POSIX requires m4
to
recognize arguments intermixed with files, even when
POSIXLY_CORRECT
is set in the environment. Most options take
effect at startup regardless of their position, but some are documented
below as taking effect after any files that occurred earlier in the
command line. The argument -- is a marker to denote the end of
options.
With short options, options that do not take arguments may be combined into a single command line argument with subsequent options, options with mandatory arguments may be provided either as a single command line argument or as two arguments, and options with optional arguments must be provided as a single argument. In other words, m4 -QPDfoo -d a -df is equivalent to m4 -Q -P -D foo -d -df -- ./a, although the latter form is considered canonical.
With long options, options with mandatory arguments may be provided with
an equal sign (‘=’) in a single argument, or as two arguments, and
options with optional arguments must be provided as a single argument.
In other words, m4 --def foo --debug a is equivalent to
m4 --define=foo --debug= -- ./a, although the latter form is
considered canonical (not to mention more robust, in case a future
version of m4
introduces an option named --default).
m4
understands the following options, grouped by functionality.
• Operation modes | Command line options for operation modes | |
• Preprocessor features | Command line options for preprocessor features | |
• Limits control | Command line options for limits control | |
• Frozen state | Command line options for frozen state | |
• Debugging options | Command line options for debugging | |
• Command line files | Specifying input files on the command line |
Next: Preprocessor features, Up: Invoking m4 [Contents][Index]
Several options control the overall operation of m4
:
--help
Print a help summary on standard output, then immediately exit
m4
without reading any input files or performing any other
actions.
--version
Print the version number of the program on standard output, then
immediately exit m4
without reading any input files or
performing any other actions.
-E
--fatal-warnings
Controls the effect of warnings. If unspecified, then execution continues and exit status is unaffected when a warning is printed. If specified exactly once, warnings become fatal; when one is issued, execution continues, but the exit status will be non-zero. If specified multiple times, then execution halts with non-zero status the first time a warning is issued. The introduction of behavior levels is new to M4 1.4.9; for behavior consistent with earlier versions, you should specify -E twice.
-i
--interactive
-e
Makes this invocation of m4
interactive. This means that all
output will be unbuffered, and interrupts will be ignored. The
spelling -e exists for compatibility with other m4
implementations, and issues a warning because it may be withdrawn in a
future version of GNU M4.
-P
--prefix-builtins
Internally modify all builtin macro names so they all start with the prefix ‘m4_’. For example, using this option, one should write ‘m4_define’ instead of ‘define’, and ‘m4___file__’ instead of ‘__file__’. This option has no effect if -R is also specified.
-Q
--quiet
--silent
Suppress warnings, such as missing or superfluous arguments in macro calls, or treating the empty string as zero.
--warn-macro-sequence[=regexp]
Issue a warning if the regular expression regexp has a non-empty
match in any macro definition (either by define
or
pushdef
). Empty matches are ignored; therefore, supplying the
empty string as regexp disables any warning. If the optional
regexp is not supplied, then the default regular expression is
‘\$\({[^}]*}\|[0-9][0-9]+\)’ (a literal ‘$’ followed by
multiple digits or by an open brace), since these sequences will
change semantics in the default operation of GNU M4 2.0 (due
to a change in how more than 9 arguments in a macro definition will be
handled, see Arguments). Providing an alternate regular
expression can provide a useful reverse lookup feature of finding
where a macro is defined to have a given definition.
-W regexp
--word-regexp=regexp
Use regexp as an alternative syntax for macro names. This
experimental option will not be present in all GNU m4
implementations (see Changeword).
Next: Limits control, Previous: Operation modes, Up: Invoking m4 [Contents][Index]
Several options allow m4
to behave more like a preprocessor.
Macro definitions and deletions can be made on the command line, the
search path can be altered, and the output file can track where the
input came from. These features occur with the following options:
-D name[=value]
--define=name[=value]
This enters name into the symbol table. If ‘=value’ is missing, the value is taken to be the empty string. The value can be any string, and the macro can be defined to take arguments, just as if it was defined from within the input. This option may be given more than once; order with respect to file names is significant, and redefining the same name loses the previous value.
-I directory
--include=directory
Make m4
search directory for included files that are not
found in the current working directory. See Search Path, for more
details. This option may be given more than once.
-s
--synclines
Generate synchronization lines, for use by the C preprocessor or other
similar tools. Order is significant with respect to file names. This
option is useful, for example, when m4
is used as a
front end to a compiler. Source file name and line number information
is conveyed by directives of the form ‘#line linenum
"file"’, which are inserted as needed into the middle of the
output. Such directives mean that the following line originated or was
expanded from the contents of input file file at line
linenum. The ‘"file"’ part is often omitted when
the file name did not change from the previous directive.
Synchronization directives are always given on complete lines by themselves. When a synchronization discrepancy occurs in the middle of an output line, the associated synchronization directive is delayed until the next newline that does not occur in the middle of a quoted string or comment.
define(`twoline', `1 2') ⇒#line 2 "stdin" ⇒ changecom(`/*', `*/') ⇒ define(`comment', `/*1 2*/') ⇒#line 5 ⇒ dnl no line hello ⇒#line 7 ⇒hello twoline ⇒1 ⇒#line 8 ⇒2 comment ⇒/*1 ⇒2*/ one comment `two three' ⇒#line 10 ⇒one /*1 ⇒2*/ two ⇒three goodbye ⇒#line 12 ⇒goodbye
-U name
--undefine=name
This deletes any predefined meaning name might have. Obviously, only predefined macros can be deleted in this way. This option may be given more than once; undefining a name that does not have a definition is silently ignored. Order is significant with respect to file names.
Next: Frozen state, Previous: Preprocessor features, Up: Invoking m4 [Contents][Index]
There are some limits within m4
that can be tuned. For
compatibility, m4
also accepts some options that control limits
in other implementations, but which are automatically unbounded (limited
only by your hardware and operating system constraints) in GNU
m4
.
-g
--gnu
Enable all the extensions in this implementation. In this release of
M4, this option is always on by default; it is currently only useful
when overriding a prior use of --traditional. However, having
GNU behavior as default makes it impossible to write a
strictly POSIX-compliant client that avoids all incompatible
GNU M4 extensions, since such a client would have to use the
non-POSIX command-line option to force full POSIX
behavior. Thus, a future version of M4 will be changed to implicitly
use the option --traditional if the environment variable
POSIXLY_CORRECT
is set. Projects that intentionally use
GNU extensions should consider using --gnu to state
their intentions, so that the project will not mysteriously break if the
user upgrades to a newer M4 and has POSIXLY_CORRECT
set in their
environment.
-G
--traditional
Suppress all the extensions made in this implementation, compared to the System V version. See Compatibility, for a list of these.
-H num
--hashsize=num
Make the internal hash table for symbol lookup be num entries big. For better performance, the number should be prime, but this is not checked. The default is 65537 entries. It should not be necessary to increase this value, unless you define an excessive number of macros.
-L num
--nesting-limit=num
Artificially limit the nesting of macro calls to num levels, stopping program execution if this limit is ever exceeded. When not specified, nesting defaults to unlimited on platforms that can detect stack overflow, and to 1024 levels otherwise. A value of zero means unlimited; but then heavily nested code could potentially cause a stack overflow.
The precise effect of this option is more correctly associated
with textual nesting than dynamic recursion. It has been useful
when some complex m4
input was generated by mechanical means, and
also in diagnosing recursive algorithms that do not scale well.
Most users never need to change this option from its default.
This option does not have the ability to break endless
rescanning loops, since these do not necessarily consume much memory
or stack space. Through clever usage of rescanning loops, one can
request complex, time-consuming computations from m4
with useful
results. Putting limitations in this area would break m4
power.
There are many pathological cases: ‘define(`a', `a')a’ is
only the simplest example (but see Compatibility). Expecting GNU
m4
to detect these would be a little like expecting a compiler
system to detect and diagnose endless loops: it is a quite hard
problem in general, if not undecidable!
-B num
-S num
-T num
These options are present for compatibility with System V m4
, but
do nothing in this implementation. They may disappear in future
releases, and issue a warning to that effect.
-N num
--diversions=num
These options are present only for compatibility with previous
versions of GNU m4
, and were controlling the number of
possible diversions which could be used at the same time. They do nothing,
because there is no fixed limit anymore. They may disappear in future
releases, and issue a warning to that effect.
Next: Debugging options, Previous: Limits control, Up: Invoking m4 [Contents][Index]
GNU m4
comes with a feature of freezing internal state
(see Frozen files). This can be used to speed up m4
execution when reusing a common initialization script.
-F file
--freeze-state=file
Once execution is finished, write out the frozen state on the specified file. It is conventional, but not required, for file to end in ‘.m4f’.
-R file
--reload-state=file
Before execution starts, recover the internal state from the specified frozen file. The options -D, -U, and -t take effect after state is reloaded, but before the input files are read.
Next: Command line files, Previous: Frozen state, Up: Invoking m4 [Contents][Index]
Finally, there are several options for aiding in debugging m4
scripts.
-d[flags]
--debug[=flags]
Set the debug-level according to the flags flags. The debug-level controls the format and amount of information presented by the debugging functions. See Debug Levels, for more details on the format and meaning of flags. If omitted, flags defaults to ‘aeq’.
--debugfile[=file]
-o file
--error-output=file
Redirect dumpdef
output, debug messages, and trace output to the
named file. Warnings, error messages, and errprint
output
are still printed to standard error. If these options are not used, or
if file is unspecified (only possible for --debugfile),
debug output goes to standard error; if file is the empty string,
debug output is discarded. See Debug Output, for more details. The
option --debugfile may be given more than once, and order is
significant with respect to file names. The spellings -o and
--error-output are misleading and inconsistent with other
GNU tools; for now they are silently accepted as synonyms of
--debugfile and only recognized once, but in a future version
of M4, using them will cause a warning to be issued.
-l num
--arglength=num
Restrict the size of the output generated by macro tracing to num characters per trace line. If unspecified or zero, output is unlimited. See Debug Levels, for more details.
-t name
--trace=name
This enables tracing for the macro name, at any point where it is defined. name need not be defined when this option is given. This option may be given more than once, and order is significant with respect to file names. See Trace, for more details.
Previous: Debugging options, Up: Invoking m4 [Contents][Index]
The remaining arguments on the command line are taken to be input file names. If no names are present, standard input is read. A file name of - is taken to mean standard input. It is conventional, but not required, for input files to end in ‘.m4’.
The input files are read in the sequence given. Standard input can be read more than once, so the file name - may appear multiple times on the command line; this makes a difference when input is from a terminal or other special file type. It is an error if an input file ends in the middle of argument collection, a comment, or a quoted string.
The options --define (-D), --undefine (-U), --synclines (-s), and --trace (-t) only take effect after processing input from any file names that occur earlier on the command line. For example, assume the file foo contains:
$ cat foo bar
The text ‘bar’ can then be redefined over multiple uses of foo:
$ m4 -Dbar=hello foo -Dbar=world foo ⇒hello ⇒world
If none of the input files invoked m4exit
(see M4exit), the
exit status of m4
will be 0 for success, 1 for general failure
(such as problems with reading an input file), and 63 for version
mismatch (see Using frozen files).
If you need to read a file whose name starts with a -, you can specify it as ‘./-file’, or use -- to mark the end of options.
Next: Macros, Previous: Invoking m4, Up: Top [Contents][Index]
As m4
reads its input, it separates it into tokens. A
token is either a name, a quoted string, or any single character, that
is not a part of either a name or a string. Input to m4
can also
contain comments. GNU m4
does not yet understand
multibyte locales; all operations are byte-oriented rather than
character-oriented (although if your locale uses a single byte
encoding, such as ISO-8859-1, you will not notice a difference).
However, m4
is eight-bit clean, so you can
use non-ASCII characters in quoted strings (see Changequote),
comments (see Changecom), and macro names (see Indir), with the
exception of the NUL character (the zero byte ‘'\0'’).
• Names | Macro names | |
• Quoted strings | Quoting input to m4
| |
• Comments | Comments in m4 input
| |
• Other tokens | Other kinds of input tokens | |
• Input processing | How m4 copies input to output
|
Next: Quoted strings, Up: Syntax [Contents][Index]
A name is any sequence of letters, digits, and the character ‘_’
(underscore), where the first character is not a digit. m4
will
use the longest such sequence found in the input. If a name has a
macro definition, it will be subject to macro expansion
(see Macros). Names are case-sensitive.
Examples of legal names are: ‘foo’, ‘_tmp’, and ‘name01’.
m4
A quoted string is a sequence of characters surrounded by quote strings, defaulting to ‘`’ and ‘'’, where the nested begin and end quotes within the string are balanced. The value of a string token is the text, with one level of quotes stripped off. Thus
`' ⇒
is the empty string, and double-quoting turns into single-quoting.
``quoted'' ⇒`quoted'
The quote characters can be changed at any time, using the builtin macro
changequote
. See Changequote, for more information.
Next: Other tokens, Previous: Quoted strings, Up: Syntax [Contents][Index]
m4
inputComments in m4
are normally delimited by the characters ‘#’
and newline. All characters between the comment delimiters are ignored,
but the entire comment (including the delimiters) is passed through to
the output—comments are not discarded by m4
.
Comments cannot be nested, so the first newline after a ‘#’ ends the comment. The commenting effect of the begin-comment string can be inhibited by quoting it.
$ m4 `quoted text' # `commented text' ⇒quoted text # `commented text' `quoting inhibits' `#' `comments' ⇒quoting inhibits # comments
The comment delimiters can be changed to any string at any time, using
the builtin macro changecom
. See Changecom, for more
information.
Next: Input processing, Previous: Comments, Up: Syntax [Contents][Index]
Any character, that is neither a part of a name, nor of a quoted string, nor a comment, is a token by itself. When not in the context of macro expansion, all of these tokens are just copied to output. However, during macro expansion, whitespace characters (space, tab, newline, formfeed, carriage return, vertical tab), parentheses (‘(’ and ‘)’), comma (‘,’), and dollar (‘$’) have additional roles, explained later.
Previous: Other tokens, Up: Syntax [Contents][Index]
m4
copies input to outputAs m4
reads the input token by token, it will copy each token
directly to the output immediately.
The exception is when it finds a word with a macro definition. In that
case m4
will calculate the macro’s expansion, possibly reading
more input to get the arguments. It then inserts the expansion in front
of the remaining input. In other words, the resulting text from a macro
call will be read and parsed into tokens again.
m4
expands a macro as soon as possible. If it finds a macro call
when collecting the arguments to another, it will expand the second call
first. This process continues until there are no more macro calls to
expand and all the input has been consumed.
For a running example, examine how m4
handles this input:
format(`Result is %d', eval(`2**15'))
First, m4
sees that the token ‘format’ is a macro name, so
it collects the tokens ‘(’, ‘`Result is %d'’, ‘,’,
and ‘ ’, before encountering another potential macro. Sure
enough, ‘eval’ is a macro name, so the nested argument collection
picks up ‘(’, ‘`2**15'’, and ‘)’, invoking the eval macro
with the lone argument of ‘2**15’. The expansion of
‘eval(2**15)’ is ‘32768’, which is then rescanned as the five
tokens ‘3’, ‘2’, ‘7’, ‘6’, and ‘8’; and
combined with the next ‘)’, the format macro now has all its
arguments, as if the user had typed:
format(`Result is %d', 32768)
The format macro expands to ‘Result is 32768’, and we have another round of scanning for the tokens ‘Result’, ‘ ’, ‘is’, ‘ ’, ‘3’, ‘2’, ‘7’, ‘6’, and ‘8’. None of these are macros, so the final output is
⇒Result is 32768
As a more complicated example, we will contrast an actual code example from the Gnulib project1, showing both a buggy approach and the desired results. The user desires to output a shell assignment statement that takes its argument and turns it into a shell variable by converting it to uppercase and prepending a prefix. The original attempt looks like this:
changequote([,])dnl define([gl_STRING_MODULE_INDICATOR], [ dnl comment GNULIB_]translit([$1],[a-z],[A-Z])[=1 ])dnl gl_STRING_MODULE_INDICATOR([strcase]) ⇒ ⇒ GNULIB_strcase=1 ⇒
Oops – the argument did not get capitalized. And although the manual
is not able to easily show it, both lines that appear empty actually
contain two trailing spaces. By stepping through the parse, it is easy
to see what happened. First, m4
sees the token
‘changequote’, which it recognizes as a macro, followed by
‘(’, ‘[’, ‘,’, ‘]’, and ‘)’ to form the
argument list. The macro expands to the empty string, but changes the
quoting characters to something more useful for generating shell code
(unbalanced ‘`’ and ‘'’ appear all the time in shell scripts,
but unbalanced ‘[]’ tend to be rare). Also in the first line,
m4
sees the token ‘dnl’, which it recognizes as a builtin
macro that consumes the rest of the line, resulting in no output for
that line.
The second line starts a macro definition. m4
sees the token
‘define’, which it recognizes as a macro, followed by a ‘(’,
‘[gl_STRING_MODULE_INDICATOR]’, and ‘,’. Because an unquoted
comma was encountered, the first argument is known to be the expansion
of the single-quoted string token, or ‘gl_STRING_MODULE_INDICATOR’.
Next, m4
sees ‘NL’, ‘ ’, and ‘ ’, but this
whitespace is discarded as part of argument collection. Then comes a
rather lengthy single-quoted string token, ‘[NL dnl
commentNL GNULIB_]’. This is followed by the token
‘translit’, which m4
recognizes as a macro name, so a nested
macro expansion has started.
The arguments to the translit
are found by the tokens ‘(’,
‘[$1]’, ‘,’, ‘[a-z]’, ‘,’, ‘[A-Z]’, and finally
‘)’. All three string arguments are expanded (or in other words,
the quotes are stripped), and since neither ‘$’ nor ‘1’ need
capitalization, the result of the macro is ‘$1’. This expansion is
rescanned, resulting in the two literal characters ‘$’ and
‘1’.
Scanning of the outer macro resumes, and picks up with ‘[=1NL ]’, and finally ‘)’. The collected pieces of expanded text are concatenated, with the end result that the macro ‘gl_STRING_MODULE_INDICATOR’ is now defined to be the sequence ‘NL dnl commentNL GNULIB_$1=1NL ’. Once again, ‘dnl’ is recognized and avoids a newline in the output.
The final line is then parsed, beginning with ‘ ’ and ‘ ’ that are output literally. Then ‘gl_STRING_MODULE_INDICATOR’ is recognized as a macro name, with an argument list of ‘(’, ‘[strcase]’, and ‘)’. Since the definition of the macro contains the sequence ‘$1’, that sequence is replaced with the argument ‘strcase’ prior to starting the rescan. The rescan sees ‘NL’ and four spaces, which are output literally, then ‘dnl’, which discards the text ‘ commentNL’. Next comes four more spaces, also output literally, and the token ‘GNULIB_strcase’, which resulted from the earlier parameter substitution. Since that is not a macro name, it is output literally, followed by the literal tokens ‘=’, ‘1’, ‘NL’, and two more spaces. Finally, the original ‘NL’ seen after the macro invocation is scanned and output literally.
Now for a corrected approach. This rearranges the use of newlines and
whitespace so that less whitespace is output (which, although harmless
to shell scripts, can be visually unappealing), and fixes the quoting
issues so that the capitalization occurs when the macro
‘gl_STRING_MODULE_INDICATOR’ is invoked, rather then when it is
defined. It also adds another layer of quoting to the first argument of
translit
, to ensure that the output will be rescanned as a string
rather than a potential uppercase macro name needing further expansion.
changequote([,])dnl define([gl_STRING_MODULE_INDICATOR], [dnl comment GNULIB_[]translit([[$1]], [a-z], [A-Z])=1dnl ])dnl gl_STRING_MODULE_INDICATOR([strcase]) ⇒ GNULIB_STRCASE=1
The parsing of the first line is unchanged. The second line sees the
name of the macro to define, then sees the discarded ‘NL’
and two spaces, as before. But this time, the next token is
‘[dnl commentNL GNULIB_[]translit([[$1]], [a-z],
[A-Z])=1dnlNL]’, which includes nested quotes, followed by
‘)’ to end the macro definition and ‘dnl’ to skip the
newline. No early expansion of translit
occurs, so the entire
string becomes the definition of the macro.
The final line is then parsed, beginning with two spaces that are
output literally, and an invocation of
gl_STRING_MODULE_INDICATOR
with the argument ‘strcase’.
Again, the ‘$1’ in the macro definition is substituted prior to
rescanning. Rescanning first encounters ‘dnl’, and discards
‘ commentNL’. Then two spaces are output literally. Next
comes the token ‘GNULIB_’, but that is not a macro, so it is
output literally. The token ‘[]’ is an empty string, so it does
not affect output. Then the token ‘translit’ is encountered.
This time, the arguments to translit
are parsed as ‘(’,
‘[[strcase]]’, ‘,’, ‘ ’, ‘[a-z]’, ‘,’, ‘ ’,
‘[A-Z]’, and ‘)’. The two spaces are discarded, and the
translit results in the desired result ‘[STRCASE]’. This is
rescanned, but since it is a string, the quotes are stripped and the
only output is a literal ‘STRCASE’.
Then the scanner sees ‘=’ and ‘1’, which are output
literally, followed by ‘dnl’ which discards the rest of the
definition of gl_STRING_MODULE_INDICATOR
. The newline at the
end of output is the literal ‘NL’ that appeared after the
invocation of the macro.
The order in which m4
expands the macros can be further explored
using the trace facilities of GNU m4
(see Trace).
Next: Definitions, Previous: Syntax, Up: Top [Contents][Index]
This chapter covers macro invocation, macro arguments and how macro expansion is treated.
• Invocation | Macro invocation | |
• Inhibiting Invocation | Preventing macro invocation | |
• Macro Arguments | Macro arguments | |
• Quoting Arguments | On Quoting Arguments to macros | |
• Macro expansion | Expanding macros |
Next: Inhibiting Invocation, Up: Macros [Contents][Index]
Macro invocations has one of the forms
name
which is a macro invocation without any arguments, or
name(arg1, arg2, …, argn)
which is a macro invocation with n arguments. Macros can have any number of arguments. All arguments are strings, but different macros might interpret the arguments in different ways.
The opening parenthesis must follow the name directly, with no spaces in between. If it does not, the macro is called with no arguments at all.
For a macro call to have no arguments, the parentheses must be left out. The macro call
name()
is a macro call with one argument, which is the empty string, not a call with no arguments.
Next: Macro Arguments, Previous: Invocation, Up: Macros [Contents][Index]
An innovation of the m4
language, compared to some of its
predecessors (like Strachey’s GPM
, for example), is the ability
to recognize macro calls without resorting to any special, prefixed
invocation character. While generally useful, this feature might
sometimes be the source of spurious, unwanted macro calls. So, GNU
m4
offers several mechanisms or techniques for inhibiting the
recognition of names as macro calls.
First of all, many builtin macros cannot meaningfully be called without arguments. As a GNU extension, for any of these macros, whenever an opening parenthesis does not immediately follow their name, the builtin macro call is not triggered. This solves the most usual cases, like for ‘include’ or ‘eval’. Later in this document, the sentence “This macro is recognized only with parameters” refers to this specific provision of GNU M4, also known as a blind builtin macro. For the builtins defined by POSIX that bear this disclaimer, POSIX specifically states that invoking those builtins without arguments is unspecified, because many other implementations simply invoke the builtin as though it were given one empty argument instead.
$ m4 eval ⇒eval eval(`1') ⇒1
There is also a command line option (--prefix-builtins, or
-P, see Invoking m4) that renames all
builtin macros with a prefix of ‘m4_’ at startup. The option has
no effect whatsoever on user defined macros. For example, with this option,
one has to write m4_dnl
and even m4_m4exit
. It also has
no effect on whether a macro requires parameters.
$ m4 -P eval ⇒eval eval(`1') ⇒eval(1) m4_eval ⇒m4_eval m4_eval(`1') ⇒1
Another alternative is to redefine problematic macros to a name less likely to cause conflicts, using Definitions.
If your version of GNU m4
has the changeword
feature
compiled in, it offers far more flexibility in specifying the
syntax of macro names, both builtin or user-defined. See Changeword,
for more information on this experimental feature.
Of course, the simplest way to prevent a name from being interpreted as a call to an existing macro is to quote it. The remainder of this section studies a little more deeply how quoting affects macro invocation, and how quoting can be used to inhibit macro invocation.
Even if quoting is usually done over the whole macro name, it can also be done over only a few characters of this name (provided, of course, that the unquoted portions are not also a macro). It is also possible to quote the empty string, but this works only inside the name. For example:
`divert' ⇒divert `d'ivert ⇒divert di`ver't ⇒divert div`'ert ⇒divert
all yield the string ‘divert’. While in both:
`'divert ⇒ divert`' ⇒
the divert
builtin macro will be called, which expands to the
empty string.
The output of macro evaluations is always rescanned. In the following
example, the input ‘x`'y’ yields the string ‘bCD’, exactly as
if m4
has been given ‘substr(ab`'cde, `1', `3')’ as input:
define(`cde', `CDE') ⇒ define(`x', `substr(ab') ⇒ define(`y', `cde, `1', `3')') ⇒ x`'y ⇒bCD
Unquoted strings on either side of a quoted string are subject to
being recognized as macro names. In the following example, quoting the
empty string allows for the second macro
to be recognized as such:
define(`macro', `m') ⇒ macro(`m')macro ⇒mmacro macro(`m')`'macro ⇒mm
Quoting may prevent recognizing as a macro name the concatenation of a macro expansion with the surrounding characters. In this example:
define(`macro', `di$1') ⇒ macro(`v')`ert' ⇒divert macro(`v')ert ⇒
the input will produce the string ‘divert’. When the quotes were
removed, the divert
builtin was called instead.
Next: Quoting Arguments, Previous: Inhibiting Invocation, Up: Macros [Contents][Index]
When a name is seen, and it has a macro definition, it will be expanded as a macro.
If the name is followed by an opening parenthesis, the arguments will be collected before the macro is called. If too few arguments are supplied, the missing arguments are taken to be the empty string. However, some builtins are documented to behave differently for a missing optional argument than for an explicit empty string. If there are too many arguments, the excess arguments are ignored. Unquoted leading whitespace is stripped off all arguments, but whitespace generated by a macro expansion or occurring after a macro that expanded to an empty string remains intact. Whitespace includes space, tab, newline, carriage return, vertical tab, and formfeed.
define(`macro', `$1') ⇒ macro( unquoted leading space lost) ⇒unquoted leading space lost macro(` quoted leading space kept') ⇒ quoted leading space kept macro( divert `unquoted space kept after expansion') ⇒ unquoted space kept after expansion macro(macro(` ')`whitespace from expansion kept') ⇒ ⇒whitespace from expansion kept macro(`unquoted trailing whitespace kept' ) ⇒unquoted trailing whitespace kept ⇒
Normally m4
will issue warnings if a builtin macro is called
with an inappropriate number of arguments, but it can be suppressed with
the --quiet command line option (or --silent, or
-Q, see Invoking m4). For user
defined macros, there is no check of the number of arguments given.
$ m4 index(`abc') error→m4:stdin:1: Warning: too few arguments to builtin `index' ⇒0 index(`abc',) ⇒0 index(`abc', `b', `ignored') error→m4:stdin:3: Warning: excess arguments to builtin `index' ignored ⇒1
$ m4 -Q index(`abc') ⇒0 index(`abc',) ⇒0 index(`abc', `b', `ignored') ⇒1
Macros are expanded normally during argument collection, and whatever commas, quotes and parentheses that might show up in the resulting expanded text will serve to define the arguments as well. Thus, if foo expands to ‘, b, c’, the macro call
bar(a foo, d)
is a macro call with four arguments, which are ‘a ’, ‘b’, ‘c’ and ‘d’. To understand why the first argument contains whitespace, remember that unquoted leading whitespace is never part of an argument, but trailing whitespace always is.
It is possible for a macro’s definition to change during argument collection, in which case the expansion uses the definition that was in effect at the time the opening ‘(’ was seen.
define(`f', `1') ⇒ f(define(`f', `2')) ⇒1 f ⇒2
It is an error if the end of file occurs while collecting arguments.
hello world ⇒hello world define( ^D error→m4:stdin:2: ERROR: end of file in argument list
Next: Macro expansion, Previous: Macro Arguments, Up: Macros [Contents][Index]
Each argument has unquoted leading whitespace removed. Within each argument, all unquoted parentheses must match. For example, if foo is a macro,
foo(() (`(') `(')
is a macro call, with one argument, whose value is ‘() (() (’. Commas separate arguments, except when they occur inside quotes, comments, or unquoted parentheses. See Pseudo Arguments, for examples.
It is common practice to quote all arguments to macros, unless you are sure you want the arguments expanded. Thus, in the above example with the parentheses, the ‘right’ way to do it is like this:
foo(`() (() (')
It is, however, in certain cases necessary (because nested expansion
must occur to create the arguments for the outer macro) or convenient
(because it uses fewer characters) to leave out quotes for some
arguments, and there is nothing wrong in doing it. It just makes life a
bit harder, if you are not careful to follow a consistent quoting style.
For consistency, this manual follows the rule of thumb that each layer
of parentheses introduces another layer of single quoting, except when
showing the consequences of quoting rules. This is done even when the
quoted string cannot be a macro, such as with integers when you have not
changed the syntax via changeword
(see Changeword).
The quoting rule of thumb of one level of quoting per parentheses has a nice property: when a macro name appears inside parentheses, you can determine when it will be expanded. If it is not quoted, it will be expanded prior to the outer macro, so that its expansion becomes the argument. If it is single-quoted, it will be expanded after the outer macro. And if it is double-quoted, it will be used as literal text instead of a macro name.
define(`active', `ACT, IVE') ⇒ define(`show', `$1 $1') ⇒ show(active) ⇒ACT ACT show(`active') ⇒ACT, IVE ACT, IVE show(``active'') ⇒active active
Previous: Quoting Arguments, Up: Macros [Contents][Index]
When the arguments, if any, to a macro call have been collected, the macro is expanded, and the expansion text is pushed back onto the input (unquoted), and reread. The expansion text from one macro call might therefore result in more macros being called, if the calls are included, completely or partially, in the first macro calls’ expansion.
Taking a very simple example, if foo expands to ‘bar’, and bar expands to ‘Hello’, the input
$ m4 -Dbar=Hello -Dfoo=bar foo ⇒Hello
will expand first to ‘bar’, and when this is reread and expanded, into ‘Hello’.
Next: Conditionals, Previous: Macros, Up: Top [Contents][Index]
Macros can be defined, redefined and deleted in several different ways. Also, it is possible to redefine a macro without losing a previous value, and bring back the original value at a later time.
• Define | Defining a new macro | |
• Arguments | Arguments to macros | |
• Pseudo Arguments | Special arguments to macros | |
• Undefine | Deleting a macro | |
• Defn | Renaming macros | |
• Pushdef | Temporarily redefining macros | |
• Indir | Indirect call of macros | |
• Builtin | Indirect call of builtins |
Next: Arguments, Up: Definitions [Contents][Index]
The normal way to define or redefine macros is to use the builtin
define
:
Defines name to expand to expansion. If expansion is not given, it is taken to be empty.
The expansion of define
is void.
The macro define
is recognized only with parameters.
The following example defines the macro foo to expand to the text ‘Hello World.’.
define(`foo', `Hello world.') ⇒ foo ⇒Hello world.
The empty line in the output is there because the newline is not
a part of the macro definition, and it is consequently copied to
the output. This can be avoided by use of the macro dnl
.
See Dnl, for details.
The first argument to define
should be quoted; otherwise, if the
macro is already defined, you will be defining a different macro. This
example shows the problems with underquoting, since we did not want to
redefine one
:
define(foo, one) ⇒ define(foo, two) ⇒ one ⇒two
GNU m4
normally replaces only the topmost
definition of a macro if it has several definitions from pushdef
(see Pushdef). Some other implementations of m4
replace all
definitions of a macro with define
. See Incompatibilities,
for more details.
As a GNU extension, the first argument to define
does
not have to be a simple word.
It can be any text string, even the empty string. A macro with a
non-standard name cannot be invoked in the normal way, as the name is
not recognized. It can only be referenced by the builtins indir
(see Indir) and defn
(see Defn).
Arrays and associative arrays can be simulated by using non-standard macro names.
Provide access to entries within an array. array
reads the entry
at location index, and array_set
assigns value to
location index.
define(`array', `defn(format(``array[%d]'', `$1'))') ⇒ define(`array_set', `define(format(``array[%d]'', `$1'), `$2')') ⇒ array_set(`4', `array element no. 4') ⇒ array_set(`17', `array element no. 17') ⇒ array(`4') ⇒array element no. 4 array(eval(`10 + 7')) ⇒array element no. 17
Change the ‘%d’ to ‘%s’ and it is an associative array.
Next: Pseudo Arguments, Previous: Define, Up: Definitions [Contents][Index]
Macros can have arguments. The nth argument is denoted by
$n
in the expansion text, and is replaced by the nth actual
argument, when the macro is expanded. Replacement of arguments happens
before rescanning, regardless of how many nesting levels of quoting
appear in the expansion. Here is an example of a macro with
two arguments.
Expands to arg2 followed by arg1, effectively exchanging their order.
define(`exch', `$2, $1') ⇒ exch(`arg1', `arg2') ⇒arg2, arg1
This can be used, for example, if you like the arguments to
define
to be reversed.
define(`exch', `$2, $1') ⇒ define(exch(``expansion text'', ``macro'')) ⇒ macro ⇒expansion text
See Quoting Arguments, for an explanation of the double quotes.
(You should try and improve this example so that clients of exch
do not have to double quote; or see Answers).
As a special case, the zeroth argument, $0
, is always the name
of the macro being expanded.
define(`test', ``Macro name: $0'') ⇒ test ⇒Macro name: test
If you want quoted text to appear as part of the expansion text, remember that quotes can be nested in quoted strings. Thus, in
define(`foo', `This is macro `foo'.') ⇒ foo ⇒This is macro foo.
The ‘foo’ in the expansion text is not expanded, since it is a quoted string, and not a name.
GNU m4
allows the number following the ‘$’ to
consist of one or more digits, allowing macros to have any number of
arguments. The extension of accepting multiple digits is incompatible
with POSIX, and is different than traditional implementations
of m4
, which only recognize one digit. Therefore, future
versions of GNU M4 will phase out this feature. To portably
access beyond the ninth argument, you can use the argn
macro
documented later (see Shift).
POSIX also states that ‘$’ followed immediately by
‘{’ in a macro definition is implementation-defined. This version
of M4 passes the literal characters ‘${’ through unchanged, but M4
2.0 will implement an optional feature similar to sh
, where
‘${11}’ expands to the eleventh argument, to replace the current
recognition of ‘$11’. Meanwhile, if you want to guarantee that you
will get a literal ‘${’ in output when expanding a macro, even
when you upgrade to M4 2.0, you can use nested quoting to your
advantage:
define(`foo', `single quoted $`'{1} output') ⇒ define(`bar', ``double quoted $'`{2} output'') ⇒ foo(`a', `b') ⇒single quoted ${1} output bar(`a', `b') ⇒double quoted ${2} output
To help you detect places in your M4 input files that might change in behavior due to the changed behavior of M4 2.0, you can use the --warn-macro-sequence command-line option (see Invoking m4) with the default regular expression. This will add a warning any time a macro definition includes ‘$’ followed by multiple digits, or by ‘{’. The warning is not enabled by default, because it triggers a number of warnings in Autoconf 2.61 (and Autoconf uses -E to treat warnings as errors), and because it will still be possible to restore older behavior in M4 2.0.
$ m4 --warn-macro-sequence define(`foo', `$001 ${1} $1') error→m4:stdin:1: Warning: definition of `foo' contains sequence `$001' error→m4:stdin:1: Warning: definition of `foo' contains sequence `${1}' ⇒ foo(`bar') ⇒bar ${1} bar
Next: Undefine, Previous: Arguments, Up: Definitions [Contents][Index]
There is a special notation for the number of actual arguments supplied, and for all the actual arguments.
The number of actual arguments in a macro call is denoted by $#
in the expansion text.
Expands to a count of the number of arguments supplied.
define(`nargs', `$#') ⇒ nargs ⇒0 nargs() ⇒1 nargs(`arg1', `arg2', `arg3') ⇒3 nargs(`commas can be quoted, like this') ⇒1 nargs(arg1#inside comments, commas do not separate arguments still arg1) ⇒1 nargs((unquoted parentheses, like this, group arguments)) ⇒1
Remember that ‘#’ defaults to the comment character; if you forget quotes to inhibit the comment behavior, your macro definition may not end where you expected.
dnl Attempt to define a macro to just `$#' define(underquoted, $#) oops) ⇒ underquoted ⇒0) ⇒oops
The notation $*
can be used in the expansion text to denote all
the actual arguments, unquoted, with commas in between. For example
define(`echo', `$*') ⇒ echo(arg1, arg2, arg3 , arg4) ⇒arg1,arg2,arg3 ,arg4
Often each argument should be quoted, and the notation $@
handles
that. It is just like $*
, except that it quotes each argument.
A simple example of that is:
define(`echo', `$@') ⇒ echo(arg1, arg2, arg3 , arg4) ⇒arg1,arg2,arg3 ,arg4
Where did the quotes go? Of course, they were eaten, when the expanded
text were reread by m4
. To show the difference, try
define(`echo1', `$*') ⇒ define(`echo2', `$@') ⇒ define(`foo', `This is macro `foo'.') ⇒ echo1(foo) ⇒This is macro This is macro foo.. echo1(`foo') ⇒This is macro foo. echo2(foo) ⇒This is macro foo. echo2(`foo') ⇒foo
See Trace, if you do not understand this. As another example of the difference, remember that comments encountered in arguments are passed untouched to the macro, and that quoting disables comments.
define(`echo1', `$*') ⇒ define(`echo2', `$@') ⇒ define(`foo', `bar') ⇒ echo1(#foo'foo foo) ⇒#foo'foo ⇒bar echo2(#foo'foo foo) ⇒#foobar ⇒bar'
A ‘$’ sign in the expansion text, that is not followed by anything
m4
understands, is simply copied to the macro expansion, as any
other text is.
define(`foo', `$$$ hello $$$') ⇒ foo ⇒$$$ hello $$$
If you want a macro to expand to something like ‘$12’, the
judicious use of nested quoting can put a safe character between the
$
and the next character, relying on the rescanning to remove the
nested quote. This will prevent m4
from interpreting the
$
sign as a reference to an argument.
define(`foo', `no nested quote: $1') ⇒ foo(`arg') ⇒no nested quote: arg define(`foo', `nested quote around $: `$'1') ⇒ foo(`arg') ⇒nested quote around $: $1 define(`foo', `nested empty quote after $: $`'1') ⇒ foo(`arg') ⇒nested empty quote after $: $1 define(`foo', `nested quote around next character: $`1'') ⇒ foo(`arg') ⇒nested quote around next character: $1 define(`foo', `nested quote around both: `$1'') ⇒ foo(`arg') ⇒nested quote around both: arg
Next: Defn, Previous: Pseudo Arguments, Up: Definitions [Contents][Index]
A macro definition can be removed with undefine
:
For each argument, remove the macro name. The macro names must necessarily be quoted, since they will be expanded otherwise.
The expansion of undefine
is void.
The macro undefine
is recognized only with parameters.
foo bar blah ⇒foo bar blah define(`foo', `some')define(`bar', `other')define(`blah', `text') ⇒ foo bar blah ⇒some other text undefine(`foo') ⇒ foo bar blah ⇒foo other text undefine(`bar', `blah') ⇒ foo bar blah ⇒foo bar blah
Undefining a macro inside that macro’s expansion is safe; the macro still expands to the definition that was in effect at the ‘(’.
define(`f', ``$0':$1') ⇒ f(f(f(undefine(`f')`hello world'))) ⇒f:f:f:hello world f(`bye') ⇒f(bye)
It is not an error for name to have no macro definition. In that
case, undefine
does nothing.
Next: Pushdef, Previous: Undefine, Up: Definitions [Contents][Index]
It is possible to rename an already defined macro. To do this, you need
the builtin defn
:
Expands to the quoted definition of each name. If an argument is not a defined macro, the expansion for that argument is empty.
If name is a user-defined macro, the quoted definition is simply
the quoted expansion text. If, instead, there is only one name
and it is a builtin, the
expansion is a special token, which points to the builtin’s internal
definition. This token is only meaningful as the second argument to
define
(and pushdef
), and is silently converted to an
empty string in most other contexts. Combining a builtin with anything
else is not supported; a warning is issued and the builtin is omitted
from the final expansion.
The macro defn
is recognized only with parameters.
Its normal use is best understood through an example, which shows how to
rename undefine
to zap
:
define(`zap', defn(`undefine')) ⇒ zap(`undefine') ⇒ undefine(`zap') ⇒undefine(zap)
In this way, defn
can be used to copy macro definitions, and also
definitions of builtin macros. Even if the original macro is removed,
the other name can still be used to access the definition.
The fact that macro definitions can be transferred also explains why you
should use $0
, rather than retyping a macro’s name in its
definition:
define(`foo', `This is `$0'') ⇒ define(`bar', defn(`foo')) ⇒ bar ⇒This is bar
Macros used as string variables should be referred through defn
,
to avoid unwanted expansion of the text:
define(`string', `The macro dnl is very useful ') ⇒ string ⇒The macro defn(`string') ⇒The macro dnl is very useful ⇒
However, it is important to remember that m4
rescanning is purely
textual. If an unbalanced end-quote string occurs in a macro
definition, the rescan will see that embedded quote as the termination
of the quoted string, and the remainder of the macro’s definition will
be rescanned unquoted. Thus it is a good idea to avoid unbalanced
end-quotes in macro definitions or arguments to macros.
define(`foo', a'a) ⇒ define(`a', `A') ⇒ define(`echo', `$@') ⇒ foo ⇒A'A defn(`foo') ⇒aA' echo(foo) ⇒AA'
On the other hand, it is possible to exploit the fact that defn
can concatenate multiple macros prior to the rescanning phase, in order
to join the definitions of macros that, in isolation, have unbalanced
quotes. This is particularly useful when one has used several macros to
accumulate text that M4 should rescan as a whole. In the example below,
note how the use of defn
on l
in isolation opens a string,
which is not closed until the next line; but used on l
and
r
together results in nested quoting.
define(`l', `<[>')define(`r', `<]>') ⇒ changequote(`[', `]') ⇒ defn([l])defn([r]) ]) ⇒<[>]defn([r]) ⇒) defn([l], [r]) ⇒<[>][<]>
Using defn
to generate special tokens for builtin macros outside
of expected contexts can sometimes trigger warnings. But most of the
time, such tokens are silently converted to the empty string.
$ m4 -d defn(`defn') ⇒ define(defn(`divnum'), `cannot redefine a builtin token') error→m4:stdin:2: Warning: define: invalid macro name ignored ⇒ divnum ⇒0 len(defn(`divnum')) ⇒0
Also note that defn
with multiple arguments can only join text
macros, not builtins, although a future version of GNU M4 may
lift this restriction.
$ m4 -d define(`a', `A')define(`AA', `b') ⇒ traceon(`defn', `define') ⇒ defn(`a', `divnum', `a') error→m4:stdin:3: Warning: cannot concatenate builtin `divnum' error→m4trace: -1- defn(`a', `divnum', `a') -> ``A'`A'' ⇒AA define(`mydivnum', defn(`divnum', `divnum'))mydivnum error→m4:stdin:4: Warning: cannot concatenate builtin `divnum' error→m4:stdin:4: Warning: cannot concatenate builtin `divnum' error→m4trace: -2- defn(`divnum', `divnum') error→m4trace: -1- define(`mydivnum', `') ⇒ traceoff(`defn', `define') ⇒
Next: Indir, Previous: Defn, Up: Definitions [Contents][Index]
It is possible to redefine a macro temporarily, reverting to the
previous definition at a later time. This is done with the builtins
pushdef
and popdef
:
Analogous to define
and undefine
.
These macros work in a stack-like fashion. A macro is temporarily
redefined with pushdef
, which replaces an existing definition of
name, while saving the previous definition, before the new one is
installed. If there is no previous definition, pushdef
behaves
exactly like define
.
If a macro has several definitions (of which only one is accessible),
the topmost definition can be removed with popdef
. If there is
no previous definition, popdef
behaves like undefine
.
The expansion of both pushdef
and popdef
is void.
The macros pushdef
and popdef
are recognized only with
parameters.
define(`foo', `Expansion one.') ⇒ foo ⇒Expansion one. pushdef(`foo', `Expansion two.') ⇒ foo ⇒Expansion two. pushdef(`foo', `Expansion three.') ⇒ pushdef(`foo', `Expansion four.') ⇒ popdef(`foo') ⇒ foo ⇒Expansion three. popdef(`foo', `foo') ⇒ foo ⇒Expansion one. popdef(`foo') ⇒ foo ⇒foo
If a macro with several definitions is redefined with define
, the
topmost definition is replaced with the new definition. If it is
removed with undefine
, all the definitions are removed,
and not only the topmost one. However, POSIX allows other
implementations that treat define
as replacing an entire stack
of definitions with a single new definition, so to be portable to other
implementations, it may be worth explicitly using popdef
and
pushdef
rather than relying on the GNU behavior of
define
.
define(`foo', `Expansion one.') ⇒ foo ⇒Expansion one. pushdef(`foo', `Expansion two.') ⇒ foo ⇒Expansion two. define(`foo', `Second expansion two.') ⇒ foo ⇒Second expansion two. undefine(`foo') ⇒ foo ⇒foo
Local variables within macros are made with pushdef
and
popdef
. At the start of the macro a new definition is pushed,
within the macro it is manipulated and at the end it is popped,
revealing the former definition.
It is possible to temporarily redefine a builtin with pushdef
and defn
.
Next: Builtin, Previous: Pushdef, Up: Definitions [Contents][Index]
Any macro can be called indirectly with indir
:
Results in a call to the macro name, which is passed the rest of the arguments args. If name is not defined, an error message is printed, and the expansion is void.
The macro indir
is recognized only with parameters.
This can be used to call macros with computed or “invalid”
names (define
allows such names to be defined):
define(`$$internal$macro', `Internal macro (name `$0')') ⇒ $$internal$macro ⇒$$internal$macro indir(`$$internal$macro') ⇒Internal macro (name $$internal$macro)
The point is, here, that larger macro packages can have private macros
defined, that will not be called by accident. They can only be
called through the builtin indir
.
One other point to observe is that argument collection occurs before
indir
invokes name, so if argument collection changes the
value of name, that will be reflected in the final expansion.
This is different than the behavior when invoking macros directly,
where the definition that was in effect before argument collection is
used.
$ m4 -d define(`f', `1') ⇒ f(define(`f', `2')) ⇒1 indir(`f', define(`f', `3')) ⇒3 indir(`f', undefine(`f')) error→m4:stdin:4: undefined macro `f' ⇒
When handed the result of defn
(see Defn) as one of its
arguments, indir
defers to the invoked name for whether a
token representing a builtin is recognized or flattened to the empty
string.
$ m4 -d indir(defn(`defn'), `divnum') error→m4:stdin:1: Warning: indir: invalid macro name ignored ⇒ indir(`define', defn(`defn'), `divnum') error→m4:stdin:2: Warning: define: invalid macro name ignored ⇒ indir(`define', `foo', defn(`divnum')) ⇒ foo ⇒0 indir(`divert', defn(`foo')) error→m4:stdin:5: empty string treated as 0 in builtin `divert' ⇒
Previous: Indir, Up: Definitions [Contents][Index]
Builtin macros can be called indirectly with builtin
:
Results in a call to the builtin name, which is passed the rest of the arguments args. If name does not name a builtin, an error message is printed, and the expansion is void.
The macro builtin
is recognized only with parameters.
This can be used even if name has been given another definition that has covered the original, or been undefined so that no macro maps to the builtin.
pushdef(`define', `hidden') ⇒ undefine(`undefine') ⇒ define(`foo', `bar') ⇒hidden foo ⇒foo builtin(`define', `foo', defn(`divnum')) ⇒ foo ⇒0 builtin(`define', `foo', `BAR') ⇒ foo ⇒BAR undefine(`foo') ⇒undefine(foo) foo ⇒BAR builtin(`undefine', `foo') ⇒ foo ⇒foo
The name argument only matches the original name of the builtin,
even when the --prefix-builtins option (or -P,
see Invoking m4) is in effect. This is different
from indir
, which only tracks current macro names.
$ m4 -P m4_builtin(`divnum') ⇒0 m4_builtin(`m4_divnum') error→m4:stdin:2: undefined builtin `m4_divnum' ⇒ m4_indir(`divnum') error→m4:stdin:3: undefined macro `divnum' ⇒ m4_indir(`m4_divnum') ⇒0
Note that indir
and builtin
can be used to invoke builtins
without arguments, even when they normally require parameters to be
recognized; but it will provoke a warning, and result in a void expansion.
builtin ⇒builtin builtin() error→m4:stdin:2: undefined builtin `' ⇒ builtin(`builtin') error→m4:stdin:3: Warning: too few arguments to builtin `builtin' ⇒ builtin(`builtin',) error→m4:stdin:4: undefined builtin `' ⇒ builtin(`builtin', ``' ') error→m4:stdin:5: undefined builtin ``' error→' ⇒ indir(`index') error→m4:stdin:7: Warning: too few arguments to builtin `index' ⇒
Next: Debugging, Previous: Definitions, Up: Top [Contents][Index]
Macros, expanding to plain text, perhaps with arguments, are not quite enough. We would like to have macros expand to different things, based on decisions taken at run-time. For that, we need some kind of conditionals. Also, we would like to have some kind of loop construct, so we could do something a number of times, or while some condition is true.
• Ifdef | Testing if a macro is defined | |
• Ifelse | If-else construct, or multibranch | |
• Shift | Recursion in m4
| |
• Forloop | Iteration by counting | |
• Foreach | Iteration by list contents | |
• Stacks | Working with definition stacks | |
• Composition | Building macros with macros |
Next: Ifelse, Up: Conditionals [Contents][Index]
There are two different builtin conditionals in m4
. The first is
ifdef
:
If name is defined as a macro, ifdef
expands to
string-1, otherwise to string-2. If string-2 is
omitted, it is taken to be the empty string (according to the normal
rules).
The macro ifdef
is recognized only with parameters.
ifdef(`foo', ``foo' is defined', ``foo' is not defined') ⇒foo is not defined define(`foo', `') ⇒ ifdef(`foo', ``foo' is defined', ``foo' is not defined') ⇒foo is defined ifdef(`no_such_macro', `yes', `no', `extra argument') error→m4:stdin:4: Warning: excess arguments to builtin `ifdef' ignored ⇒no
Next: Shift, Previous: Ifdef, Up: Conditionals [Contents][Index]
The other conditional, ifelse
, is much more powerful. It can be
used as a way to introduce a long comment, as an if-else construct, or
as a multibranch, depending on the number of arguments supplied:
Used with only one argument, the ifelse
simply discards it and
produces no output.
If called with three or four arguments, ifelse
expands into
equal, if string-1 and string-2 are equal (character
for character), otherwise it expands to not-equal. A final fifth
argument is ignored, after triggering a warning.
If called with six or more arguments, and string-1 and
string-2 are equal, ifelse
expands into equal-1,
otherwise the first three arguments are discarded and the processing
starts again.
The macro ifelse
is recognized only with parameters.
Using only one argument is a common m4
idiom for introducing a
block comment, as an alternative to repeatedly using dnl
. This
special usage is recognized by GNU m4
, so that in this
case, the warning about missing arguments is never triggered.
ifelse(`some comments') ⇒ ifelse(`foo', `bar') error→m4:stdin:2: Warning: too few arguments to builtin `ifelse' ⇒
Using three or four arguments provides decision points.
ifelse(`foo', `bar', `true') ⇒ ifelse(`foo', `foo', `true') ⇒true define(`foo', `bar') ⇒ ifelse(foo, `bar', `true', `false') ⇒true ifelse(foo, `foo', `true', `false') ⇒false
Notice how the first argument was used unquoted; it is common to compare the expansion of a macro with a string. With this macro, you can now reproduce the behavior of blind builtins, where the macro is recognized only with arguments.
define(`foo', `ifelse(`$#', `0', ``$0'', `arguments:$#')') ⇒ foo ⇒foo foo() ⇒arguments:1 foo(`a', `b', `c') ⇒arguments:3
For an example of a way to make defining blind macros easier, see Composition.
The macro ifelse
can take more than four arguments. If given more
than four arguments, ifelse
works like a case
or switch
statement in traditional programming languages. If string-1 and
string-2 are equal, ifelse
expands into equal-1, otherwise
the procedure is repeated with the first three arguments discarded. This
calls for an example:
ifelse(`foo', `bar', `third', `gnu', `gnats') error→m4:stdin:1: Warning: excess arguments to builtin `ifelse' ignored ⇒gnu ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth') ⇒ ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth', `seventh') ⇒seventh ifelse(`foo', `bar', `3', `gnu', `gnats', `6', `7', `8') error→m4:stdin:4: Warning: excess arguments to builtin `ifelse' ignored ⇒7
Naturally, the normal case will be slightly more advanced than these
examples. A common use of ifelse
is in macros implementing loops
of various kinds.
Next: Forloop, Previous: Ifelse, Up: Conditionals [Contents][Index]
m4
There is no direct support for loops in m4
, but macros can be
recursive. There is no limit on the number of recursion levels, other
than those enforced by your hardware and operating system.
Loops can be programmed using recursion and the conditionals described previously.
There is a builtin macro, shift
, which can, among other things,
be used for iterating through the actual arguments to a macro:
Takes any number of arguments, and expands to all its arguments except arg1, separated by commas, with each argument quoted.
The macro shift
is recognized only with parameters.
shift ⇒shift shift(`bar') ⇒ shift(`foo', `bar', `baz') ⇒bar,baz
An example of the use of shift
is this macro:
Takes any number of arguments, and reverses their order.
It is implemented as:
define(`reverse', `ifelse(`$#', `0', , `$#', `1', ``$1'', `reverse(shift($@)), `$1'')') ⇒ reverse ⇒ reverse(`foo') ⇒foo reverse(`foo', `bar', `gnats', `and gnus') ⇒and gnus, gnats, bar, foo
While not a very interesting macro, it does show how simple loops can be
made with shift
, ifelse
and recursion. It also shows
that shift
is usually used with ‘$@’. Another example of
this is an implementation of a short-circuiting conditional operator.
Similar to ifelse
, where an equal comparison between the first
two strings results in the third, otherwise the first three arguments
are discarded and the process repeats. The difference is that each
test-<n> is expanded only when it is encountered. This means that
every third argument to cond
is normally given one more level of
quoting than the corresponding argument to ifelse
.
Here is the implementation of cond
, along with a demonstration of
how it can short-circuit the side effects in side
. Notice how
all the unquoted side effects happen regardless of how many comparisons
are made with ifelse
, compared with only the relevant effects
with cond
.
define(`cond', `ifelse(`$#', `1', `$1', `ifelse($1, `$2', `$3', `$0(shift(shift(shift($@))))')')')dnl define(`side', `define(`counter', incr(counter))$1')dnl define(`example1', `define(`counter', `0')dnl ifelse(side(`$1'), `yes', `one comparison: ', side(`$1'), `no', `two comparisons: ', side(`$1'), `maybe', `three comparisons: ', `side(`default answer: ')')counter')dnl define(`example2', `define(`counter', `0')dnl cond(`side(`$1')', `yes', `one comparison: ', `side(`$1')', `no', `two comparisons: ', `side(`$1')', `maybe', `three comparisons: ', `side(`default answer: ')')counter')dnl example1(`yes') ⇒one comparison: 3 example1(`no') ⇒two comparisons: 3 example1(`maybe') ⇒three comparisons: 3 example1(`feeling rather indecisive today') ⇒default answer: 4 example2(`yes') ⇒one comparison: 1 example2(`no') ⇒two comparisons: 2 example2(`maybe') ⇒three comparisons: 3 example2(`feeling rather indecisive today') ⇒default answer: 4
Another common task that requires iteration is joining a list of arguments into a single string.
Generate a single-quoted string, consisting of each arg separated
by separator. While joinall
always outputs a
separator between arguments, join
avoids the
separator for an empty arg.
Here are some examples of its usage, based on the implementation m4-1.4.19/examples/join.m4 distributed in this package:
$ m4 -I examples include(`join.m4') ⇒ join,join(`-'),join(`-', `'),join(`-', `', `') ⇒,,, joinall,joinall(`-'),joinall(`-', `'),joinall(`-', `', `') ⇒,,,- join(`-', `1') ⇒1 join(`-', `1', `2', `3') ⇒1-2-3 join(`', `1', `2', `3') ⇒123 join(`-', `', `1', `', `', `2', `') ⇒1-2 joinall(`-', `', `1', `', `', `2', `') ⇒-1---2- join(`,', `1', `2', `3') ⇒1,2,3 define(`nargs', `$#')dnl nargs(join(`,', `1', `2', `3')) ⇒1
Examining the implementation shows some interesting points about several m4 programming idioms.
$ m4 -I examples undivert(`join.m4')dnl ⇒divert(`-1') ⇒# join(sep, args) - join each non-empty ARG into a single ⇒# string, with each element separated by SEP ⇒define(`join', ⇒`ifelse(`$#', `2', ``$2'', ⇒ `ifelse(`$2', `', `', ``$2'_')$0(`$1', shift(shift($@)))')') ⇒define(`_join', ⇒`ifelse(`$#$2', `2', `', ⇒ `ifelse(`$2', `', `', ``$1$2'')$0(`$1', shift(shift($@)))')') ⇒# joinall(sep, args) - join each ARG, including empty ones, ⇒# into a single string, with each element separated by SEP ⇒define(`joinall', ``$2'_$0(`$1', shift($@))') ⇒define(`_joinall', ⇒`ifelse(`$#', `2', `', ``$1$3'$0(`$1', shift(shift($@)))')') ⇒divert`'dnl
First, notice that this implementation creates helper macros
_join
and _joinall
. This division of labor makes it
easier to output the correct number of separator instances:
join
and joinall
are responsible for the first argument,
without a separator, while _join
and _joinall
are
responsible for all remaining arguments, always outputting a separator
when outputting an argument.
Next, observe how join
decides to iterate to itself, because the
first arg was empty, or to output the argument and swap over to
_join
. If the argument is non-empty, then the nested
ifelse
results in an unquoted ‘_’, which is concatenated
with the ‘$0’ to form the next macro name to invoke. The
joinall
implementation is simpler since it does not have to
suppress empty arg; it always executes once then defers to
_joinall
.
Another important idiom is the idea that separator is reused for
each iteration. Each iteration has one less argument, but rather than
discarding ‘$1’ by iterating with $0(shift($@))
, the macro
discards ‘$2’ by using $0(`$1', shift(shift($@)))
.
Next, notice that it is possible to compare more than one condition in a
single ifelse
test. The test of ‘$#$2’ against ‘2’
allows _join
to iterate for two separate reasons—either there
are still more than two arguments, or there are exactly two arguments
but the last argument is not empty.
Finally, notice that these macros require exactly two arguments to
terminate recursion, but that they still correctly result in empty
output when given no args (i.e., zero or one macro argument). On
the first pass when there are too few arguments, the shift
results in no output, but leaves an empty string to serve as the
required second argument for the second pass. Put another way,
‘`$1', shift($@)’ is not the same as ‘$@’, since only the
former guarantees at least two arguments.
Sometimes, a recursive algorithm requires adding quotes to each element, or treating multiple arguments as a single element:
Takes any number of arguments, and adds quoting. With quote
,
only one level of quoting is added, effectively removing whitespace
after commas and turning multiple arguments into a single string. With
dquote
, two levels of quoting are added, one around each element,
and one around the list. And with dquote_elt
, two levels of
quoting are added around each element.
An actual implementation of these three macros is distributed as m4-1.4.19/examples/quote.m4 in this package. First, let’s examine their usage:
$ m4 -I examples include(`quote.m4') ⇒ -quote-dquote-dquote_elt- ⇒---- -quote()-dquote()-dquote_elt()- ⇒--`'-`'- -quote(`1')-dquote(`1')-dquote_elt(`1')- ⇒-1-`1'-`1'- -quote(`1', `2')-dquote(`1', `2')-dquote_elt(`1', `2')- ⇒-1,2-`1',`2'-`1',`2'- define(`n', `$#')dnl -n(quote(`1', `2'))-n(dquote(`1', `2'))-n(dquote_elt(`1', `2'))- ⇒-1-1-2- dquote(dquote_elt(`1', `2')) ⇒``1'',``2'' dquote_elt(dquote(`1', `2')) ⇒``1',`2''
The last two lines show that when given two arguments, dquote
results in one string, while dquote_elt
results in two. Now,
examine the implementation. Note that quote
and
dquote_elt
make decisions based on their number of arguments, so
that when called without arguments, they result in nothing instead of a
quoted empty string; this is so that it is possible to distinguish
between no arguments and an empty first argument. dquote
, on the
other hand, results in a string no matter what, since it is still
possible to tell whether it was invoked without arguments based on the
resulting string.
$ m4 -I examples undivert(`quote.m4')dnl ⇒divert(`-1') ⇒# quote(args) - convert args to single-quoted string ⇒define(`quote', `ifelse(`$#', `0', `', ``$*'')') ⇒# dquote(args) - convert args to quoted list of quoted strings ⇒define(`dquote', ``$@'') ⇒# dquote_elt(args) - convert args to list of double-quoted strings ⇒define(`dquote_elt', `ifelse(`$#', `0', `', `$#', `1', ```$1''', ⇒ ```$1'',$0(shift($@))')') ⇒divert`'dnl
It is worth pointing out that ‘quote(args)’ is more efficient than ‘joinall(`,', args)’ for producing the same output.
One more useful macro based on shift
allows portably selecting
an arbitrary argument (usually greater than the ninth argument), without
relying on the GNU extension of multi-digit arguments
(see Arguments).
Expands to argument n out of the remaining arguments. n must be a positive number. Usually invoked as ‘argn(`n',$@)’.
It is implemented as:
define(`argn', `ifelse(`$1', 1, ``$2'', `argn(decr(`$1'), shift(shift($@)))')') ⇒ argn(`1', `a') ⇒a define(`foo', `argn(`11', $@)') ⇒ foo(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k', `l') ⇒k
Next: Foreach, Previous: Shift, Up: Conditionals [Contents][Index]
Here is an example of a loop macro that implements a simple for loop.
Takes the name in iterator, which must be a valid macro name, and
successively assign it each integer value from start to end,
inclusive. For each assignment to iterator, append text to
the expansion of the forloop
. text may refer to
iterator. Any definition of iterator prior to this
invocation is restored.
It can, for example, be used for simple counting:
$ m4 -I examples include(`forloop.m4') ⇒ forloop(`i', `1', `8', `i ') ⇒1 2 3 4 5 6 7 8
For-loops can be nested, like:
$ m4 -I examples include(`forloop.m4') ⇒ forloop(`i', `1', `4', `forloop(`j', `1', `8', ` (i, j)') ') ⇒ (1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6) (1, 7) (1, 8) ⇒ (2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6) (2, 7) (2, 8) ⇒ (3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6) (3, 7) (3, 8) ⇒ (4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6) (4, 7) (4, 8) ⇒
The implementation of the forloop
macro is fairly
straightforward. The forloop
macro itself is simply a wrapper,
which saves the previous definition of the first argument, calls the
internal macro _forloop
, and re-establishes the saved
definition of the first argument.
The macro _forloop
expands the fourth argument once, and
tests to see if the iterator has reached the final value. If it has
not finished, it increments the iterator (using the predefined macro
incr
, see Incr), and recurses.
Here is an actual implementation of forloop
, distributed as
m4-1.4.19/examples/forloop.m4 in this package:
$ m4 -I examples undivert(`forloop.m4')dnl ⇒divert(`-1') ⇒# forloop(var, from, to, stmt) - simple version ⇒define(`forloop', `pushdef(`$1', `$2')_forloop($@)popdef(`$1')') ⇒define(`_forloop', ⇒ `$4`'ifelse($1, `$3', `', `define(`$1', incr($1))$0($@)')') ⇒divert`'dnl
Notice the careful use of quotes. Certain macro arguments are left unquoted, each for its own reason. Try to find out why these arguments are left unquoted, and see what happens if they are quoted. (As presented, these two macros are useful but not very robust for general use. They lack even basic error handling for cases like start less than end, end not numeric, or iterator not being a macro name. See if you can improve these macros; or see Answers).
Next: Stacks, Previous: Forloop, Up: Conditionals [Contents][Index]
Here is an example of a loop macro that implements list iteration.
Takes the name in iterator, which must be a valid macro name, and
successively assign it each value from paren-list or
quote-list. In foreach
, paren-list is a
comma-separated list of elements contained in parentheses. In
foreachq
, quote-list is a comma-separated list of elements
contained in a quoted string. For each assignment to iterator,
append text to the overall expansion. text may refer to
iterator. Any definition of iterator prior to this
invocation is restored.
As an example, this displays each word in a list inside of a sentence,
using an implementation of foreach
distributed as
m4-1.4.19/examples/foreach.m4, and foreachq
in m4-1.4.19/examples/foreachq.m4.
$ m4 -I examples include(`foreach.m4') ⇒ foreach(`x', (foo, bar, foobar), `Word was: x ')dnl ⇒Word was: foo ⇒Word was: bar ⇒Word was: foobar include(`foreachq.m4') ⇒ foreachq(`x', `foo, bar, foobar', `Word was: x ')dnl ⇒Word was: foo ⇒Word was: bar ⇒Word was: foobar
It is possible to be more complex; each element of the paren-list or quote-list can itself be a list, to pass as further arguments to a helper macro. This example generates a shell case statement:
$ m4 -I examples include(`foreach.m4') ⇒ define(`_case', ` $1) $2=" $1";; ')dnl define(`_cat', `$1$2')dnl case $`'1 in ⇒case $1 in foreach(`x', `(`(`a', `vara')', `(`b', `varb')', `(`c', `varc')')', `_cat(`_case', x)')dnl ⇒ a) ⇒ vara=" a";; ⇒ b) ⇒ varb=" b";; ⇒ c) ⇒ varc=" c";; esac ⇒esac
The implementation of the foreach
macro is a bit more involved;
it is a wrapper around two helper macros. First, _arg1
is
needed to grab the first element of a list. Second,
_foreach
implements the recursion, successively walking
through the original list. Here is a simple implementation of
foreach
:
$ m4 -I examples undivert(`foreach.m4')dnl ⇒divert(`-1') ⇒# foreach(x, (item_1, item_2, ..., item_n), stmt) ⇒# parenthesized list, simple version ⇒define(`foreach', `pushdef(`$1')_foreach($@)popdef(`$1')') ⇒define(`_arg1', `$1') ⇒define(`_foreach', `ifelse(`$2', `()', `', ⇒ `define(`$1', _arg1$2)$3`'$0(`$1', (shift$2), `$3')')') ⇒divert`'dnl
Unfortunately, that implementation is not robust to macro names as list
elements. Each iteration of _foreach
is stripping another
layer of quotes, leading to erratic results if list elements are not
already fully expanded. The first cut at implementing foreachq
takes this into account. Also, when using quoted elements in a
paren-list, the overall list must be quoted. A quote-list
has the nice property of requiring fewer characters to create a list
containing the same quoted elements. To see the difference between the
two macros, we attempt to pass double-quoted macro names in a list,
expecting the macro name on output after one layer of quotes is removed
during list iteration and the final layer removed during the final
rescan:
$ m4 -I examples define(`a', `1')define(`b', `2')define(`c', `3') ⇒ include(`foreach.m4') ⇒ include(`foreachq.m4') ⇒ foreach(`x', `(``a'', ``(b'', ``c)'')', `x ') ⇒1 ⇒(2)1 ⇒ ⇒, x ⇒) foreachq(`x', ```a'', ``(b'', ``c)''', `x ')dnl ⇒a ⇒(b ⇒c)
Obviously, foreachq
did a better job; here is its implementation:
$ m4 -I examples undivert(`foreachq.m4')dnl ⇒include(`quote.m4')dnl ⇒divert(`-1') ⇒# foreachq(x, `item_1, item_2, ..., item_n', stmt) ⇒# quoted list, simple version ⇒define(`foreachq', `pushdef(`$1')_foreachq($@)popdef(`$1')') ⇒define(`_arg1', `$1') ⇒define(`_foreachq', `ifelse(quote($2), `', `', ⇒ `define(`$1', `_arg1($2)')$3`'$0(`$1', `shift($2)', `$3')')') ⇒divert`'dnl
Notice that _foreachq
had to use the helper macro
quote
defined earlier (see Shift), to ensure that the
embedded ifelse
call does not go haywire if a list element
contains a comma. Unfortunately, this implementation of foreachq
has its own severe flaw. Whereas the foreach
implementation was
linear, this macro is quadratic in the number of list elements, and is
much more likely to trip up the limit set by the command line option
--nesting-limit (or -L, see Invoking m4). Additionally, this implementation does not expand
‘defn(`iterator')’ very well, when compared with
foreach
.
$ m4 -I examples include(`foreach.m4')include(`foreachq.m4') ⇒ foreach(`name', `(`a', `b')', ` defn(`name')') ⇒ a b foreachq(`name', ``a', `b'', ` defn(`name')') ⇒ _arg1(`a', `b') _arg1(shift(`a', `b'))
It is possible to have robust iteration with linear behavior and sane iterator contents for either list style. See if you can learn from the best elements of both of these implementations to create robust macros (or see Answers).
Next: Composition, Previous: Foreach, Up: Conditionals [Contents][Index]
Thanks to pushdef
, manipulation of a stack is an intrinsic
operation in m4
. Normally, only the topmost definition in a
stack is important, but sometimes, it is desirable to manipulate the
entire definition stack.
For each of the pushdef
definitions associated with macro,
invoke the macro action with a single argument of that definition.
stack_foreach
visits the oldest definition first, while
stack_foreach_lifo
visits the current definition first.
action should not modify or dereference macro. There are a
few special macros, such as defn
, which cannot be used as the
macro parameter.
A sample implementation of these macros is distributed in the file m4-1.4.19/examples/stack.m4.
$ m4 -I examples include(`stack.m4') ⇒ pushdef(`a', `1')pushdef(`a', `2')pushdef(`a', `3') ⇒ define(`show', ``$1' ') ⇒ stack_foreach(`a', `show')dnl ⇒1 ⇒2 ⇒3 stack_foreach_lifo(`a', `show')dnl ⇒3 ⇒2 ⇒1
Now for the implementation. Note the definition of a helper macro,
_stack_reverse
, which destructively swaps the contents of one
stack of definitions into the reverse order in the temporary macro
‘tmp-$1’. By calling the helper twice, the original order is
restored back into the macro ‘$1’; since the operation is
destructive, this explains why ‘$1’ must not be modified or
dereferenced during the traversal. The caller can then inject
additional code to pass the definition currently being visited to
‘$2’. The choice of helper names is intentional; since ‘-’ is
not valid as part of a macro name, there is no risk of conflict with a
valid macro name, and the code is guaranteed to use defn
where
necessary. Finally, note that any macro used in the traversal of a
pushdef
stack, such as pushdef
or defn
, cannot be
handled by stack_foreach
, since the macro would temporarily be
undefined during the algorithm.
$ m4 -I examples undivert(`stack.m4')dnl ⇒divert(`-1') ⇒# stack_foreach(macro, action) ⇒# Invoke ACTION with a single argument of each definition ⇒# from the definition stack of MACRO, starting with the oldest. ⇒define(`stack_foreach', ⇒`_stack_reverse(`$1', `tmp-$1')'dnl ⇒`_stack_reverse(`tmp-$1', `$1', `$2(defn(`$1'))')') ⇒# stack_foreach_lifo(macro, action) ⇒# Invoke ACTION with a single argument of each definition ⇒# from the definition stack of MACRO, starting with the newest. ⇒define(`stack_foreach_lifo', ⇒`_stack_reverse(`$1', `tmp-$1', `$2(defn(`$1'))')'dnl ⇒`_stack_reverse(`tmp-$1', `$1')') ⇒define(`_stack_reverse', ⇒`ifdef(`$1', `pushdef(`$2', defn(`$1'))$3`'popdef(`$1')$0($@)')') ⇒divert`'dnl
Previous: Stacks, Up: Conditionals [Contents][Index]
Since m4 is a macro language, it is possible to write macros that can build other macros. First on the list is a way to automate the creation of blind macros.
Defines name as a blind macro, such that name will expand to
value only when given explicit arguments. value should not
be the result of defn
(see Defn). This macro is only
recognized with parameters, and results in an empty string.
Defining a macro to define another macro can be a bit tricky. We want
to use a literal ‘$#’ in the argument to the nested define
.
However, if ‘$’ and ‘#’ are adjacent in the definition of
define_blind
, then it would be expanded as the number of
arguments to define_blind
rather than the intended number of
arguments to name. The solution is to pass the difficult
characters through extra arguments to a helper macro
_define_blind
. When composing macros, it is a common idiom to
need a helper macro to concatenate text that forms parameters in the
composed macro, rather than interpreting the text as a parameter of the
composing macro.
As for the limitation against using defn
, there are two reasons.
If a macro was previously defined with define_blind
, then it can
safely be renamed to a new blind macro using plain define
; using
define_blind
to rename it just adds another layer of
ifelse
, occupying memory and slowing down execution. And if a
macro is a builtin, then it would result in an attempt to define a macro
consisting of both text and a builtin token; this is not supported, and
the builtin token is flattened to an empty string.
With that explanation, here’s the definition, and some sample usage.
Notice that define_blind
is itself a blind macro.
$ m4 -d define(`define_blind', `ifelse(`$#', `0', ``$0'', `_$0(`$1', `$2', `$'`#', `$'`0')')') ⇒ define(`_define_blind', `define(`$1', `ifelse(`$3', `0', ``$4'', `$2')')') ⇒ define_blind ⇒define_blind define_blind(`foo', `arguments were $*') ⇒ foo ⇒foo foo(`bar') ⇒arguments were bar define(`blah', defn(`foo')) ⇒ blah ⇒blah blah(`a', `b') ⇒arguments were a,b defn(`blah') ⇒ifelse(`$#', `0', ``$0'', `arguments were $*')
Another interesting composition tactic is argument currying, or factoring a macro that takes multiple arguments for use in a context that provides exactly one argument.
Expand to a macro call that takes exactly one argument, then appends that argument to the original arguments and invokes macro with the resulting list of arguments.
A demonstration of currying makes the intent of this macro a little more
obvious. The macro stack_foreach
mentioned earlier is an example
of a context that provides exactly one argument to a macro name. But
coupled with currying, we can invoke reverse
with two arguments
for each definition of a macro stack. This example uses the file
m4-1.4.19/examples/curry.m4 included in the
distribution.
$ m4 -I examples include(`curry.m4')include(`stack.m4') ⇒ define(`reverse', `ifelse(`$#', `0', , `$#', `1', ``$1'', `reverse(shift($@)), `$1'')') ⇒ pushdef(`a', `1')pushdef(`a', `2')pushdef(`a', `3') ⇒ stack_foreach(`a', `:curry(`reverse', `4')') ⇒:1, 4:2, 4:3, 4 curry(`curry', `reverse', `1')(`2')(`3') ⇒3, 2, 1
Now for the implementation. Notice how curry
leaves off with a
macro name but no open parenthesis, while still in the middle of
collecting arguments for ‘$1’. The macro _curry
is the
helper macro that takes one argument, then adds it to the list and
finally supplies the closing parenthesis. The use of a comma inside the
shift
call allows currying to also work for a macro that takes
one argument, although it often makes more sense to invoke that macro
directly rather than going through curry
.
$ m4 -I examples undivert(`curry.m4')dnl ⇒divert(`-1') ⇒# curry(macro, args) ⇒# Expand to a macro call that takes one argument, then invoke ⇒# macro(args, extra). ⇒define(`curry', `$1(shift($@,)_$0') ⇒define(`_curry', ``$1')') ⇒divert`'dnl
Unfortunately, with M4 1.4.x, curry
is unable to handle builtin
tokens, which are silently flattened to the empty string when passed
through another text macro. This limitation will be lifted in a future
release of M4.
Putting the last few concepts together, it is possible to copy or rename an entire stack of macro definitions.
Ensure that dest is undefined, then define it to the same stack of
definitions currently in source. copy
leaves source
unchanged, while rename
undefines source. There are only a
few macros, such as copy
or defn
, which cannot be copied
via this macro.
The implementation is relatively straightforward (although since it uses
curry
, it is unable to copy builtin macros, such as the second
definition of a
as a synonym for divnum
. See if you can
design a version that works around this limitation, or see Answers).
$ m4 -I examples include(`curry.m4')include(`stack.m4') ⇒ define(`rename', `copy($@)undefine(`$1')')dnl define(`copy', `ifdef(`$2', `errprint(`$2 already defined ')m4exit(`1')', `stack_foreach(`$1', `curry(`pushdef', `$2')')')')dnl pushdef(`a', `1')pushdef(`a', defn(`divnum'))pushdef(`a', `2') ⇒ copy(`a', `b') ⇒ rename(`b', `c') ⇒ a b c ⇒2 b 2 popdef(`a', `c')c a ⇒ 0 popdef(`a', `c')a c ⇒1 1
Next: Input Control, Previous: Conditionals, Up: Top [Contents][Index]
When writing macros for m4
, they often do not work as intended on
the first try (as is the case with most programming languages).
Fortunately, there is support for macro debugging in m4
.
• Dumpdef | Displaying macro definitions | |
• Trace | Tracing macro calls | |
• Debug Levels | Controlling debugging output | |
• Debug Output | Saving debugging output |
If you want to see what a name expands into, you can use the builtin
dumpdef
:
Accepts any number of arguments. If called without any arguments, it displays the definitions of all known names, otherwise it displays the definitions of the names given. The output is printed to the current debug file (usually standard error), and is sorted by name. If an unknown name is encountered, a warning is printed.
The expansion of dumpdef
is void.
$ m4 -d define(`foo', `Hello world.') ⇒ dumpdef(`foo') error→foo: ⇒ dumpdef(`define') error→define: ⇒
The last example shows how builtin macros definitions are displayed. The definition that is dumped corresponds to what would occur if the macro were to be called at that point, even if other definitions are still live due to redefining a macro during argument collection.
$ m4 -d pushdef(`f', ``$0'1')pushdef(`f', ``$0'2') ⇒ f(popdef(`f')dumpdef(`f')) error→f: ⇒f2 f(popdef(`f')dumpdef(`f')) error→m4:stdin:3: undefined macro `f' ⇒f1
See Debug Levels, for information on controlling the details of the display.
Next: Debug Levels, Previous: Dumpdef, Up: Debugging [Contents][Index]
It is possible to trace macro calls and expansions through the builtins
traceon
and traceoff
:
When called without any arguments, traceon
and traceoff
will turn tracing on and off, respectively, for all currently defined
macros.
When called with arguments, only the macros listed in names are affected, whether or not they are currently defined.
The expansion of traceon
and traceoff
is void.
Whenever a traced macro is called and the arguments have been collected, the call is displayed. If the expansion of the macro call is not void, the expansion can be displayed after the call. The output is printed to the current debug file (defaulting to standard error, see Debug Output).
$ m4 -d define(`foo', `Hello World.') ⇒ define(`echo', `$@') ⇒ traceon(`foo', `echo') ⇒ foo error→m4trace: -1- foo -> `Hello World.' ⇒Hello World. echo(`gnus', `and gnats') error→m4trace: -1- echo(`gnus', `and gnats') -> ``gnus',`and gnats'' ⇒gnus,and gnats
The number between dashes is the depth of the expansion. It is one most
of the time, signifying an expansion at the outermost level, but it
increases when macro arguments contain unquoted macro calls. The
maximum number that will appear between dashes is controlled by the
option --nesting-limit (or -L, see Invoking m4). Additionally, the option --trace (or
-t) can be used to invoke traceon(name)
before
parsing input.
$ m4 -L 3 -t ifelse ifelse(`one level') error→m4trace: -1- ifelse ⇒ ifelse(ifelse(ifelse(`three levels'))) error→m4trace: -3- ifelse error→m4trace: -2- ifelse error→m4trace: -1- ifelse ⇒ ifelse(ifelse(ifelse(ifelse(`four levels')))) error→m4:stdin:3: recursion limit of 3 exceeded, use -L<N> to change it
Tracing by name is an attribute that is preserved whether the macro is defined or not. This allows the selection of macros to trace before those macros are defined.
$ m4 -d traceoff(`foo') ⇒ traceon(`foo') ⇒ foo ⇒foo defn(`foo') ⇒ define(`foo', `bar') ⇒ foo error→m4trace: -1- foo -> `bar' ⇒bar undefine(`foo') ⇒ ifdef(`foo', `yes', `no') ⇒no indir(`foo') error→m4:stdin:9: undefined macro `foo' ⇒ define(`foo', `blah') ⇒ foo error→m4trace: -1- foo -> `blah' ⇒blah traceoff ⇒ foo ⇒blah
Tracing even works on builtins. However, defn
(see Defn)
does not transfer tracing status.
$ m4 -d traceon(`traceon') ⇒ traceon(`traceoff') error→m4trace: -1- traceon(`traceoff') ⇒ traceoff(`traceoff') error→m4trace: -1- traceoff(`traceoff') ⇒ traceoff(`traceon') ⇒ traceon(`eval', `m4_divnum') ⇒ define(`m4_eval', defn(`eval')) ⇒ define(`m4_divnum', defn(`divnum')) ⇒ eval(divnum) error→m4trace: -1- eval(`0') -> `0' ⇒0 m4_eval(m4_divnum) error→m4trace: -2- m4_divnum -> `0' ⇒0
See Debug Levels, for information on controlling the details of the
display. The format of the trace output is not specified by
POSIX, and varies between implementations of m4
.
Next: Debug Output, Previous: Trace, Up: Debugging [Contents][Index]
The -d option to m4
(or --debug,
see Invoking m4) controls the amount of details
presented in three
categories of output. Trace output is requested by traceon
(see Trace), and each line is prefixed by ‘m4trace:’ in
relation to a macro invocation. Debug output tracks useful events not
associated with a macro invocation, and each line is prefixed by
‘m4debug:’. Finally, dumpdef
(see Dumpdef) output is
affected, with no prefix added to the output lines.
The flags following the option can be one or more of the following:
a
In trace output, show the actual arguments that were collected before
invoking the macro. This applies to all macro calls if the ‘t’
flag is used, otherwise only the macros covered by calls of
traceon
. Arguments are subject to length truncation specified by
the command line option --arglength (or -l).
c
In trace output, show several trace lines for each macro call. A line is shown when the macro is seen, but before the arguments are collected; a second line when the arguments have been collected and a third line after the call has completed.
e
In trace output, show the expansion of each macro call, if it is not
void. This applies to all macro calls if the ‘t’ flag is used,
otherwise only the macros covered by calls of traceon
. The
expansion is subject to length truncation specified by the command line
option --arglength (or -l).
f
In debug and trace output, include the name of the current input file in the output line.
i
In debug output, print a message each time the current input file is changed.
l
In debug and trace output, include the current input line number in the output line.
p
In debug output, print a message when a named file is found through the path search mechanism (see Search Path), giving the actual file name used.
q
In trace and dumpdef output, quote actual arguments and macro expansions in the display with the current quotes. This is useful in connection with the ‘a’ and ‘e’ flags above.
t
In trace output, trace all macro calls made in this invocation of
m4
, regardless of the settings of traceon
.
x
In trace output, add a unique ‘macro call id’ to each line of the trace output. This is useful in connection with the ‘c’ flag above.
V
A shorthand for all of the above flags.
If no flags are specified with the -d option, the default is ‘aeq’. The examples throughout this manual assume the default flags.
There is a builtin macro debugmode
, which allows on-the-fly control of
the debugging output format:
The argument flags should be a subset of the letters listed above. As special cases, if the argument starts with a ‘+’, the flags are added to the current debug flags, and if it starts with a ‘-’, they are removed. If no argument is present, all debugging flags are cleared (as if no -d was given), and with an empty argument the flags are reset to the default of ‘aeq’.
The expansion of debugmode
is void.
$ m4 define(`foo', `FOO') ⇒ traceon(`foo') ⇒ debugmode() ⇒ foo error→m4trace: -1- foo -> `FOO' ⇒FOO debugmode ⇒ foo error→m4trace: -1- foo ⇒FOO debugmode(`+l') ⇒ foo error→m4trace:8: -1- foo ⇒FOO
The following example demonstrates the behavior of length truncation, when specified on the command line. Note that each argument and the final result are individually truncated. Also, the special tokens for builtin functions are not truncated.
$ m4 -d -l 6 define(`echo', `$@')debugmode(`+t') ⇒ echo(`1', `long string') error→m4trace: -1- echo(`1', `long s...') -> ``1',`l...' ⇒1,long string indir(`echo', defn(`changequote')) error→m4trace: -2- defn(`change...') error→m4trace: -1- indir(`echo', <changequote>) -> ``'' ⇒
This example shows the effects of the debug flags that are not related to macro tracing.
$ m4 -dip -I examples error→m4debug: input read from stdin include(`foo')dnl error→m4debug: path search for `foo' found `examples/foo' error→m4debug: input read from examples/foo ⇒bar error→m4debug: input reverted to stdin, line 1 ^D error→m4debug: input exhausted
Previous: Debug Levels, Up: Debugging [Contents][Index]
Debug and tracing output can be redirected to files using either the
--debugfile option to m4
(see Invoking m4), or with the builtin macro debugfile
:
Sends all further debug and trace output to file, opened in append
mode. If file is the empty string, debug and trace output are
discarded. If debugfile
is called without any arguments, debug
and trace output are sent to standard error. This does not affect
warnings, error messages, or errprint
output, which are
always sent to standard error. If file cannot be opened, the
current debug file is unchanged, and an error is issued.
The expansion of debugfile
is void.
$ m4 -d traceon(`divnum') ⇒ divnum(`extra') error→m4:stdin:2: Warning: excess arguments to builtin `divnum' ignored error→m4trace: -1- divnum(`extra') -> `0' ⇒0 debugfile() ⇒ divnum(`extra') error→m4:stdin:4: Warning: excess arguments to builtin `divnum' ignored ⇒0 debugfile ⇒ divnum error→m4trace: -1- divnum -> `0' ⇒0
Next: File Inclusion, Previous: Debugging, Up: Top [Contents][Index]
This chapter describes various builtin macros for controlling the input
to m4
.
• Dnl | Deleting whitespace in input | |
• Changequote | Changing the quote characters | |
• Changecom | Changing the comment delimiters | |
• Changeword | Changing the lexical structure of words | |
• M4wrap | Saving text until end of input |
Next: Changequote, Up: Input Control [Contents][Index]
The builtin dnl
stands for “Discard to Next Line”:
All characters, up to and including the next newline, are discarded without performing any macro expansion. A warning is issued if the end of the file is encountered without a newline.
The expansion of dnl
is void.
It is often used in connection with define
, to remove the
newline that follows the call to define
. Thus
define(`foo', `Macro `foo'.')dnl A very simple macro, indeed. foo ⇒Macro foo.
The input up to and including the next newline is discarded, as opposed to the way comments are treated (see Comments).
Usually, dnl
is immediately followed by an end of line or some
other whitespace. GNU m4
will produce a warning diagnostic if
dnl
is followed by an open parenthesis. In this case, dnl
will collect and process all arguments, looking for a matching close
parenthesis. All predictable side effects resulting from this
collection will take place. dnl
will return no output. The
input following the matching close parenthesis up to and including the
next newline, on whatever line containing it, will still be discarded.
dnl(`args are ignored, but side effects occur', define(`foo', `like this')) while this text is ignored: undefine(`foo') error→m4:stdin:1: Warning: excess arguments to builtin `dnl' ignored See how `foo' was defined, foo? ⇒See how foo was defined, like this?
If the end of file is encountered without a newline character, a warning is issued and dnl stops consuming input.
m4wrap(`m4wrap(`2 hi ')0 hi dnl 1 hi') ⇒ define(`hi', `HI') ⇒ ^D error→m4:stdin:1: Warning: end of file treated as newline ⇒0 HI 2 HI
Next: Changecom, Previous: Dnl, Up: Input Control [Contents][Index]
The default quote delimiters can be changed with the builtin
changequote
:
This sets start as the new begin-quote delimiter and end as
the new end-quote delimiter. If both arguments are missing, the default
quotes (`
and '
) are used. If start is void, then
quoting is disabled. Otherwise, if end is missing or void, the
default end-quote delimiter ('
) is used. The quote delimiters
can be of any length.
The expansion of changequote
is void.
changequote(`[', `]') ⇒ define([foo], [Macro [foo].]) ⇒ foo ⇒Macro foo.
The quotation strings can safely contain non-ASCII characters.
define(`a', `b') ⇒ «a» ⇒«b» changequote(`«', `»') ⇒ «a» ⇒a
If no single character is appropriate, start and end can be of any length. Other implementations cap the delimiter length to five characters, but GNU has no inherent limit.
changequote(`[[[', `]]]') ⇒ define([[[foo]]], [[[Macro [[[[[foo]]]]].]]]) ⇒ foo ⇒Macro [[foo]].
Calling changequote
with start as the empty string will
effectively disable the quoting mechanism, leaving no way to quote text.
However, using an empty string is not portable, as some other
implementations of m4
revert to the default quoting, while others
preserve the prior non-empty delimiter. If start is not empty,
then an empty end will use the default end-quote delimiter of
‘'’, as otherwise, it would be impossible to end a quoted string.
Again, this is not portable, as some other m4
implementations
reuse start as the end-quote delimiter, while others preserve the
previous non-empty value. Omitting both arguments restores the default
begin-quote and end-quote delimiters; fortunately this behavior is
portable to all implementations of m4
.
define(`foo', `Macro `FOO'.') ⇒ changequote(`', `') ⇒ foo ⇒Macro `FOO'. `foo' ⇒`Macro `FOO'.' changequote(`,) ⇒ foo ⇒Macro FOO.
There is no way in m4
to quote a string containing an unmatched
begin-quote, except using changequote
to change the current
quotes.
If the quotes should be changed from, say, ‘[’ to ‘[[’,
temporary quote characters have to be defined. To achieve this, two
calls of changequote
must be made, one for the temporary quotes
and one for the new quotes.
Macros are recognized in preference to the begin-quote string, so if a
prefix of start can be recognized as part of a potential macro
name, the quoting mechanism is effectively disabled. Unless you use
changeword
(see Changeword), this means that start
should not begin with a letter, digit, or ‘_’ (underscore).
However, even though quoted strings are not recognized, the quote
characters can still be discerned in macro expansion and in trace
output.
define(`echo', `$@') ⇒ define(`hi', `HI') ⇒ changequote(`q', `Q') ⇒ q hi Q hi ⇒q HI Q HI echo(hi) ⇒qHIQ changequote ⇒ changequote(`-', `EOF') ⇒ - hi EOF hi ⇒ hi HI changequote ⇒ changequote(`1', `2') ⇒ hi1hi2 ⇒hi1hi2 hi 1hi2 ⇒HI hi
Quotes are recognized in preference to argument collection. In particular, if start is a single ‘(’, then argument collection is effectively disabled. For portability with other implementations, it is a good idea to avoid ‘(’, ‘,’, and ‘)’ as the first character in start.
define(`echo', `$#:$@:') ⇒ define(`hi', `HI') ⇒ changequote(`(',`)') ⇒ echo(hi) ⇒0::hi changequote ⇒ changequote(`((', `))') ⇒ echo(hi) ⇒1:HI: echo((hi)) ⇒0::hi changequote ⇒ changequote(`,', `)') ⇒ echo(hi,hi)bye) ⇒1:HIhibye:
However, if you are not worried about portability, using ‘(’ and
‘)’ as quoting characters has an interesting property—you can use
it to compute a quoted string containing the expansion of any quoted
text, as long as the expansion results in both balanced quotes and
balanced parentheses. The trick is realizing expand
uses
‘$1’ unquoted, to trigger its expansion using the normal quoting
characters, but uses extra parentheses to group unquoted commas that
occur in the expansion without consuming whitespace following those
commas. Then _expand
uses changequote
to convert the
extra parentheses back into quoting characters. Note that it takes two
more changequote
invocations to restore the original quotes.
Contrast the behavior on whitespace when using ‘$*’, via
quote
, to attempt the same task.
changequote(`[', `]')dnl define([a], [1, (b)])dnl define([b], [2])dnl define([quote], [[$*]])dnl define([expand], [_$0(($1))])dnl define([_expand], [changequote([(], [)])$1changequote`'changequote(`[', `]')])dnl expand([a, a, [a, a], [[a, a]]]) ⇒1, (2), 1, (2), a, a, [a, a] quote(a, a, [a, a], [[a, a]]) ⇒1,(2),1,(2),a, a,[a, a]
If end is a prefix of start, the end-quote will be recognized in preference to a nested begin-quote. In particular, changing the quotes to have the same string for start and end disables nesting of quotes. When quote nesting is disabled, it is impossible to double-quote strings across macro expansions, so using the same string is not done very often.
define(`hi', `HI') ⇒ changequote(`""', `"') ⇒ ""hi"""hi" ⇒hihi ""hi" ""hi" ⇒hi hi ""hi"" "hi" ⇒hi" "HI" changequote ⇒ `hi`hi'hi' ⇒hi`hi'hi changequote(`"', `"') ⇒ "hi"hi"hi" ⇒hiHIhi
It is an error if the end of file occurs within a quoted string.
`hello world' ⇒hello world `dangling quote ^D error→m4:stdin:2: ERROR: end of file in string
ifelse(`dangling quote ^D error→m4:stdin:1: ERROR: end of file in string
Next: Changeword, Previous: Changequote, Up: Input Control [Contents][Index]
The default comment delimiters can be changed with the builtin
macro changecom
:
This sets start as the new begin-comment delimiter and end as the new end-comment delimiter. If both arguments are missing, or start is void, then comments are disabled. Otherwise, if end is missing or void, the default end-comment delimiter of newline is used. The comment delimiters can be of any length.
The expansion of changecom
is void.
define(`comment', `COMMENT') ⇒ # A normal comment ⇒# A normal comment changecom(`/*', `*/') ⇒ # Not a comment anymore ⇒# Not a COMMENT anymore But: /* this is a comment now */ while this is not a comment ⇒But: /* this is a comment now */ while this is not a COMMENT
Note how comments are copied to the output, much as if they were quoted strings. If you want the text inside a comment expanded, quote the begin-comment delimiter.
Calling changecom
without any arguments, or with start as
the empty string, will effectively disable the commenting mechanism. To
restore the original comment start of ‘#’, you must explicitly ask
for it. If start is not empty, then an empty end will use
the default end-comment delimiter of newline, as otherwise, it would be
impossible to end a comment. However, this is not portable, as some
other m4
implementations preserve the previous non-empty
delimiters instead.
define(`comment', `COMMENT') ⇒ changecom ⇒ # Not a comment anymore ⇒# Not a COMMENT anymore changecom(`#', `') ⇒ # comment again ⇒# comment again
The comment strings can safely contain non-ASCII characters.
define(`a', `b') ⇒ «a» ⇒«b» changecom(`«', `»') ⇒ «a» ⇒«a»
If no single character is appropriate, start and end can be of any length. Other implementations cap the delimiter length to five characters, but GNU has no inherent limit.
Comments are recognized in preference to macros. However, this is not compatible with other implementations, where macros and even quoting takes precedence over comments, so it may change in a future release. For portability, this means that start should not begin with a letter, digit, or ‘_’ (underscore), and that neither the start-quote nor the start-comment string should be a prefix of the other.
define(`hi', `HI') ⇒ define(`hi1hi2', `hello') ⇒ changecom(`q', `Q') ⇒ q hi Q hi ⇒q hi Q HI changecom(`1', `2') ⇒ hi1hi2 ⇒hello hi 1hi2 ⇒HI 1hi2
Comments are recognized in preference to argument collection. In particular, if start is a single ‘(’, then argument collection is effectively disabled. For portability with other implementations, it is a good idea to avoid ‘(’, ‘,’, and ‘)’ as the first character in start.
define(`echo', `$#:$*:$@:') ⇒ define(`hi', `HI') ⇒ changecom(`(',`)') ⇒ echo(hi) ⇒0:::(hi) changecom ⇒ changecom(`((', `))') ⇒ echo(hi) ⇒1:HI:HI: echo((hi)) ⇒0:::((hi)) changecom(`,', `)') ⇒ echo(hi,hi)bye) ⇒1:HI,hi)bye:HI,hi)bye: changecom ⇒ echo(hi,`,`'hi',hi) ⇒3:HI,,HI,HI:HI,,`'hi,HI: echo(hi,`,`'hi',hi`'changecom(`,,', `hi')) ⇒3:HI,,`'hi,HI:HI,,`'hi,HI:
It is an error if the end of file occurs within a comment.
changecom(`/*', `*/') ⇒ /*dangling comment ^D error→m4:stdin:2: ERROR: end of file in comment
Next: M4wrap, Previous: Changecom, Up: Input Control [Contents][Index]
The macro
changeword
and all associated functionality is experimental. It is only available if the --enable-changeword option was given toconfigure
, at GNUm4
installation time. The functionality will go away in the future, to be replaced by other new features that are more efficient at providing the same capabilities. Do not rely on it. Please direct your comments about it the same way you would do for bugs.
A file being processed by m4
is split into quoted strings, words
(potential macro names) and simple tokens (any other single character).
Initially a word is defined by the following regular expression:
[_a-zA-Z][_a-zA-Z0-9]*
Using changeword
, you can change this regular expression:
Changes the regular expression for recognizing macro names to be regex. If regex is empty, use ‘[_a-zA-Z][_a-zA-Z0-9]*’. regex must obey the constraint that every prefix of the desired final pattern is also accepted by the regular expression. If regex contains grouping parentheses, the macro invoked is the portion that matched the first group, rather than the entire matching string.
The expansion of changeword
is void.
The macro changeword
is recognized only with parameters.
Relaxing the lexical rules of m4
might be useful (for example) if
you wanted to apply translations to a file of numbers:
ifdef(`changeword', `', `errprint(` skipping: no changeword support ')m4exit(`77')')dnl changeword(`[_a-zA-Z0-9]+') ⇒ define(`1', `0')1 ⇒0
Tightening the lexical rules is less useful, because it will generally make some of the builtins unavailable. You could use it to prevent accidental call of builtins, for example:
ifdef(`changeword', `', `errprint(` skipping: no changeword support ')m4exit(`77')')dnl define(`_indir', defn(`indir')) ⇒ changeword(`_[_a-zA-Z0-9]*') ⇒ esyscmd(`foo') ⇒esyscmd(foo) _indir(`esyscmd', `echo hi') ⇒hi ⇒
Because m4
constructs its words a character at a time, there
is a restriction on the regular expressions that may be passed to
changeword
. This is that if your regular expression accepts
‘foo’, it must also accept ‘f’ and ‘fo’.
ifdef(`changeword', `', `errprint(` skipping: no changeword support ')m4exit(`77')')dnl define(`foo ', `bar ') ⇒ dnl This example wants to recognize changeword, dnl, and `foo\n'. dnl First, we check that our regexp will match. regexp(`changeword', `[cd][a-z]*\|foo[ ]') ⇒0 regexp(`foo ', `[cd][a-z]*\|foo[ ]') ⇒0 regexp(`f', `[cd][a-z]*\|foo[ ]') ⇒-1 foo ⇒foo changeword(`[cd][a-z]*\|foo[ ]') ⇒ dnl Even though `foo\n' matches, we forgot to allow `f'. foo ⇒foo changeword(`[cd][a-z]*\|fo*[ ]?') ⇒ dnl Now we can call `foo\n'. foo ⇒bar
changeword
has another function. If the regular expression
supplied contains any grouped subexpressions, then text outside
the first of these is discarded before symbol lookup. So:
ifdef(`changeword', `', `errprint(` skipping: no changeword support ')m4exit(`77')')dnl ifdef(`__unix__', , `errprint(` skipping: syscmd does not have unix semantics ')m4exit(`77')')dnl changecom(`/*', `*/')dnl define(`foo', `bar')dnl changeword(`#\([_a-zA-Z0-9]*\)') ⇒ #esyscmd(`echo foo \#foo') ⇒foo bar ⇒
m4
now requires a ‘#’ mark at the beginning of every
macro invocation, so one can use m4
to preprocess plain
text without losing various words like ‘divert’.
In m4
, macro substitution is based on text, while in TeX, it
is based on tokens. changeword
can throw this difference into
relief. For example, here is the same idea represented in TeX and
m4
. First, the TeX version:
\def\a{\message{Hello}} \catcode`\@=0 \catcode`\\=12 @a @bye ⇒Hello
Then, the m4
version:
ifdef(`changeword', `', `errprint(` skipping: no changeword support ')m4exit(`77')')dnl define(`a', `errprint(`Hello')')dnl changeword(`@\([_a-zA-Z0-9]*\)') ⇒ @a ⇒errprint(Hello)
In the TeX example, the first line defines a macro a
to
print the message ‘Hello’. The second line defines @ to
be usable instead of \ as an escape character. The third line
defines \ to be a normal printing character, not an escape.
The fourth line invokes the macro a
. So, when TeX is run
on this file, it displays the message ‘Hello’.
When the m4
example is passed through m4
, it outputs
‘errprint(Hello)’. The reason for this is that TeX does
lexical analysis of macro definition when the macro is defined.
m4
just stores the text, postponing the lexical analysis until
the macro is used.
You should note that using changeword
will slow m4
down
by a factor of about seven, once it is changed to something other
than the default regular expression. You can invoke changeword
with the empty string to restore the default word definition, and regain
the parsing speed.
Previous: Changeword, Up: Input Control [Contents][Index]
It is possible to ‘save’ some text until the end of the normal input has
been seen. Text can be saved, to be read again by m4
when the
normal input has been exhausted. This feature is normally used to
initiate cleanup actions before normal exit, e.g., deleting temporary
files.
To save input text, use the builtin m4wrap
:
Stores string in a safe place, to be reread when end of input is reached. As a GNU extension, additional arguments are concatenated with a space to the string.
The expansion of m4wrap
is void.
The macro m4wrap
is recognized only with parameters.
define(`cleanup', `This is the `cleanup' action. ') ⇒ m4wrap(`cleanup') ⇒ This is the first and last normal input line. ⇒This is the first and last normal input line. ^D ⇒This is the cleanup action.
The saved input is only reread when the end of normal input is seen, and
not if m4exit
is used to exit m4
.
It is safe to call m4wrap
from saved text, but then the order in
which the saved text is reread is undefined. If m4wrap
is not used
recursively, the saved pieces of text are reread in the opposite order
in which they were saved (LIFO—last in, first out). However, this
behavior is likely to change in a future release, to match
POSIX, so you should not depend on this order.
It is possible to emulate POSIX behavior even with older versions of GNU M4 by including the file m4-1.4.19/examples/wrapfifo.m4 from the distribution:
$ m4 -I examples undivert(`wrapfifo.m4')dnl ⇒dnl Redefine m4wrap to have FIFO semantics. ⇒define(`_m4wrap_level', `0')dnl ⇒define(`m4wrap', ⇒`ifdef(`m4wrap'_m4wrap_level, ⇒ `define(`m4wrap'_m4wrap_level, ⇒ defn(`m4wrap'_m4wrap_level)`$1')', ⇒ `builtin(`m4wrap', `define(`_m4wrap_level', ⇒ incr(_m4wrap_level))dnl ⇒m4wrap'_m4wrap_level)dnl ⇒define(`m4wrap'_m4wrap_level, `$1')')')dnl include(`wrapfifo.m4') ⇒ m4wrap(`a`'m4wrap(`c ', `d')')m4wrap(`b') ⇒ ^D ⇒abc
It is likewise possible to emulate LIFO behavior without resorting to
the GNU M4 extension of builtin
, by including the file
m4-1.4.19/examples/wraplifo.m4 from the
distribution. (Unfortunately, both examples shown here share some
subtle bugs. See if you can find and correct them; or see Answers).
$ m4 -I examples undivert(`wraplifo.m4')dnl ⇒dnl Redefine m4wrap to have LIFO semantics. ⇒define(`_m4wrap_level', `0')dnl ⇒define(`_m4wrap', defn(`m4wrap'))dnl ⇒define(`m4wrap', ⇒`ifdef(`m4wrap'_m4wrap_level, ⇒ `define(`m4wrap'_m4wrap_level, ⇒ `$1'defn(`m4wrap'_m4wrap_level))', ⇒ `_m4wrap(`define(`_m4wrap_level', incr(_m4wrap_level))dnl ⇒m4wrap'_m4wrap_level)dnl ⇒define(`m4wrap'_m4wrap_level, `$1')')')dnl include(`wraplifo.m4') ⇒ m4wrap(`a`'m4wrap(`c ', `d')')m4wrap(`b') ⇒ ^D ⇒bac
Here is an example of implementing a factorial function using
m4wrap
:
define(`f', `ifelse(`$1', `0', `Answer: 0!=1 ', eval(`$1>1'), `0', `Answer: $2$1=eval(`$2$1') ', `m4wrap(`f(decr(`$1'), `$2$1*')')')') ⇒ f(`10') ⇒ ^D ⇒Answer: 10*9*8*7*6*5*4*3*2*1=3628800
Invocations of m4wrap
at the same recursion level are
concatenated and rescanned as usual:
define(`aa', `AA ') ⇒ m4wrap(`a')m4wrap(`a') ⇒ ^D ⇒AA
however, the transition between recursion levels behaves like an end of file condition between two input files.
m4wrap(`m4wrap(`)')len(abc') ⇒ ^D error→m4:stdin:1: ERROR: end of file in argument list
Next: Diversions, Previous: Input Control, Up: Top [Contents][Index]
m4
allows you to include named files at any point in the input.
• Include | Including named files | |
• Search Path | Searching for include files |
Next: Search Path, Up: File Inclusion [Contents][Index]
There are two builtin macros in m4
for including files:
Both macros cause the file named file to be read by
m4
. When the end of the file is reached, input is resumed from
the previous input file.
The expansion of include
and sinclude
is therefore the
contents of file.
If file does not exist, is a directory, or cannot otherwise be
read, the expansion is void,
and include
will fail with an error while sinclude
is
silent. The empty string counts as a file that does not exist.
The macros include
and sinclude
are recognized only with
parameters.
include(`none') error→m4:stdin:1: cannot open `none': No such file or directory ⇒ include() error→m4:stdin:2: cannot open `': No such file or directory ⇒ sinclude(`none') ⇒ sinclude() ⇒
The rest of this section assumes that m4
is invoked with the
-I option (see Invoking m4)
pointing to the m4-1.4.19/examples
directory shipped as part of the GNU m4
package. The
file m4-1.4.19/examples/incl.m4 in the distribution
contains the lines:
$ cat examples/incl.m4 ⇒Include file start ⇒foo ⇒Include file end
Normally file inclusion is used to insert the contents of a file
into the input stream. The contents of the file will be read by
m4
and macro calls in the file will be expanded:
$ m4 -I examples define(`foo', `FOO') ⇒ include(`incl.m4') ⇒Include file start ⇒FOO ⇒Include file end ⇒
The fact that include
and sinclude
expand to the contents
of the file can be used to define macros that operate on entire files.
Here is an example, which defines ‘bar’ to expand to the contents
of incl.m4:
$ m4 -I examples define(`bar', include(`incl.m4')) ⇒ This is `bar': >>bar<< ⇒This is bar: >>Include file start ⇒foo ⇒Include file end ⇒<<
This use of include
is not trivial, though, as files can contain
quotes, commas, and parentheses, which can interfere with the way the
m4
parser works. GNU m4
seamlessly concatenates
the file contents with the next character, even if the included file
ended in the middle of a comment, string, or macro call. These
conditions are only treated as end of file errors if specified as input
files on the command line.
In GNU m4
, an alternative method of reading files is
using undivert
(see Undivert) on a named file.
Previous: Include, Up: File Inclusion [Contents][Index]
GNU m4
allows included files to be found in other directories
than the current working directory.
If the --prepend-include or -B command-line option was
provided (see Invoking m4), those
directories are searched first, in reverse order that those options were
listed on the command line. Then m4
looks in the current working
directory. Next comes the directories specified with the
--include or -I option, in the order found on the
command line. Finally, if the M4PATH
environment variable is set,
it is expected to contain a colon-separated list of directories, which
will be searched in order.
If the automatic search for include-files causes trouble, the ‘p’ debug flag (see Debug Levels) can help isolate the problem.
Next: Text handling, Previous: File Inclusion, Up: Top [Contents][Index]
Diversions are a way of temporarily saving output. The output of
m4
can at any time be diverted to a temporary file, and be
reinserted into the output stream, undiverted, again at a later
time.
Numbered diversions are counted from 0 upwards, diversion number 0
being the normal output stream. GNU
m4
tries to keep diversions in memory. However, there is a
limit to the overall memory usable by all diversions taken together
(512K, currently). When this maximum is about to be exceeded,
a temporary file is opened to receive the contents of the biggest
diversion still in memory, freeing this memory for other diversions.
When creating the temporary file, m4
honors the value of the
environment variable TMPDIR
, and falls back to /tmp.
Thus, the amount of available disk space provides the only real limit on
the number and aggregate size of diversions.
Diversions make it possible to generate output in a different order than the input was read. It is possible to implement topological sorting dependencies. For example, GNU Autoconf makes use of diversions under the hood to ensure that the expansion of a prerequisite macro appears in the output prior to the expansion of a dependent macro, regardless of which order the two macros were invoked in the user’s input file.
• Divert | Diverting output | |
• Undivert | Undiverting output | |
• Divnum | Diversion numbers | |
• Cleardivert | Discarding diverted text |
Next: Undivert, Up: Diversions [Contents][Index]
Output is diverted using divert
:
The current diversion is changed to number. If number is left out or empty, it is assumed to be zero. If number cannot be parsed, the diversion is unchanged.
The expansion of divert
is void.
When all the m4
input will have been processed, all existing
diversions are automatically undiverted, in numerical order.
divert(`1') This text is diverted. divert ⇒ This text is not diverted. ⇒This text is not diverted. ^D ⇒ ⇒This text is diverted.
Several calls of divert
with the same argument do not overwrite
the previous diverted text, but append to it. Diversions are printed
after any wrapped text is expanded.
define(`text', `TEXT') ⇒ divert(`1')`diverted text.' divert ⇒ m4wrap(`Wrapped text precedes ') ⇒ ^D ⇒Wrapped TEXT precedes diverted text.
If output is diverted to a negative diversion, it is simply discarded.
This can be used to suppress unwanted output. A common example of
unwanted output is the trailing newlines after macro definitions. Here
is a common programming idiom in m4
for avoiding them.
divert(`-1') define(`foo', `Macro `foo'.') define(`bar', `Macro `bar'.') divert ⇒
Traditional implementations only supported ten diversions. But as a GNU extension, diversion numbers can be as large as positive integers will allow, rather than treating a multi-digit diversion number as a request to discard text.
divert(eval(`1<<28'))world divert(`2')hello ^D ⇒hello ⇒world
Note that divert
is an English word, but also an active macro
without arguments. When processing plain text, the word might appear in
normal text and be unintentionally swallowed as a macro invocation. One
way to avoid this is to use the -P option to rename all
builtins (see Invoking m4). Another is to write
a wrapper that requires a parameter to be recognized.
We decided to divert the stream for irrigation. ⇒We decided to the stream for irrigation. define(`divert', `ifelse(`$#', `0', ``$0'', `builtin(`$0', $@)')') ⇒ divert(`-1') Ignored text. divert(`0') ⇒ We decided to divert the stream for irrigation. ⇒We decided to divert the stream for irrigation.
Next: Divnum, Previous: Divert, Up: Diversions [Contents][Index]
Diverted text can be undiverted explicitly using the builtin
undivert
:
Undiverts the numeric diversions given by the arguments, in the order given. If no arguments are supplied, all diversions are undiverted, in numerical order.
As a GNU extension, diversions may contain non-numeric strings, which are treated as the names of files to copy into the output without expansion. A warning is issued if a file could not be opened.
The expansion of undivert
is void.
divert(`1') This text is diverted. divert ⇒ This text is not diverted. ⇒This text is not diverted. undivert(`1') ⇒ ⇒This text is diverted. ⇒
Notice the last two blank lines. One of them comes from the newline
following undivert
, the other from the newline that followed the
divert
! A diversion often starts with a blank line like this.
When diverted text is undiverted, it is not reread by m4
,
but rather copied directly to the current output, and it is therefore
not an error to undivert into a diversion. Undiverting the empty string
is the same as specifying diversion 0; in either case nothing happens
since the output has already been flushed.
divert(`1')diverted text divert ⇒ undivert() ⇒ undivert(`0') ⇒ undivert ⇒diverted text ⇒ divert(`1')more divert(`2')undivert(`1')diverted text`'divert ⇒ undivert(`1') ⇒ undivert(`2') ⇒more ⇒diverted text
When a diversion has been undiverted, the diverted text is discarded, and it is not possible to bring back diverted text more than once.
divert(`1') This text is diverted first. divert(`0')undivert(`1')dnl ⇒ ⇒This text is diverted first. undivert(`1') ⇒ divert(`1') This text is also diverted but not appended. divert(`0')undivert(`1')dnl ⇒ ⇒This text is also diverted but not appended.
Attempts to undivert the current diversion are silently ignored. Thus, when the current diversion is not 0, the current diversion does not get rearranged among the other diversions.
divert(`1')one divert(`2')two divert(`3')three divert(`2')undivert`'dnl divert`'undivert`'dnl ⇒two ⇒one ⇒three
GNU m4
allows named files to be undiverted. Given a
non-numeric argument, the contents of the file named will be copied,
uninterpreted, to the current output. This complements the builtin
include
(see Include). To illustrate the difference, assume
the file foo contains:
$ cat foo bar
then
define(`bar', `BAR') ⇒ undivert(`foo') ⇒bar ⇒ include(`foo') ⇒BAR ⇒
If the file is not found (or cannot be read), an error message is issued, and the expansion is void. It is possible to intermix files and diversion numbers.
divert(`1')diversion one divert(`2')undivert(`foo')dnl divert(`3')diversion three divert`'dnl undivert(`1', `2', `foo', `3')dnl ⇒diversion one ⇒bar ⇒bar ⇒diversion three
Next: Cleardivert, Previous: Undivert, Up: Diversions [Contents][Index]
The current diversion is tracked by the builtin divnum
:
Expands to the number of the current diversion.
Initial divnum ⇒Initial 0 divert(`1') Diversion one: divnum divert(`2') Diversion two: divnum ^D ⇒ ⇒Diversion one: 1 ⇒ ⇒Diversion two: 2
Previous: Divnum, Up: Diversions [Contents][Index]
Often it is not known, when output is diverted, whether the diverted
text is actually needed. Since all non-empty diversion are brought back
on the main output stream when the end of input is seen, a method of
discarding a diversion is needed. If all diversions should be
discarded, the easiest is to end the input to m4
with
‘divert(`-1')’ followed by an explicit ‘undivert’:
divert(`1') Diversion one: divnum divert(`2') Diversion two: divnum divert(`-1') undivert ^D
No output is produced at all.
Clearing selected diversions can be done with the following macro:
Discard the contents of each of the listed numeric diversions.
define(`cleardivert', `pushdef(`_n', divnum)divert(`-1')undivert($@)divert(_n)popdef(`_n')') ⇒
It is called just like undivert
, but the effect is to clear the
diversions, given by the arguments. (This macro has a nasty bug! You
should try to see if you can find it and correct it; or see Answers).
Next: Arithmetic, Previous: Diversions, Up: Top [Contents][Index]
There are a number of builtins in m4
for manipulating text in
various ways, extracting substrings, searching, substituting, and so on.
• Len | Calculating length of strings | |
• Index macro | Searching for substrings | |
• Regexp | Searching for regular expressions | |
• Substr | Extracting substrings | |
• Translit | Translating characters | |
• Patsubst | Substituting text by regular expression | |
• Format | Formatting strings (printf-like) |
Next: Index macro, Up: Text handling [Contents][Index]
The length of a string can be calculated by len
:
Expands to the length of string, as a decimal number.
The macro len
is recognized only with parameters.
len() ⇒0 len(`abcdef') ⇒6
Next: Regexp, Previous: Len, Up: Text handling [Contents][Index]
Searching for substrings is done with index
:
Expands to the index of the first occurrence of substring in
string. The first character in string has index 0. If
substring does not occur in string, index
expands to
‘-1’.
The macro index
is recognized only with parameters.
index(`gnus, gnats, and armadillos', `nat') ⇒7 index(`gnus, gnats, and armadillos', `dag') ⇒-1
Omitting substring evokes a warning, but still produces output; contrast this with an empty substring.
index(`abc') error→m4:stdin:1: Warning: too few arguments to builtin `index' ⇒0 index(`abc', `') ⇒0 index(`abc', `b') ⇒1
Next: Substr, Previous: Index macro, Up: Text handling [Contents][Index]
Searching for regular expressions is done with the builtin
regexp
:
Searches for regexp in string. The syntax for regular expressions is the same as in GNU Emacs, which is similar to BRE, Basic Regular Expressions in POSIX. See Syntax of Regular Expressions in the GNU Emacs Manual. Support for ERE, Extended Regular Expressions is not available, but will be added in GNU M4 2.0.
If replacement is omitted, regexp
expands to the index of
the first match of regexp in string. If regexp does
not match anywhere in string, it expands to -1.
If replacement is supplied, and there was a match, regexp
changes the expansion to this argument, with ‘\n’ substituted
by the text matched by the nth parenthesized sub-expression of
regexp, up to nine sub-expressions. The escape ‘\&’ is
replaced by the text of the entire regular expression matched. For
all other characters, ‘\’ treats the next character literally. A
warning is issued if there were fewer sub-expressions than the
‘\n’ requested, or if there is a trailing ‘\’. If there
was no match, regexp
expands to the empty string.
The macro regexp
is recognized only with parameters.
regexp(`GNUs not Unix', `\<[a-z]\w+') ⇒5 regexp(`GNUs not Unix', `\<Q\w*') ⇒-1 regexp(`GNUs not Unix', `\w\(\w+\)$', `*** \& *** \1 ***') ⇒*** Unix *** nix *** regexp(`GNUs not Unix', `\<Q\w*', `*** \& *** \1 ***') ⇒
Here are some more examples on the handling of backslash:
regexp(`abc', `\(b\)', `\\\10\a') ⇒\b0a regexp(`abc', `b', `\1\') error→m4:stdin:2: Warning: sub-expression 1 not present error→m4:stdin:2: Warning: trailing \ ignored in replacement ⇒ regexp(`abc', `\(\(d\)?\)\(c\)', `\1\2\3\4\5\6') error→m4:stdin:3: Warning: sub-expression 4 not present error→m4:stdin:3: Warning: sub-expression 5 not present error→m4:stdin:3: Warning: sub-expression 6 not present ⇒c
Omitting regexp evokes a warning, but still produces output; contrast this with an empty regexp argument.
regexp(`abc') error→m4:stdin:1: Warning: too few arguments to builtin `regexp' ⇒0 regexp(`abc', `') ⇒0 regexp(`abc', `', `\\def') ⇒\def
Next: Translit, Previous: Regexp, Up: Text handling [Contents][Index]
Substrings are extracted with substr
:
Expands to the substring of string, which starts at index from, and extends for length characters, or to the end of string, if length is omitted. The starting index of a string is always 0. The expansion is empty if there is an error parsing from or length, if from is beyond the end of string, or if length is negative.
The macro substr
is recognized only with parameters.
substr(`gnus, gnats, and armadillos', `6') ⇒gnats, and armadillos substr(`gnus, gnats, and armadillos', `6', `5') ⇒gnats
Omitting from evokes a warning, but still produces output.
substr(`abc') error→m4:stdin:1: Warning: too few arguments to builtin `substr' ⇒abc substr(`abc',) error→m4:stdin:2: empty string treated as 0 in builtin `substr' ⇒abc
Next: Patsubst, Previous: Substr, Up: Text handling [Contents][Index]
Character translation is done with translit
:
Expands to string, with each character that occurs in chars translated into the character from replacement with the same index.
If replacement is shorter than chars, the excess characters of chars are deleted from the expansion; if chars is shorter, the excess characters in replacement are silently ignored. If replacement is omitted, all characters in string that are present in chars are deleted from the expansion. If a character appears more than once in chars, only the first instance is used in making the translation. Only a single translation pass is made, even if characters in replacement also appear in chars.
As a GNU extension, both chars and replacement can contain character-ranges, e.g., ‘a-z’ (meaning all lowercase letters) or ‘0-9’ (meaning all digits). To include a dash ‘-’ in chars or replacement, place it first or last in the entire string, or as the last character of a range. Back-to-back ranges can share a common endpoint. It is not an error for the last character in the range to be ‘larger’ than the first. In that case, the range runs backwards, i.e., ‘9-0’ means the string ‘9876543210’. The expansion of a range is dependent on the underlying encoding of characters, so using ranges is not always portable between machines.
The macro translit
is recognized only with parameters.
translit(`GNUs not Unix', `A-Z') ⇒s not nix translit(`GNUs not Unix', `a-z', `A-Z') ⇒GNUS NOT UNIX translit(`GNUs not Unix', `A-Z', `z-a') ⇒tmfs not fnix translit(`+,-12345', `+--1-5', `<;>a-c-a') ⇒<;>abcba translit(`abcdef', `aabdef', `bcged') ⇒bgced
In the ASCII encoding, the first example deletes all uppercase letters, the second converts lowercase to uppercase, and the third ‘mirrors’ all uppercase letters, while converting them to lowercase. The two first cases are by far the most common, even though they are not portable to EBCDIC or other encodings. The fourth example shows a range ending in ‘-’, as well as back-to-back ranges. The final example shows that ‘a’ is mapped to ‘b’, not ‘c’; the resulting ‘b’ is not further remapped to ‘g’; the ‘d’ and ‘e’ are swapped, and the ‘f’ is discarded.
Omitting chars evokes a warning, but still produces output.
translit(`abc') error→m4:stdin:1: Warning: too few arguments to builtin `translit' ⇒abc
Next: Format, Previous: Translit, Up: Text handling [Contents][Index]
Global substitution in a string is done by patsubst
:
Searches string for matches of regexp, and substitutes replacement for each match. The syntax for regular expressions is the same as in GNU Emacs (see Regexp).
The parts of string that are not covered by any match of regexp are copied to the expansion. Whenever a match is found, the search proceeds from the end of the match, so a character from string will never be substituted twice. If regexp matches a string of zero length, the start position for the search is incremented, to avoid infinite loops.
When a replacement is to be made, replacement is inserted into the expansion, with ‘\n’ substituted by the text matched by the nth parenthesized sub-expression of patsubst, for up to nine sub-expressions. The escape ‘\&’ is replaced by the text of the entire regular expression matched. For all other characters, ‘\’ treats the next character literally. A warning is issued if there were fewer sub-expressions than the ‘\n’ requested, or if there is a trailing ‘\’.
The replacement argument can be omitted, in which case the text matched by regexp is deleted.
The macro patsubst
is recognized only with parameters.
patsubst(`GNUs not Unix', `^', `OBS: ') ⇒OBS: GNUs not Unix patsubst(`GNUs not Unix', `\<', `OBS: ') ⇒OBS: GNUs OBS: not OBS: Unix patsubst(`GNUs not Unix', `\w*', `(\&)') ⇒(GNUs)() (not)() (Unix)() patsubst(`GNUs not Unix', `\w+', `(\&)') ⇒(GNUs) (not) (Unix) patsubst(`GNUs not Unix', `[A-Z][a-z]+') ⇒GN not patsubst(`GNUs not Unix', `not', `NOT\') error→m4:stdin:6: Warning: trailing \ ignored in replacement ⇒GNUs NOT Unix
Here is a slightly more realistic example, which capitalizes individual
words or whole sentences, by substituting calls of the macros
upcase
and downcase
into the strings.
Expand to text, but with capitalization changed: upcase
changes all letters to upper case, downcase
changes all letters
to lower case, and capitalize
changes the first character of each
word to upper case and the remaining characters to lower case.
First, an example of their usage, using implementations distributed in m4-1.4.19/examples/capitalize.m4.
$ m4 -I examples include(`capitalize.m4') ⇒ upcase(`GNUs not Unix') ⇒GNUS NOT UNIX downcase(`GNUs not Unix') ⇒gnus not unix capitalize(`GNUs not Unix') ⇒Gnus Not Unix
Now for the implementation. There is a helper macro _capitalize
which puts only its first word in mixed case. Then capitalize
merely parses out the words, and replaces them with an invocation of
_capitalize
. (As presented here, the capitalize
macro has
some subtle flaws. You should try to see if you can find and correct
them; or see Answers).
$ m4 -I examples undivert(`capitalize.m4')dnl ⇒divert(`-1') ⇒# upcase(text) ⇒# downcase(text) ⇒# capitalize(text) ⇒# change case of text, simple version ⇒define(`upcase', `translit(`$*', `a-z', `A-Z')') ⇒define(`downcase', `translit(`$*', `A-Z', `a-z')') ⇒define(`_capitalize', ⇒ `regexp(`$1', `^\(\w\)\(\w*\)', ⇒ `upcase(`\1')`'downcase(`\2')')') ⇒define(`capitalize', `patsubst(`$1', `\w+', `_$0(`\&')')') ⇒divert`'dnl
While regexp
replaces the whole input with the replacement as
soon as there is a match, patsubst
replaces each
occurrence of a match and preserves non-matching pieces:
define(`patreg', `patsubst($@) regexp($@)')dnl patreg(`bar foo baz Foo', `foo\|Foo', `FOO') ⇒bar FOO baz FOO ⇒FOO patreg(`aba abb 121', `\(.\)\(.\)\1', `\2\1\2') ⇒bab abb 212 ⇒bab
Omitting regexp evokes a warning, but still produces output; contrast this with an empty regexp argument.
patsubst(`abc') error→m4:stdin:1: Warning: too few arguments to builtin `patsubst' ⇒abc patsubst(`abc', `') ⇒abc patsubst(`abc', `', `\\-') ⇒\-a\-b\-c\-
Previous: Patsubst, Up: Text handling [Contents][Index]
Formatted output can be made with format
:
Works much like the C function printf
. The first argument
format-string can contain ‘%’ specifications which are
satisfied by additional arguments, and the expansion of format
is
the formatted string.
The macro format
is recognized only with parameters.
Its use is best described by a few examples:
define(`foo', `The brown fox jumped over the lazy dog') ⇒ format(`The string "%s" uses %d characters', foo, len(foo)) ⇒The string "The brown fox jumped over the lazy dog" uses 38 characters format(`%*.*d', `-1', `-1', `1') ⇒1 format(`%.0f', `56789.9876') ⇒56790 len(format(`%-*X', `5000', `1')) ⇒5000 ifelse(format(`%010F', `infinity'), ` INF', `success', format(`%010F', `infinity'), ` INFINITY', `success', format(`%010F', `infinity')) ⇒success ifelse(format(`%.1A', `1.999'), `0X1.0P+1', `success', format(`%.1A', `1.999'), `0X2.0P+0', `success', format(`%.1A', `1.999')) ⇒success format(`%g', `0xa.P+1') ⇒20
Using the forloop
macro defined earlier (see Forloop), this
example shows how format
can be used to produce tabular output.
$ m4 -I examples include(`forloop.m4') ⇒ forloop(`i', `1', `10', `format(`%6d squared is %10d ', i, eval(i**2))') ⇒ 1 squared is 1 ⇒ 2 squared is 4 ⇒ 3 squared is 9 ⇒ 4 squared is 16 ⇒ 5 squared is 25 ⇒ 6 squared is 36 ⇒ 7 squared is 49 ⇒ 8 squared is 64 ⇒ 9 squared is 81 ⇒ 10 squared is 100 ⇒
The builtin format
is modeled after the ANSI C ‘printf’
function, and supports these ‘%’ specifiers: ‘c’, ‘s’,
‘d’, ‘o’, ‘x’, ‘X’, ‘u’, ‘a’, ‘A’,
‘e’, ‘E’, ‘f’, ‘F’, ‘g’, ‘G’, and
‘%’; it supports field widths and precisions, and the flags
‘+’, ‘-’, ‘ ’, ‘0’, ‘#’, and ‘'’. For
integer specifiers, the width modifiers ‘hh’, ‘h’, and
‘l’ are recognized, and for floating point specifiers, the width
modifier ‘l’ is recognized. Items not yet supported include
positional arguments, the ‘n’, ‘p’, ‘S’, and ‘C’
specifiers, the ‘z’, ‘t’, ‘j’, ‘L’ and ‘ll’
modifiers, and any platform extensions available in the native
printf
. For more details on the functioning of printf
,
see the C Library Manual, or the POSIX specification (for
example, ‘%a’ is supported even on platforms that haven’t yet
implemented C99 hexadecimal floating point output natively).
Unrecognized specifiers result in a warning. It is anticipated that a
future release of GNU m4
will support more specifiers,
and give better warnings when various problems such as overflow are
encountered. Likewise, escape sequences are not yet recognized.
format(`%p', `0') error→m4:stdin:1: Warning: unrecognized specifier in `%p' ⇒
Next: Shell commands, Previous: Text handling, Up: Top [Contents][Index]
Integer arithmetic is included in m4
, with a C-like syntax. As
convenient shorthands, there are builtins for simple increment and
decrement operations.
• Incr | Decrement and increment operators | |
• Eval | Evaluating integer expressions |
Next: Eval, Up: Arithmetic [Contents][Index]
Increment and decrement of integers are supported using the builtins
incr
and decr
:
Expand to the numerical value of number, incremented or decremented, respectively, by one. Except for the empty string, the expansion is empty if number could not be parsed.
The macros incr
and decr
are recognized only with
parameters.
incr(`4') ⇒5 decr(`7') ⇒6 incr() error→m4:stdin:3: empty string treated as 0 in builtin `incr' ⇒1 decr() error→m4:stdin:4: empty string treated as 0 in builtin `decr' ⇒-1
Previous: Incr, Up: Arithmetic [Contents][Index]
Integer expressions are evaluated with eval
:
Expands to the value of expression. The expansion is empty if a problem is encountered while parsing the arguments. If specified, radix and width control the format of the output.
Calculations are done with 32-bit signed numbers. Overflow silently results in wraparound. A warning is issued if division by zero is attempted, or if expression could not be parsed.
Expressions can contain the following operators, listed in order of decreasing precedence.
Parentheses
Unary plus and minus, and bitwise and logical negation
Exponentiation
Multiplication, division, and modulo
Addition and subtraction
Shift left or right
Relational operators
Equality operators
Bitwise and
Bitwise exclusive-or
Bitwise or
Logical and
Logical or
The macro eval
is recognized only with parameters.
All binary operators, except exponentiation, are left associative. C
operators that perform variable assignment, such as ‘+=’ or
‘--’, are not implemented, since eval
only operates on
constants, not variables. Attempting to use them results in an error.
However, since traditional implementations treated ‘=’ as an
undocumented alias for ‘==’ as opposed to an assignment operator,
this usage is supported as a special case. Be aware that a future
version of GNU M4 may support assignment semantics as an
extension when POSIX mode is not requested, and that using
‘=’ to check equality is not portable.
eval(`2 = 2') error→m4:stdin:1: Warning: recommend ==, not =, for equality operator ⇒1 eval(`++0') error→m4:stdin:2: invalid operator in eval: ++0 ⇒ eval(`0 |= 1') error→m4:stdin:3: invalid operator in eval: 0 |= 1 ⇒
Note that some older m4
implementations use ‘^’ as an
alternate operator for the exponentiation, although POSIX
requires the C behavior of bitwise exclusive-or. The precedence of the
negation operators, ‘~’ and ‘!’, was traditionally lower than
equality. The unary operators could not be used reliably more than once
on the same term without intervening parentheses. The traditional
precedence of the equality operators ‘==’ and ‘!=’ was
identical instead of lower than the relational operators such as
‘<’, even through GNU M4 1.4.8. Starting with version
1.4.9, GNU M4 correctly follows POSIX precedence
rules. M4 scripts designed to be portable between releases must be
aware that parentheses may be required to enforce C precedence rules.
Likewise, division by zero, even in the unused branch of a
short-circuiting operator, is not always well-defined in other
implementations.
Following are some examples where the current version of M4 follows C
precedence rules, but where older versions and some other
implementations of m4
require explicit parentheses to get the
correct result:
eval(`1 == 2 > 0') ⇒1 eval(`(1 == 2) > 0') ⇒0 eval(`! 0 * 2') ⇒2 eval(`! (0 * 2)') ⇒1 eval(`1 | 1 ^ 1') ⇒1 eval(`(1 | 1) ^ 1') ⇒0 eval(`+ + - ~ ! ~ 0') ⇒1 eval(`2 || 1 / 0') ⇒1 eval(`0 || 1 / 0') error→m4:stdin:9: divide by zero in eval: 0 || 1 / 0 ⇒ eval(`0 && 1 % 0') ⇒0 eval(`2 && 1 % 0') error→m4:stdin:11: modulo by zero in eval: 2 && 1 % 0 ⇒
As a GNU extension, the operator ‘**’ performs integral exponentiation. The operator is right-associative, and if evaluated, the exponent must be non-negative, and at least one of the arguments must be non-zero, or a warning is issued.
eval(`2 ** 3 ** 2') ⇒512 eval(`(2 ** 3) ** 2') ⇒64 eval(`0 ** 1') ⇒0 eval(`2 ** 0') ⇒1 eval(`0 ** 0') ⇒ error→m4:stdin:5: divide by zero in eval: 0 ** 0 eval(`4 ** -2') error→m4:stdin:6: negative exponent in eval: 4 ** -2 ⇒
Within expression, (but not radix or width), numbers without a special prefix are decimal. A simple ‘0’ prefix introduces an octal number. ‘0x’ introduces a hexadecimal number. As GNU extensions, ‘0b’ introduces a binary number. ‘0r’ introduces a number expressed in any radix between 1 and 36: the prefix should be immediately followed by the decimal expression of the radix, a colon, then the digits making the number. For radix 1, leading zeros are ignored, and all remaining digits must be ‘1’; for all other radices, the digits are ‘0’, ‘1’, ‘2’, …. Beyond ‘9’, the digits are ‘a’, ‘b’ … up to ‘z’. Lower and upper case letters can be used interchangeably in numbers prefixes and as number digits.
Parentheses may be used to group subexpressions whenever needed. For the
relational operators, a true relation returns 1
, and a false
relation return 0
.
Here are a few examples of use of eval
.
eval(`-3 * 5') ⇒-15 eval(`-99 / 10') ⇒-9 eval(`-99 % 10') ⇒-9 eval(`99 % -10') ⇒9 eval(index(`Hello world', `llo') >= 0) ⇒1 eval(`0r1:0111 + 0b100 + 0r3:12') ⇒12 define(`square', `eval(`($1) ** 2')') ⇒ square(`9') ⇒81 square(square(`5')` + 1') ⇒676 define(`foo', `666') ⇒ eval(`foo / 6') error→m4:stdin:11: bad expression in eval: foo / 6 ⇒ eval(foo / 6) ⇒111
As the last two lines show, eval
does not handle macro
names, even if they expand to a valid expression (or part of a valid
expression). Therefore all macros must be expanded before they are
passed to eval
.
Some calculations are not portable to other implementations, since they
have undefined semantics in C, but GNU m4
has
well-defined behavior on overflow. When shifting, an out-of-range shift
amount is implicitly brought into the range of 32-bit signed integers
using an implicit bit-wise and with 0x1f).
define(`max_int', eval(`0x7fffffff')) ⇒ define(`min_int', incr(max_int)) ⇒ eval(min_int` < 0') ⇒1 eval(max_int` > 0') ⇒1 ifelse(eval(min_int` / -1'), min_int, `overflow occurred') ⇒overflow occurred min_int ⇒-2147483648 eval(`0x80000000 % -1') ⇒0 eval(`-4 >> 1') ⇒-2 eval(`-4 >> 33') ⇒-2
If radix is specified, it specifies the radix to be used in the
expansion. The default radix is 10; this is also the case if
radix is the empty string. A warning results if the radix is
outside the range of 1 through 36, inclusive. The result of eval
is always taken to be signed. No radix prefix is output, and for
radices greater than 10, the digits are lower case. The width
argument specifies the minimum output width, excluding any negative
sign. The result is zero-padded to extend the expansion to the
requested width. A warning results if the width is negative. If
radix or width is out of bounds, the expansion of
eval
is empty.
eval(`666', `10') ⇒666 eval(`666', `11') ⇒556 eval(`666', `6') ⇒3030 eval(`666', `6', `10') ⇒0000003030 eval(`-666', `6', `10') ⇒-0000003030 eval(`10', `', `0') ⇒10 `0r1:'eval(`10', `1', `11') ⇒0r1:01111111111 eval(`10', `16') ⇒a eval(`1', `37') error→m4:stdin:9: radix 37 in builtin `eval' out of range ⇒ eval(`1', , `-1') error→m4:stdin:10: negative width to builtin `eval' ⇒ eval() error→m4:stdin:11: empty string treated as 0 in builtin `eval' ⇒0
Next: Miscellaneous, Previous: Arithmetic, Up: Top [Contents][Index]
There are a few builtin macros in m4
that allow you to run shell
commands from within m4
.
Note that the definition of a valid shell command is system dependent.
On UNIX systems, this is the typical /bin/sh
. But on other
systems, such as native Windows, the shell has a different syntax of
commands that it understands. Some examples in this chapter assume
/bin/sh
, and also demonstrate how to quit early with a known
exit value if this is not the case.
• Platform macros | Determining the platform | |
• Syscmd | Executing simple commands | |
• Esyscmd | Reading the output of commands | |
• Sysval | Exit status | |
• Mkstemp | Making temporary files |
Next: Syscmd, Up: Shell commands [Contents][Index]
Sometimes it is desirable for an input file to know which platform
m4
is running on. GNU m4
provides several
macros that are predefined to expand to the empty string; checking for
their existence will confirm platform details.
Each of these macros is conditionally defined as needed to describe the
environment of m4
. If defined, each macro expands to the empty
string. For now, these macros silently ignore all arguments, but in a
future release of M4, they might warn if arguments are present.
When GNU extensions are in effect (that is, when you did not
use the -G option, see Invoking m4),
GNU m4
will define the macro __gnu__
to
expand to the empty string.
$ m4 __gnu__ ⇒ __gnu__(`ignored') ⇒ Extensions are ifdef(`__gnu__', `active', `inactive') ⇒Extensions are active
$ m4 -G __gnu__ ⇒__gnu__ __gnu__(`ignored') ⇒__gnu__(ignored) Extensions are ifdef(`__gnu__', `active', `inactive') ⇒Extensions are inactive
On UNIX systems, GNU m4
will define __unix__
by default, or unix
when the -G option is specified.
On native Windows systems, GNU m4
will define
__windows__
by default, or windows
when the
-G option is specified.
On OS/2 systems, GNU m4
will define __os2__
by default, or os2
when the -G option is specified.
If GNU m4
does not provide a platform macro for your system,
please report that as a bug.
define(`provided', `0') ⇒ ifdef(`__unix__', `define(`provided', incr(provided))') ⇒ ifdef(`__windows__', `define(`provided', incr(provided))') ⇒ ifdef(`__os2__', `define(`provided', incr(provided))') ⇒ provided ⇒1
Next: Esyscmd, Previous: Platform macros, Up: Shell commands [Contents][Index]
Any shell command can be executed, using syscmd
:
Executes shell-command as a shell command.
The expansion of syscmd
is void, not the output from
shell-command! Output or error messages from shell-command
are not read by m4
. See Esyscmd, if you need to process the
command output.
Prior to executing the command, m4
flushes its buffers.
The default standard input, output and error of shell-command are
the same as those of m4
.
By default, the shell-command will be used as the argument to the
-c option of the /bin/sh
shell (or the version of
sh
specified by ‘command -p getconf PATH’, if your system
supports that). If you prefer a different shell, the
configure
script can be given the option
--with-syscmd-shell=location to set the location of an
alternative shell at GNU m4
installation; the
alternative shell must still support -c.
The macro syscmd
is recognized only with parameters.
define(`foo', `FOO') ⇒ syscmd(`echo foo') ⇒foo ⇒
Note how the expansion of syscmd
keeps the trailing newline of
the command, as well as using the newline that appeared after the macro.
The following is an example of shell-command using the same
standard input as m4
:
$ echo "m4wrap(\`syscmd(\`cat')')" | m4 ⇒
It tells m4
to read all of its input before executing the wrapped
text, then hand a valid (albeit emptied) pipe as standard input for the
cat
subcommand. Therefore, you should be careful when using
standard input (either by specifying no files, or by passing ‘-’ as
a file name on the command line, see Invoking
m4), and also invoking subcommands via syscmd
or esyscmd
that consume data from standard input. When standard input is a
seekable file, the subprocess will pick up with the next character not
yet processed by m4
; when it is a pipe or other non-seekable
file, there is no guarantee how much data will already be buffered by
m4
and thus unavailable to the child.
Next: Sysval, Previous: Syscmd, Up: Shell commands [Contents][Index]
If you want m4
to read the output of a shell command, use
esyscmd
:
Expands to the standard output of the shell command shell-command.
Prior to executing the command, m4
flushes its buffers.
The default standard input and standard error of shell-command are
the same as those of m4
. The error output of shell-command
is not a part of the expansion: it will appear along with the error
output of m4
.
By default, the shell-command will be used as the argument to the
-c option of the /bin/sh
shell (or the version of
sh
specified by ‘command -p getconf PATH’, if your system
supports that). If you prefer a different shell, the
configure
script can be given the option
--with-syscmd-shell=location to set the location of an
alternative shell at GNU m4
installation; the
alternative shell must still support -c.
The macro esyscmd
is recognized only with parameters.
define(`foo', `FOO') ⇒ esyscmd(`echo foo') ⇒FOO ⇒
Note how the expansion of esyscmd
keeps the trailing newline of
the command, as well as using the newline that appeared after the macro.
Just as with syscmd
, care must be exercised when sharing standard
input between m4
and the child process of esyscmd
.
Next: Mkstemp, Previous: Esyscmd, Up: Shell commands [Contents][Index]
To see whether a shell command succeeded, use sysval
:
Expands to the exit status of the last shell command run with
syscmd
or esyscmd
. Expands to 0 if no command has been
run yet.
sysval ⇒0 syscmd(`false') ⇒ ifelse(sysval, `0', `zero', `non-zero') ⇒non-zero syscmd(`exit 2') ⇒ sysval ⇒2 syscmd(`true') ⇒ sysval ⇒0 esyscmd(`false') ⇒ ifelse(sysval, `0', `zero', `non-zero') ⇒non-zero esyscmd(`echo dnl && exit 127') ⇒ sysval ⇒127 esyscmd(`true') ⇒ sysval ⇒0
sysval
results in 127 if there was a problem executing the
command, for example, if the system-imposed argument length is exceeded,
or if there were not enough resources to fork. It is not possible to
distinguish between failed execution and successful execution that had
an exit status of 127, unless there was output from the child process.
On UNIX platforms, where it is possible to detect when command execution is terminated by a signal, rather than a normal exit, the result is the signal number shifted left by eight bits.
dnl This test assumes kill is a shell builtin, and that signals are dnl recognizable. ifdef(`__unix__', , `errprint(` skipping: syscmd does not have unix semantics ')m4exit(`77')')dnl changequote(`[', `]') ⇒ syscmd([/bin/sh -c 'kill -9 $$'; st=$?; test $st = 137 || test $st = 265]) ⇒ ifelse(sysval, [0], , [errprint([ skipping: shell does not send signal 9 ])m4exit([77])])dnl syscmd([kill -9 $$]) ⇒ sysval ⇒2304 syscmd() ⇒ sysval ⇒0 esyscmd([kill -9 $$]) ⇒ sysval ⇒2304
Previous: Sysval, Up: Shell commands [Contents][Index]
Commands specified to syscmd
or esyscmd
might need a
temporary file, for output or for some other purpose. There is a
builtin macro, mkstemp
, for making a temporary file:
Expands to the quoted name of a new, empty file, made from the string
template, which should end with the string ‘XXXXXX’. The six
‘X’ characters are then replaced with random characters matching
the regular expression ‘[a-zA-Z0-9._-]’, in order to make the file
name unique. If fewer than six ‘X’ characters are found at the end
of template
, the result will be longer than the template. The
created file will have access permissions as if by chmod =rw,go=,
meaning that the current umask of the m4
process is taken into
account, and at most only the current user can read and write the file.
The traditional behavior, standardized by POSIX, is that
maketemp
merely replaces the trailing ‘X’ with the process
id, without creating a file or quoting the expansion, and without
ensuring that the resulting
string is a unique file name. In part, this means that using the same
template twice in the same input file will result in the same
expansion. This behavior is a security hole, as it is very easy for
another process to guess the name that will be generated, and thus
interfere with a subsequent use of syscmd
trying to manipulate
that file name. Hence, POSIX has recommended that all new
implementations of m4
provide the secure mkstemp
builtin,
and that users of m4
check for its existence.
The expansion is void and an error issued if a temporary file could not be created.
The macros mkstemp
and maketemp
are recognized only with
parameters.
If you try this next example, you will most likely get different output for the two file names, since the replacement characters are randomly chosen:
$ m4 define(`tmp', `oops') ⇒ maketemp(`/tmp/fooXXXXXX') ⇒/tmp/fooa07346 ifdef(`mkstemp', `define(`maketemp', defn(`mkstemp'))', `define(`mkstemp', defn(`maketemp'))dnl errprint(`warning: potentially insecure maketemp implementation ')') ⇒ mkstemp(`doc') ⇒docQv83Uw
Unless you use the --traditional command line option (or
-G, see Invoking m4), the GNU
version of maketemp
is secure. This means that using the same
template to multiple calls will generate multiple files. However, we
recommend that you use the new mkstemp
macro, introduced in
GNU M4 1.4.8, which is secure even in traditional mode. Also,
as of M4 1.4.11, the secure implementation quotes the resulting file
name, so that you are guaranteed to know what file was created even if
the random file name happens to match an existing macro. Notice that
this example is careful to use defn
to avoid unintended expansion
of ‘foo’.
$ m4 define(`foo', `errprint(`oops')') ⇒ syscmd(`rm -f foo-??????')sysval ⇒0 define(`file1', maketemp(`foo-XXXXXX'))dnl ifelse(esyscmd(`echo \` foo-?????? \''), ` foo-?????? ', `no file', `created') ⇒created define(`file2', maketemp(`foo-XX'))dnl define(`file3', mkstemp(`foo-XXXXXX'))dnl ifelse(len(defn(`file1')), len(defn(`file2')), `same length', `different') ⇒same length ifelse(defn(`file1'), defn(`file2'), `same', `different file') ⇒different file ifelse(defn(`file2'), defn(`file3'), `same', `different file') ⇒different file ifelse(defn(`file1'), defn(`file3'), `same', `different file') ⇒different file syscmd(`rm 'defn(`file1') defn(`file2') defn(`file3')) ⇒ sysval ⇒0
Next: Frozen files, Previous: Shell commands, Up: Top [Contents][Index]
This chapter describes various builtins, that do not really belong in any of the previous chapters.
• Errprint | Printing error messages | |
• Location | Printing current location | |
• M4exit | Exiting from m4
|
Next: Location, Up: Miscellaneous [Contents][Index]
You can print error messages using errprint
:
Prints message and the rest of the arguments to standard error, separated by spaces. Standard error is used, regardless of the --debugfile option (see Invoking m4).
The expansion of errprint
is void.
The macro errprint
is recognized only with parameters.
errprint(`Invalid arguments to forloop ') error→Invalid arguments to forloop ⇒ errprint(`1')errprint(`2',`3 ') error→12 3 ⇒
A trailing newline is not printed automatically, so it should be
supplied as part of the argument, as in the example. Unfortunately, the
exact output of errprint
is not very portable to other m4
implementations: POSIX requires that all arguments be printed,
but some implementations of m4
only print the first.
Furthermore, some BSD implementations always append a newline
for each errprint
call, regardless of whether the last argument
already had one, and POSIX is silent on whether this is
acceptable.
Next: M4exit, Previous: Errprint, Up: Miscellaneous [Contents][Index]
To make it possible to specify the location of an error, three utility builtins exist:
Expand to the quoted name of the current input file, the
current input line number in that file, and the quoted name of the
current invocation of m4
.
errprint(__program__:__file__:__line__: `input error ') error→m4:stdin:1: input error ⇒
Line numbers start at 1 for each file. If the file was found due to the
-I option or M4PATH
environment variable, that is
reflected in the file name. The syncline option (-s,
see Invoking m4), and the
‘f’ and ‘l’ flags of debugmode
(see Debug Levels),
also use this notion of current file and line. Redefining the three
location macros has no effect on syncline, debug, warning, or error
message output.
This example reuses the file incl.m4 mentioned earlier (see Include):
$ m4 -I examples define(`foo', ``$0' called at __file__:__line__') ⇒ foo ⇒foo called at stdin:2 include(`incl.m4') ⇒Include file start ⇒foo called at examples/incl.m4:2 ⇒Include file end ⇒
The location of macros invoked during the rescanning of macro expansion
text corresponds to the location in the file where the expansion was
triggered, regardless of how many newline characters the expansion text
contains. As of GNU M4 1.4.8, the location of text wrapped
with m4wrap
(see M4wrap) is the point at which the
m4wrap
was invoked. Previous versions, however, behaved as
though wrapped text came from line 0 of the file “”.
define(`echo', `$@') ⇒ define(`foo', `echo(__line__ __line__)') ⇒ echo(__line__ __line__) ⇒4 ⇒5 m4wrap(`foo ') ⇒ foo(errprint(__line__ __line__ )) error→8 error→9 ⇒8 ⇒8 __line__ ⇒11 m4wrap(`__line__ ') ⇒ ^D ⇒12 ⇒6 ⇒6
The __program__
macro behaves like ‘$0’ in shell
terminology. If you invoke m4
through an absolute path or a link
with a different spelling, rather than by relying on a PATH
search
for plain ‘m4’, it will affect how __program__
expands.
The intent is that you can use it to produce error messages with the
same formatting that m4
produces internally. It can also be used
within syscmd
(see Syscmd) to pick the same version of
m4
that is currently running, rather than whatever version of
m4
happens to be first in PATH
. It was first introduced in
GNU M4 1.4.6.
Previous: Location, Up: Miscellaneous [Contents][Index]
m4
If you need to exit from m4
before the entire input has been
read, you can use m4exit
:
Causes m4
to exit, with exit status code. If code is
left out, the exit status is zero. If code cannot be parsed, or
is outside the range of 0 to 255, the exit status is one. No further
input is read, and all wrapped and diverted text is discarded.
m4wrap(`This text is lost due to `m4exit'.') ⇒ divert(`1') So is this. divert ⇒ m4exit And this is never read.
A common use of this is to abort processing:
Abort processing with an error message and non-zero status. Prefix message with details about where the error occurred, and print the resulting string to standard error.
define(`fatal_error', `errprint(__program__:__file__:__line__`: fatal error: $* ')m4exit(`1')') ⇒ fatal_error(`this is a BAD one, buster') error→m4:stdin:4: fatal error: this is a BAD one, buster
After this macro call, m4
will exit with exit status 1. This macro
is only intended for error exits, since the normal exit procedures are
not followed, i.e., diverted text is not undiverted, and saved text
(see M4wrap) is not reread. (This macro could be made more robust
to earlier versions of m4
. You should try to see if you can find
weaknesses and correct them; or see Answers).
Note that it is still possible for the exit status to be different than
what was requested by m4exit
. If m4
detects some other
error, such as a write error on standard output, the exit status will be
non-zero even if m4exit
requested zero.
If standard input is seekable, then the file will be positioned at the
next unread character. If it is a pipe or other non-seekable file,
then there are no guarantees how much data m4
might have read
into buffers, and thus discarded.
Next: Compatibility, Previous: Miscellaneous, Up: Top [Contents][Index]
Some bigger m4
applications may be built over a common base
containing hundreds of definitions and other costly initializations.
Usually, the common base is kept in one or more declarative files,
which files are listed on each m4
invocation prior to the
user’s input file, or else each input file uses include
.
Reading the common base of a big application, over and over again, may
be time consuming. GNU m4
offers some machinery to
speed up the start of an application using lengthy common bases.
• Using frozen files | Using frozen files | |
• Frozen file format | Frozen file format |
Next: Frozen file format, Up: Frozen files [Contents][Index]
Suppose a user has a library of m4
initializations in
base.m4, which is then used with multiple input files:
$ m4 base.m4 input1.m4 $ m4 base.m4 input2.m4 $ m4 base.m4 input3.m4
Rather than spending time parsing the fixed contents of base.m4 every time, the user might rather execute:
$ m4 -F base.m4f base.m4
once, and further execute, as often as needed:
$ m4 -R base.m4f input1.m4 $ m4 -R base.m4f input2.m4 $ m4 -R base.m4f input3.m4
with the varying input. The first call, containing the -F
option, only reads and executes file base.m4, defining
various application macros and computing other initializations.
Once the input file base.m4 has been completely processed, GNU
m4
produces in base.m4f a frozen file, that is, a
file which contains a kind of snapshot of the m4
internal state.
Later calls, containing the -R option, are able to reload
the internal state of m4
, from base.m4f,
prior to reading any other input files. This means
instead of starting with a virgin copy of m4
, input will be
read after having effectively recovered the effect of a prior run.
In our example, the effect is the same as if file base.m4 has
been read anew. However, this effect is achieved a lot faster.
Only one frozen file may be created or read in any one m4
invocation. It is not possible to recover two frozen files at once.
However, frozen files may be updated incrementally, through using
-R and -F options simultaneously. For example, if
some care is taken, the command:
$ m4 file1.m4 file2.m4 file3.m4 file4.m4
could be broken down in the following sequence, accumulating the same output:
$ m4 -F file1.m4f file1.m4 $ m4 -R file1.m4f -F file2.m4f file2.m4 $ m4 -R file2.m4f -F file3.m4f file3.m4 $ m4 -R file3.m4f file4.m4
Some care is necessary because not every effort has been made for
this to work in all cases. In particular, the trace attribute of
macros is not handled, nor the current setting of changeword
.
Currently, m4wrap
and sysval
also have problems.
Also, interactions for some options of m4
, being used in one call
and not in the next, have not been fully analyzed yet. On the other
end, you may be confident that stacks of pushdef
definitions
are handled correctly, as well as undefined or renamed builtins, and
changed strings for quotes or comments. And future releases of
GNU M4 will improve on the utility of frozen files.
When an m4
run is to be frozen, the automatic undiversion
which takes place at end of execution is inhibited. Instead, all
positively numbered diversions are saved into the frozen file.
The active diversion number is also transmitted.
A frozen file to be reloaded need not reside in the current directory.
It is looked up the same way as an include
file (see Search Path).
If the frozen file was generated with a newer version of m4
, and
contains directives that an older m4
cannot parse, attempting to
load the frozen file with option -R will cause m4
to
exit with status 63 to indicate version mismatch.
Previous: Using frozen files, Up: Frozen files [Contents][Index]
Frozen files are sharable across architectures. It is safe to write
a frozen file on one machine and read it on another, given that the
second machine uses the same or newer version of GNU m4
.
It is conventional, but not required, to give a frozen file the suffix
of .m4f
.
These are simple (editable) text files, made up of directives, each starting with a capital letter and ending with a newline (NL). Wherever a directive is expected, the character ‘#’ introduces a comment line; empty lines are also ignored if they are not part of an embedded string. In the following descriptions, each len refers to the length of the corresponding strings str in the next line of input. Numbers are always expressed in decimal. There are no escape characters. The directives are:
C len1 , len2 NL str1 str2 NL
Uses str1 and str2 as the begin-comment and end-comment strings. If omitted, then ‘#’ and NL are the comment delimiters.
D number, len NL str NL
Selects diversion number, making it current, then copy
str in the current diversion. number may be a negative
number for a non-existing diversion. To merely specify an active
selection, use this command with an empty str. With 0 as the
diversion number, str will be issued on standard output
at reload time. GNU m4
will not produce the ‘D’
directive with non-zero length for diversion 0, but this can be done
with manual edits. This directive may
appear more than once for the same diversion, in which case the
diversion is the concatenation of the various uses. If omitted, then
diversion 0 is current.
F len1 , len2 NL str1 str2 NL
Defines, through pushdef
, a definition for str1
expanding to the function whose builtin name is str2. If the
builtin does not exist (for example, if the frozen file was produced by
a copy of m4
compiled with changeword support, but the version
of m4
reloading was compiled without it), the reload is silent,
but any subsequent use of the definition of str1 will result in
a warning. This directive may appear more than once for the same name,
and its order, along with ‘T’, is important. If omitted, you will
have no access to any builtins.
Q len1 , len2 NL str1 str2 NL
Uses str1 and str2 as the begin-quote and end-quote strings. If omitted, then ‘`’ and ‘'’ are the quote delimiters.
T len1 , len2 NL str1 str2 NL
Defines, though pushdef
, a definition for str1
expanding to the text given by str2. This directive may appear
more than once for the same name, and its order, along with ‘F’, is
important.
V number NL
Confirms the format of the file. m4
1.4.19 only creates
and understands frozen files where number is 1. This directive
must be the first non-comment in the file, and may not appear more than
once.
Next: Answers, Previous: Frozen files, Up: Top [Contents][Index]
m4
This chapter describes the many of the differences between this
implementation of m4
, and of other implementations found under
UNIX, such as System V Release 4, Solaris, and BSD flavors.
In particular, it lists the known differences and extensions to
POSIX. However, the list is not necessarily comprehensive.
At the time of this writing, POSIX 2001 (also known as IEEE
Std 1003.1-2001) is the latest standard, although a new version of
POSIX is under development and includes several proposals for
modifying what m4
is required to do. The requirements for
m4
are shared between SUSv3 and POSIX, and
can be viewed at
https://www.opengroup.org/onlinepubs/000095399/utilities/m4.html.
• Extensions | Extensions in GNU M4 | |
• Incompatibilities | Facilities in System V m4 not in GNU M4 | |
• Other Incompatibilities | Other incompatibilities |
Next: Incompatibilities, Up: Compatibility [Contents][Index]
This version of m4
contains a few facilities that do not exist
in System V m4
. These extra facilities are all suppressed by
using the -G command line option (see Invoking m4), unless overridden by other command line options.
$n
notation for macro arguments, n can contain
several digits, while the System V m4
only accepts one digit.
This allows macros in GNU m4
to take any number of
arguments, and not only nine (see Arguments).
This means that define(`foo', `$11')
is ambiguous between
implementations. To portably choose between grabbing the first
parameter and appending 1 to the expansion, or grabbing the eleventh
parameter, you can do the following:
define(`a1', `A1') ⇒ dnl First argument, concatenated with 1 define(`_1', `$1')define(`first1', `_1($@)1') ⇒ dnl Eleventh argument, portable define(`_9', `$9')define(`eleventh', `_9(shift(shift($@)))') ⇒ dnl Eleventh argument, GNU style define(`Eleventh', `$11') ⇒ first1(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k') ⇒A1 eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k') ⇒k Eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k') ⇒k
Also see the argn
macro (see Shift).
divert
(see Divert) macro can manage more than 9
diversions. GNU m4
treats all positive numbers as valid
diversions, rather than discarding diversions greater than 9.
include
and sinclude
are sought in a
user specified search path, if they are not found in the working
directory. The search path is specified by the -I option and the
M4PATH
environment variable (see Search Path).
undivert
can be non-numeric, in which case the named
file will be included uninterpreted in the output (see Undivert).
format
builtin, which
is modeled after the C library function printf
(see Format).
regexp
(see Regexp) and patsubst
(see Patsubst) builtins. Some BSD implementations use
extended regular expressions instead.
m4
with
esyscmd
(see Esyscmd).
builtin
(see Builtin).
indir
(see Indir).
__program__
,
__file__
, and __line__
(see Location).
dumpdef
and macro tracing can be
controlled with debugmode
(see Debug Levels).
debugfile
(see Debug Output).
maketemp
(see Mkstemp) macro behaves like mkstemp
,
creating a new file with a unique name on every invocation, rather than
following the insecure behavior of replacing the trailing ‘X’
characters with the m4
process id.
The debugging and tracing facilities in GNU m4
are much
more extensive than in most other versions of m4
.
Next: Other Incompatibilities, Previous: Extensions, Up: Compatibility [Contents][Index]
m4
not in GNU m4
The version of m4
from System V contains a few facilities that
have not been implemented in GNU m4
yet. Additionally,
POSIX requires some behaviors that GNU m4
has not
implemented yet. Relying on these behaviors is non-portable, as a
future release of GNU m4
may change.
defn
,
without any clarification on how defn
behaves when one of the
multiple arguments names a builtin. System V m4
and some other
implementations allow mixing builtins and text macros into a single
macro. GNU m4
only supports joining multiple text
arguments, although a future implementation may lift this restriction to
behave more like System V. The only portable way to join text macros
with builtins is via helper macros and implicit concatenation of macro
results.
eval
(see Eval) when an argument cannot be parsed).
m4
correctly handles multiple instances
of ‘-’ on the command line.
m4wrap
(see M4wrap) to act in FIFO
(first-in, first-out) order, but GNU m4
currently uses
LIFO order. Furthermore, POSIX states that only the first
argument to m4wrap
is saved for later evaluation, but
GNU m4
saves and processes all arguments, with output
separated by spaces.
a`'define`'b
would expand to ab
. But
GNU m4
ignores certain builtins if they have missing
arguments, giving adefineb
for the above example.
define(`f',`1')
(see Define)
by undefining the entire stack of previous definitions, and if doing
undefine(`f')
first. GNU m4
replaces just the top
definition on the stack, as if doing popdef(`f')
followed by
pushdef(`f',`1')
. POSIX allows either behavior.
syscmd
(see Syscmd) to evaluate
command output for macro expansion, but this was a mistake that is
anticipated to be corrected in the next version of POSIX.
GNU m4
follows traditional behavior in syscmd
where output is not rescanned, and provides the extension esyscmd
that does scan the output.
changequote(arg)
(see Changequote) to use newline as the close quote, but this was a
bug, and the next version of POSIX is anticipated to state
that using empty strings or just one argument is unspecified.
Meanwhile, the GNU m4
behavior of treating an empty
end-quote delimiter as ‘'’ is not portable, as Solaris treats it as
repeating the start-quote delimiter, and BSD treats it as leaving the
previous end-quote delimiter unchanged. For predictable results, never
call changequote with just one argument, or with empty strings for
arguments.
changecom(arg,)
(see Changecom) to make it impossible to end a comment, but this is
a bug, and the next version of POSIX is anticipated to state
that using empty strings is unspecified. Meanwhile, the GNU
m4
behavior of treating an empty end-comment delimiter as newline
is not portable, as BSD treats it as leaving the previous end-comment
delimiter unchanged. It is also impossible in BSD implementations to
disable comments, even though that is required by POSIX. For
predictable results, never call changecom with empty strings for
arguments.
m4
give macros a higher precedence than
comments when parsing, meaning that if the start delimiter given to
changecom
(see Changecom) starts with a macro name, comments
are effectively disabled. POSIX does not specify what the
precedence is, so this version of GNU m4
parser
recognizes comments, then macros, then quoted strings.
m4
, but
gives an error message that the end of file was encountered inside a
macro with GNU m4
. On the other hand, traditional
implementations do end of file processing for files included with
include
or sinclude
(see Include), while GNU
m4
seamlessly integrates the content of those files. Thus
include(`a.m4')include(`b.m4')
will output ‘3’ instead of
giving an error.
m4
treats traceon
(see Trace) without
arguments as a global variable, independent of named macro tracing.
Also, once a macro is undefined, named tracing of that macro is lost.
On the other hand, when GNU m4
encounters
traceon
without
arguments, it turns tracing on for all existing definitions at the time,
but does not trace future definitions; traceoff
without arguments
turns tracing off for all definitions regardless of whether they were
also traced by name; and tracing by name, such as with -tfoo at
the command line or traceon(`foo')
in the input, is an attribute
that is preserved even if the macro is currently undefined.
Additionally, while POSIX requires trace output, it makes no
demands on the formatting of that output. Parsing trace output is not
guaranteed to be reliable, even between different releases of
GNU M4; however, the intent is that any future changes in
trace output will only occur under the direction of additional
debugmode
flags (see Debug Levels).
eval
(see Eval) to treat all
operators with the same precedence as C. However, earlier versions of
GNU m4
followed the traditional behavior of other
m4
implementations, where bitwise and logical negation (‘~’
and ‘!’) have lower precedence than equality operators; and where
equality operators (‘==’ and ‘!=’) had the same precedence as
relational operators (such as ‘<’). Use explicit parentheses to
ensure proper precedence. As extensions to POSIX,
GNU m4
gives well-defined semantics to operations that
C leaves undefined, such as when overflow occurs, when shifting negative
numbers, or when performing division by zero. POSIX also
requires ‘=’ to cause an error, but many traditional
implementations allowed it as an alias for ‘==’.
translit
(see Translit) to
treat each character of the second and third arguments literally.
However, it is anticipated that the next version of POSIX will
allow the GNU m4
behavior of treating ‘-’ as a
range operator.
m4
to honor the locale environment
variables of LANG
, LC_ALL
, LC_CTYPE
,
LC_MESSAGES
, and NLSPATH
, but this has not yet been
implemented in GNU m4
.
m4
follows
tradition and ignores all leading unquoted whitespace.
POSIXLY_CORRECT
and enables the option
--gnu
by default (see Invoking m4), a
client desiring to be strictly compliant has no way to disable
GNU extensions that conflict with POSIX when
directly invoking the compiled m4
. A future version of
GNU
M4 will honor the environment variable POSIXLY_CORRECT
,
implicitly enabling --traditional if it is set, in order to
allow a strictly-compliant client. In the meantime, a client needing
strict POSIX compliance can use the workaround of invoking a
shell script wrapper, where the wrapper then adds --traditional
to the arguments passed to the compiled m4
.
Previous: Incompatibilities, Up: Compatibility [Contents][Index]
There are a few other incompatibilities between this implementation of
m4
, and the System V version.
m4
implements sync lines differently from System V
m4
, when text is being diverted. GNU m4
outputs
the sync lines when the text is being diverted, and System V m4
when the diverted text is being brought back.
The problem is which lines and file names should be attached to text
that is being, or has been, diverted. System V m4
regards all
the diverted text as being generated by the source line containing the
undivert
call, whereas GNU m4
regards the
diverted text as being generated at the time it is diverted.
The sync line option is used mostly when using m4
as
a front end to a compiler. If a diverted line causes a compiler error,
the error messages should most probably refer to the place where the
diversion was made, and not where it was inserted again.
divert(2)2 divert(1)1 divert`'0 ⇒#line 3 "stdin" ⇒0 ^D ⇒#line 2 "stdin" ⇒1 ⇒#line 1 "stdin" ⇒2
The current m4
implementation has a limitation that the syncline
output at the start of each diversion occurs no matter what, even if the
previous diversion did not end with a newline. This goes contrary to
the claim that synclines appear on a line by themselves, so this
limitation may be corrected in a future version of m4
. In the
meantime, when using -s, it is wisest to make sure all
diversions end with newline.
m4
makes no attempt at prohibiting self-referential
definitions like:
define(`x', `x') ⇒ define(`x', `x ') ⇒
There is nothing inherently wrong with defining ‘x’ to
return ‘x’. The wrong thing is to expand ‘x’ unquoted,
because that would cause an infinite rescan loop.
In m4
, one might use macros to hold strings, as we do for
variables in other programming languages, further checking them with:
ifelse(defn(`holder'), `value', …)
In cases like this one, an interdiction for a macro to hold its own name
would be a useless limitation. Of course, this leaves more rope for the
GNU m4
user to hang himself! Rescanning hangs may be
avoided through careful programming, a little like for endless loops in
traditional programming languages.
Next: Copying This Package, Previous: Compatibility, Up: Top [Contents][Index]
Some of the examples in this manuals are buggy or not very robust, for demonstration purposes. Improved versions of these composite macros are presented here.
• Improved exch | Solution for exch
| |
• Improved forloop | Solution for forloop
| |
• Improved foreach | Solution for foreach
| |
• Improved copy | Solution for copy
| |
• Improved m4wrap | Solution for m4wrap
| |
• Improved cleardivert | Solution for cleardivert
| |
• Improved capitalize | Solution for capitalize
| |
• Improved fatal_error | Solution for fatal_error
|
Next: Improved forloop, Up: Answers [Contents][Index]
exch
The exch
macro (see Arguments) as presented requires clients
to double quote their arguments. A nicer definition, which lets
clients follow the rule of thumb of one level of quoting per level of
parentheses, involves adding quotes in the definition of exch
, as
follows:
define(`exch', ``$2', `$1'') ⇒ define(exch(`expansion text', `macro')) ⇒ macro ⇒expansion text
Next: Improved foreach, Previous: Improved exch, Up: Answers [Contents][Index]
forloop
The forloop
macro (see Forloop) as presented earlier can go
into an infinite loop if given an iterator that is not parsed as a macro
name. It does not do any sanity checking on its numeric bounds, and
only permits decimal numbers for bounds. Here is an improved version,
shipped as m4-1.4.19/examples/forloop2.m4; this
version also optimizes overhead by calling four macros instead of six
per iteration (excluding those in text), by not dereferencing the
iterator in the helper _forloop
.
$ m4 -d -I examples undivert(`forloop2.m4')dnl ⇒divert(`-1') ⇒# forloop(var, from, to, stmt) - improved version: ⇒# works even if VAR is not a strict macro name ⇒# performs sanity check that FROM is larger than TO ⇒# allows complex numerical expressions in TO and FROM ⇒define(`forloop', `ifelse(eval(`($2) <= ($3)'), `1', ⇒ `pushdef(`$1')_$0(`$1', eval(`$2'), ⇒ eval(`$3'), `$4')popdef(`$1')')') ⇒define(`_forloop', ⇒ `define(`$1', `$2')$4`'ifelse(`$2', `$3', `', ⇒ `$0(`$1', incr(`$2'), `$3', `$4')')') ⇒divert`'dnl include(`forloop2.m4') ⇒ forloop(`i', `2', `1', `no iteration occurs') ⇒ forloop(`', `1', `2', ` odd iterator name') ⇒ odd iterator name odd iterator name forloop(`i', `5 + 5', `0xc', ` 0x`'eval(i, `16')') ⇒ 0xa 0xb 0xc forloop(`i', `a', `b', `non-numeric bounds') error→m4:stdin:6: bad expression in eval (bad input): (a) <= (b) ⇒
One other change to notice is that the improved version used ‘_$0’
rather than ‘_foreach’ to invoke the helper routine. In general,
this is a good practice to follow, because then the set of macros can be
uniformly transformed. The following example shows a transformation
that doubles the current quoting and appends a suffix ‘2’ to each
transformed macro. If foreach
refers to the literal
‘_foreach’, then foreach2
invokes _foreach
instead of
the intended _foreach2
, and the mixing of quoting paradigms leads
to an infinite recursion loop in this example.
$ m4 -d -L 9 -I examples define(`arg1', `$1')include(`forloop2.m4')include(`quote.m4') ⇒ define(`double', `define(`$1'`2', arg1(patsubst(dquote(defn(`$1')), `[`']', `\&\&')))') ⇒ double(`forloop')double(`_forloop')defn(`forloop2') ⇒ifelse(eval(``($2) <= ($3)''), ``1'', ⇒ ``pushdef(``$1'')_$0(``$1'', eval(``$2''), ⇒ eval(``$3''), ``$4'')popdef(``$1'')'') forloop(i, 1, 5, `ifelse(')forloop(i, 1, 5, `)') ⇒ changequote(`[', `]')changequote([``], ['']) ⇒ forloop2(i, 1, 5, ``ifelse('')forloop2(i, 1, 5, ``)'') ⇒ changequote`'include(`forloop.m4') ⇒ double(`forloop')double(`_forloop')defn(`forloop2') ⇒pushdef(``$1'', ``$2'')_forloop($@)popdef(``$1'') forloop(i, 1, 5, `ifelse(')forloop(i, 1, 5, `)') ⇒ changequote(`[', `]')changequote([``], ['']) ⇒ forloop2(i, 1, 5, ``ifelse('')forloop2(i, 1, 5, ``)'') error→m4:stdin:12: recursion limit of 9 exceeded, use -L<N> to change it
One more optimization is still possible. Instead of repeatedly
assigning a variable then invoking or dereferencing it, it is possible
to pass the current iterator value as a single argument. Coupled with
curry
if other arguments are needed (see Composition), or
with helper macros if the argument is needed in more than one place in
the expansion, the output can be generated with three, rather than four,
macros of overhead per iteration. Notice how the file
m4-1.4.19/examples/forloop3.m4 rearranges the
arguments of the helper _forloop
to take two arguments that are
placed around the current value. By splitting a balanced set of
parantheses across multiple arguments, the helper macro can now be
shared by forloop
and the new forloop_arg
.
$ m4 -I examples include(`forloop3.m4') ⇒ undivert(`forloop3.m4')dnl ⇒divert(`-1') ⇒# forloop_arg(from, to, macro) - invoke MACRO(value) for ⇒# each value between FROM and TO, without define overhead ⇒define(`forloop_arg', `ifelse(eval(`($1) <= ($2)'), `1', ⇒ `_forloop(`$1', eval(`$2'), `$3(', `)')')') ⇒# forloop(var, from, to, stmt) - refactored to share code ⇒define(`forloop', `ifelse(eval(`($2) <= ($3)'), `1', ⇒ `pushdef(`$1')_forloop(eval(`$2'), eval(`$3'), ⇒ `define(`$1',', `)$4')popdef(`$1')')') ⇒define(`_forloop', ⇒ `$3`$1'$4`'ifelse(`$1', `$2', `', ⇒ `$0(incr(`$1'), `$2', `$3', `$4')')') ⇒divert`'dnl forloop(`i', `1', `3', ` i') ⇒ 1 2 3 define(`echo', `$@') ⇒ forloop_arg(`1', `3', ` echo') ⇒ 1 2 3 include(`curry.m4') ⇒ forloop_arg(`1', `3', `curry(`pushdef', `a')') ⇒ a ⇒3 popdef(`a')a ⇒2 popdef(`a')a ⇒1 popdef(`a')a ⇒a
Of course, it is possible to make even more improvements, such as
adding an optional step argument, or allowing iteration through
descending sequences. GNU Autoconf provides some of these
additional bells and whistles in its m4_for
macro.
Next: Improved copy, Previous: Improved forloop, Up: Answers [Contents][Index]
foreach
The foreach
and foreachq
macros (see Foreach) as
presented earlier each have flaws. First, we will examine and fix the
quadratic behavior of foreachq
:
$ m4 -I examples include(`foreachq.m4') ⇒ traceon(`shift')debugmode(`aq') ⇒ foreachq(`x', ``1', `2', `3', `4'', `x ')dnl ⇒1 error→m4trace: -3- shift(`1', `2', `3', `4') error→m4trace: -2- shift(`1', `2', `3', `4') ⇒2 error→m4trace: -4- shift(`1', `2', `3', `4') error→m4trace: -3- shift(`2', `3', `4') error→m4trace: -3- shift(`1', `2', `3', `4') error→m4trace: -2- shift(`2', `3', `4') ⇒3 error→m4trace: -5- shift(`1', `2', `3', `4') error→m4trace: -4- shift(`2', `3', `4') error→m4trace: -3- shift(`3', `4') error→m4trace: -4- shift(`1', `2', `3', `4') error→m4trace: -3- shift(`2', `3', `4') error→m4trace: -2- shift(`3', `4') ⇒4 error→m4trace: -6- shift(`1', `2', `3', `4') error→m4trace: -5- shift(`2', `3', `4') error→m4trace: -4- shift(`3', `4') error→m4trace: -3- shift(`4')
Each successive iteration was adding more quoted shift
invocations, and the entire list contents were passing through every
iteration. In general, when recursing, it is a good idea to make the
recursion use fewer arguments, rather than adding additional quoted
uses of shift
. By doing so, m4
uses less memory, invokes
fewer macros, is less likely to run into machine limits, and most
importantly, performs faster. The fixed version of foreachq
can
be found in m4-1.4.19/examples/foreachq2.m4:
$ m4 -I examples include(`foreachq2.m4') ⇒ undivert(`foreachq2.m4')dnl ⇒include(`quote.m4')dnl ⇒divert(`-1') ⇒# foreachq(x, `item_1, item_2, ..., item_n', stmt) ⇒# quoted list, improved version ⇒define(`foreachq', `pushdef(`$1')_$0($@)popdef(`$1')') ⇒define(`_arg1q', ``$1'') ⇒define(`_rest', `ifelse(`$#', `1', `', `dquote(shift($@))')') ⇒define(`_foreachq', `ifelse(`$2', `', `', ⇒ `define(`$1', _arg1q($2))$3`'$0(`$1', _rest($2), `$3')')') ⇒divert`'dnl traceon(`shift')debugmode(`aq') ⇒ foreachq(`x', ``1', `2', `3', `4'', `x ')dnl ⇒1 error→m4trace: -3- shift(`1', `2', `3', `4') ⇒2 error→m4trace: -3- shift(`2', `3', `4') ⇒3 error→m4trace: -3- shift(`3', `4') ⇒4
Note that the fixed version calls unquoted helper macros in
_foreachq
to trim elements immediately; those helper macros
in turn must re-supply the layer of quotes lost in the macro invocation.
Contrast the use of _arg1q
, which quotes the first list
element, with _arg1
of the earlier implementation that
returned the first list element directly. Additionally, by calling the
helper method immediately, the ‘defn(`iterator')’ no longer
contains unexpanded macros.
The astute m4 programmer might notice that the solution above still uses
more memory and macro invocations, and thus more time, than strictly
necessary. Note that ‘$2’, which contains an arbitrarily long
quoted list, is expanded and rescanned three times per iteration of
_foreachq
. Furthermore, every iteration of the algorithm
effectively unboxes then reboxes the list, which costs a couple of macro
invocations. It is possible to rewrite the algorithm for a bit more
speed by swapping the order of the arguments to _foreachq
in
order to operate on an unboxed list in the first place, and by using the
fixed-length ‘$#’ instead of an arbitrary length list as the key to
end recursion. The result is an overhead of six macro invocations per
loop (excluding any macros in text), instead of eight. This
alternative approach is available as
m4-1.4.19/examples/foreach3.m4:
$ m4 -I examples include(`foreachq3.m4') ⇒ undivert(`foreachq3.m4')dnl ⇒divert(`-1') ⇒# foreachq(x, `item_1, item_2, ..., item_n', stmt) ⇒# quoted list, alternate improved version ⇒define(`foreachq', `ifelse(`$2', `', `', ⇒ `pushdef(`$1')_$0(`$1', `$3', `', $2)popdef(`$1')')') ⇒define(`_foreachq', `ifelse(`$#', `3', `', ⇒ `define(`$1', `$4')$2`'$0(`$1', `$2', ⇒ shift(shift(shift($@))))')') ⇒divert`'dnl traceon(`shift')debugmode(`aq') ⇒ foreachq(`x', ``1', `2', `3', `4'', `x ')dnl ⇒1 error→m4trace: -4- shift(`x', `x error→', `', `1', `2', `3', `4') error→m4trace: -3- shift(`x error→', `', `1', `2', `3', `4') error→m4trace: -2- shift(`', `1', `2', `3', `4') ⇒2 error→m4trace: -4- shift(`x', `x error→', `1', `2', `3', `4') error→m4trace: -3- shift(`x error→', `1', `2', `3', `4') error→m4trace: -2- shift(`1', `2', `3', `4') ⇒3 error→m4trace: -4- shift(`x', `x error→', `2', `3', `4') error→m4trace: -3- shift(`x error→', `2', `3', `4') error→m4trace: -2- shift(`2', `3', `4') ⇒4 error→m4trace: -4- shift(`x', `x error→', `3', `4') error→m4trace: -3- shift(`x error→', `3', `4') error→m4trace: -2- shift(`3', `4')
In the current version of M4, every instance of ‘$@’ is rescanned
as it is encountered. Thus, the foreachq3.m4 alternative uses
much less memory than foreachq2.m4, and executes as much as 10%
faster, since each iteration encounters fewer ‘$@’. However, the
implementation of rescanning every byte in ‘$@’ is quadratic in
the number of bytes scanned (for example, making the broken version in
foreachq.m4 cubic, rather than quadratic, in behavior). A future
release of M4 will improve the underlying implementation by reusing
results of previous scans, so that both styles of foreachq
can
become linear in the number of bytes scanned. Notice how the
implementation injects an empty argument prior to expanding ‘$2’
within foreachq
; the helper macro _foreachq
then ignores
the third argument altogether, and ends recursion when there are three
arguments left because there was nothing left to pass through
shift
. Thus, each iteration only needs one ifelse
, rather
than the two conditionals used in the version from foreachq2.m4.
So far, all of the implementations of foreachq
presented have
been quadratic with M4 1.4.x. But forloop
is linear, because
each iteration parses a constant amount of arguments. So, it is
possible to design a variant that uses forloop
to do the
iteration, then uses ‘$@’ only once at the end, giving a linear
result even with older M4 implementations. This implementation relies
on the GNU extension that ‘$10’ expands to the tenth
argument rather than the first argument concatenated with ‘0’. The
trick is to define an intermediate macro that repeats the text
m4_define(`$1', `$n')$2`'
, with ‘n’ set to successive
integers corresponding to each argument. The helper macro
_foreachq_
is needed in order to generate the literal sequences
such as ‘$1’ into the intermediate macro, rather than expanding
them as the arguments of _foreachq
. With this approach, no
shift
calls are even needed! Even though there are seven macros
of overhead per iteration instead of six in foreachq3.m4, the
linear scaling is apparent at relatively small list sizes. However,
this approach will need adjustment when a future version of M4 follows
POSIX by no longer treating ‘$10’ as the tenth argument;
the anticipation is that ‘${10}’ can be used instead, although
that alternative syntax is not yet supported.
$ m4 -I examples include(`foreachq4.m4') ⇒ undivert(`foreachq4.m4')dnl ⇒include(`forloop2.m4')dnl ⇒divert(`-1') ⇒# foreachq(x, `item_1, item_2, ..., item_n', stmt) ⇒# quoted list, version based on forloop ⇒define(`foreachq', ⇒`ifelse(`$2', `', `', `_$0(`$1', `$3', $2)')') ⇒define(`_foreachq', ⇒`pushdef(`$1', forloop(`$1', `3', `$#', ⇒ `$0_(`1', `2', indir(`$1'))')`popdef( ⇒ `$1')')indir(`$1', $@)') ⇒define(`_foreachq_', ⇒``define(`$$1', `$$3')$$2`''') ⇒divert`'dnl traceon(`shift')debugmode(`aq') ⇒ foreachq(`x', ``1', `2', `3', `4'', `x ')dnl ⇒1 ⇒2 ⇒3 ⇒4
For yet another approach, the improved version of foreach
,
available in m4-1.4.19/examples/foreach2.m4, simply
overquotes the arguments to _foreach
to begin with, using
dquote_elt
. Then _foreach
can just use
_arg1
to remove the extra layer of quoting that was added up
front:
$ m4 -I examples include(`foreach2.m4') ⇒ undivert(`foreach2.m4')dnl ⇒include(`quote.m4')dnl ⇒divert(`-1') ⇒# foreach(x, (item_1, item_2, ..., item_n), stmt) ⇒# parenthesized list, improved version ⇒define(`foreach', `pushdef(`$1')_$0(`$1', ⇒ (dquote(dquote_elt$2)), `$3')popdef(`$1')') ⇒define(`_arg1', `$1') ⇒define(`_foreach', `ifelse(`$2', `(`')', `', ⇒ `define(`$1', _arg1$2)$3`'$0(`$1', (dquote(shift$2)), `$3')')') ⇒divert`'dnl traceon(`shift')debugmode(`aq') ⇒ foreach(`x', `(`1', `2', `3', `4')', `x ')dnl error→m4trace: -4- shift(`1', `2', `3', `4') error→m4trace: -4- shift(`2', `3', `4') error→m4trace: -4- shift(`3', `4') ⇒1 error→m4trace: -3- shift(``1'', ``2'', ``3'', ``4'') ⇒2 error→m4trace: -3- shift(``2'', ``3'', ``4'') ⇒3 error→m4trace: -3- shift(``3'', ``4'') ⇒4 error→m4trace: -3- shift(``4'')
It is likewise possible to write a variant of foreach
that
performs in linear time on M4 1.4.x; the easiest method is probably
writing a version of foreach
that unboxes its list, then invokes
_foreachq
as previously defined in foreachq4.m4.
In summary, recursion over list elements is trickier than it appeared at
first glance, but provides a powerful idiom within m4
processing.
As a final demonstration, both list styles are now able to handle
several scenarios that would wreak havoc on one or both of the original
implementations. This points out one other difference between the
list styles. foreach
evaluates unquoted list elements only once,
in preparation for calling _foreach
, similary for
foreachq
as provided by foreachq3.m4 or
foreachq4.m4. But
foreachq
, as provided by foreachq2.m4,
evaluates unquoted list elements twice while visiting the first list
element, once in _arg1q
and once in _rest
. When
deciding which list style to use, one must take into account whether
repeating the side effects of unquoted list elements will have any
detrimental effects.
$ m4 -I examples include(`foreach2.m4') ⇒ include(`foreachq2.m4') ⇒ dnl 0-element list: foreach(`x', `', `<x>') / foreachq(`x', `', `<x>') ⇒ / dnl 1-element list of empty element foreach(`x', `()', `<x>') / foreachq(`x', ``'', `<x>') ⇒<> / <> dnl 2-element list of empty elements foreach(`x', `(`',`')', `<x>') / foreachq(`x', ``',`'', `<x>') ⇒<><> / <><> dnl 1-element list of a comma foreach(`x', `(`,')', `<x>') / foreachq(`x', ``,'', `<x>') ⇒<,> / <,> dnl 2-element list of unbalanced parentheses foreach(`x', `(`(', `)')', `<x>') / foreachq(`x', ``(', `)'', `<x>') ⇒<(><)> / <(><)> define(`ab', `oops')dnl using defn(`iterator') foreach(`x', `(`a', `b')', `defn(`x')') /dnl foreachq(`x', ``a', `b'', `defn(`x')') ⇒ab / ab define(`active', `ACT, IVE') ⇒ traceon(`active') ⇒ dnl list of unquoted macros; expansion occurs before recursion foreach(`x', `(active, active)', `<x> ')dnl error→m4trace: -4- active -> `ACT, IVE' error→m4trace: -4- active -> `ACT, IVE' ⇒<ACT> ⇒<IVE> ⇒<ACT> ⇒<IVE> foreachq(`x', `active, active', `<x> ')dnl error→m4trace: -3- active -> `ACT, IVE' error→m4trace: -3- active -> `ACT, IVE' ⇒<ACT> error→m4trace: -3- active -> `ACT, IVE' error→m4trace: -3- active -> `ACT, IVE' ⇒<IVE> ⇒<ACT> ⇒<IVE> dnl list of quoted macros; expansion occurs during recursion foreach(`x', `(`active', `active')', `<x> ')dnl error→m4trace: -1- active -> `ACT, IVE' ⇒<ACT, IVE> error→m4trace: -1- active -> `ACT, IVE' ⇒<ACT, IVE> foreachq(`x', ``active', `active'', `<x> ')dnl error→m4trace: -1- active -> `ACT, IVE' ⇒<ACT, IVE> error→m4trace: -1- active -> `ACT, IVE' ⇒<ACT, IVE> dnl list of double-quoted macro names; no expansion foreach(`x', `(``active'', ``active'')', `<x> ')dnl ⇒<active> ⇒<active> foreachq(`x', ```active'', ``active''', `<x> ')dnl ⇒<active> ⇒<active>
Next: Improved m4wrap, Previous: Improved foreach, Up: Answers [Contents][Index]
copy
The macro copy
presented above
is unable to handle builtin tokens with M4 1.4.x, because it tries to
pass the builtin token through the macro curry
, where it is
silently flattened to an empty string (see Composition). Rather
than using the problematic curry
to work around the limitation
that stack_foreach
expects to invoke a macro that takes exactly
one argument, we can write a new macro that lets us form the exact
two-argument pushdef
call sequence needed, so that we are no
longer passing a builtin token through a text macro.
For each of the pushdef
definitions associated with macro,
expand the sequence ‘pre`'definition`'post’.
Additionally, expand sep between definitions.
stack_foreach_sep
visits the oldest definition first, while
stack_foreach_sep_lifo
visits the current definition first. The
expansion may dereference macro, but should not modify it. There
are a few special macros, such as defn
, which cannot be used as
the macro parameter.
Note that stack_foreach(`macro', `action')
is
equivalent to stack_foreach_sep(`macro', `action(',
`)')
. By supplying explicit parentheses, split among the pre and
post arguments to stack_foreach_sep
, it is now possible to
construct macro calls with more than one argument, without passing
builtin tokens through a macro call. It is likewise possible to
directly reference the stack definitions without a macro call, by
leaving pre and post empty. Thus, in addition to fixing
copy
on builtin tokens, it also executes with fewer macro
invocations.
The new macro also adds a separator that is only output after the first
iteration of the helper _stack_reverse_sep
, implemented by
prepending the original sep to pre and omitting a sep
argument in subsequent iterations. Note that the empty string that
separates sep from pre is provided as part of the fourth
argument when originally calling _stack_reverse_sep
, and not by
writing $4`'$3
as the third argument in the recursive call; while
the other approach would give the same output, it does so at the expense
of increasing the argument size on each iteration of
_stack_reverse_sep
, which results in quadratic instead of linear
execution time. The improved stack walking macros are available in
m4-1.4.19/examples/stack_sep.m4:
$ m4 -I examples include(`stack_sep.m4') ⇒ define(`copy', `ifdef(`$2', `errprint(`$2 already defined ')m4exit(`1')', `stack_foreach_sep(`$1', `pushdef(`$2',', `)')')')dnl pushdef(`a', `1')pushdef(`a', defn(`divnum')) ⇒ copy(`a', `b') ⇒ b ⇒0 popdef(`b') ⇒ b ⇒1 pushdef(`c', `1')pushdef(`c', `2') ⇒ stack_foreach_sep_lifo(`c', `', `', `, ') ⇒2, 1 undivert(`stack_sep.m4')dnl ⇒divert(`-1') ⇒# stack_foreach_sep(macro, pre, post, sep) ⇒# Invoke PRE`'defn`'POST with a single argument of each definition ⇒# from the definition stack of MACRO, starting with the oldest, and ⇒# separated by SEP between definitions. ⇒define(`stack_foreach_sep', ⇒`_stack_reverse_sep(`$1', `tmp-$1')'dnl ⇒`_stack_reverse_sep(`tmp-$1', `$1', `$2`'defn(`$1')$3', `$4`'')') ⇒# stack_foreach_sep_lifo(macro, pre, post, sep) ⇒# Like stack_foreach_sep, but starting with the newest definition. ⇒define(`stack_foreach_sep_lifo', ⇒`_stack_reverse_sep(`$1', `tmp-$1', `$2`'defn(`$1')$3', `$4`'')'dnl ⇒`_stack_reverse_sep(`tmp-$1', `$1')') ⇒define(`_stack_reverse_sep', ⇒`ifdef(`$1', `pushdef(`$2', defn(`$1'))$3`'popdef(`$1')$0( ⇒ `$1', `$2', `$4$3')')') ⇒divert`'dnl
Next: Improved cleardivert, Previous: Improved copy, Up: Answers [Contents][Index]
m4wrap
The replacement m4wrap
versions presented above, designed to
guarantee FIFO or LIFO order regardless of the underlying M4
implementation, share a bug when dealing with wrapped text that looks
like parameter expansion. Note how the invocation of
m4wrapn
interprets these parameters, while using the
builtin preserves them for their intended use.
$ m4 -I examples include(`wraplifo.m4') ⇒ m4wrap(`define(`foo', ``$0:'-$1-$*-$#-')foo(`a', `b') ') ⇒ builtin(`m4wrap', ``'define(`bar', ``$0:'-$1-$*-$#-')bar(`a', `b') ') ⇒ ^D ⇒bar:-a-a,b-2- ⇒m4wrap0:---0-
Additionally, the computation of _m4wrap_level
and creation of
multiple m4wrapn
placeholders in the original examples is
more expensive in time and memory than strictly necessary. Notice how
the improved version grabs the wrapped text via defn
to avoid
parameter expansion, then undefines _m4wrap_text
, before
stripping a level of quotes with _arg1
to expand the text. That
way, each level of wrapping reuses the single placeholder, which starts
each nesting level in an undefined state.
Finally, it is worth emulating the GNU M4 extension of saving
all arguments to m4wrap
, separated by a space, rather than saving
just the first argument. This is done with the join
macro
documented previously (see Shift). The improved LIFO example is
shipped as m4-1.4.19/examples/wraplifo2.m4, and can
easily be converted to a FIFO solution by swapping the adjacent
invocations of joinall
and defn
.
$ m4 -I examples include(`wraplifo2.m4') ⇒ undivert(`wraplifo2.m4')dnl ⇒dnl Redefine m4wrap to have LIFO semantics, improved example. ⇒include(`join.m4')dnl ⇒define(`_m4wrap', defn(`m4wrap'))dnl ⇒define(`_arg1', `$1')dnl ⇒define(`m4wrap', ⇒`ifdef(`_$0_text', ⇒ `define(`_$0_text', joinall(` ', $@)defn(`_$0_text'))', ⇒ `_$0(`_arg1(defn(`_$0_text')undefine(`_$0_text'))')dnl ⇒define(`_$0_text', joinall(` ', $@))')')dnl m4wrap(`define(`foo', ``$0:'-$1-$*-$#-')foo(`a', `b') ') ⇒ m4wrap(`lifo text m4wrap(`nested', `', `$@ ')') ⇒ ^D ⇒lifo text ⇒foo:-a-a,b-2- ⇒nested $@
Next: Improved capitalize, Previous: Improved m4wrap, Up: Answers [Contents][Index]
cleardivert
The cleardivert
macro (see Cleardivert) cannot, as it stands, be
called without arguments to clear all pending diversions. That is
because using undivert with an empty string for an argument is different
than using it with no arguments at all. Compare the earlier definition
with one that takes the number of arguments into account:
define(`cleardivert', `pushdef(`_n', divnum)divert(`-1')undivert($@)divert(_n)popdef(`_n')') ⇒ divert(`1')one divert ⇒ cleardivert ⇒ undivert ⇒one ⇒ define(`cleardivert', `pushdef(`_num', divnum)divert(`-1')ifelse(`$#', `0', `undivert`'', `undivert($@)')divert(_num)popdef(`_num')') ⇒ divert(`2')two divert ⇒ cleardivert ⇒ undivert ⇒
Next: Improved fatal_error, Previous: Improved cleardivert, Up: Answers [Contents][Index]
capitalize
The capitalize
macro (see Patsubst) as presented earlier does
not allow clients to follow the quoting rule of thumb. Consider the
three macros active
, Active
, and ACTIVE
, and the
difference between calling capitalize
with the expansion of a
macro, expanding the result of a case change, and changing the case of a
double-quoted string:
$ m4 -I examples include(`capitalize.m4')dnl define(`active', `act1, ive')dnl define(`Active', `Act2, Ive')dnl define(`ACTIVE', `ACT3, IVE')dnl upcase(active) ⇒ACT1,IVE upcase(`active') ⇒ACT3, IVE upcase(``active'') ⇒ACTIVE downcase(ACTIVE) ⇒act3,ive downcase(`ACTIVE') ⇒act1, ive downcase(``ACTIVE'') ⇒active capitalize(active) ⇒Act1 capitalize(`active') ⇒Active capitalize(``active'') ⇒_capitalize(`active') define(`A', `OOPS') ⇒ capitalize(active) ⇒OOPSct1 capitalize(`active') ⇒OOPSctive
First, when capitalize
is called with more than one argument, it
was throwing away later arguments, whereas upcase
and
downcase
used ‘$*’ to collect them all. The fix is simple:
use ‘$*’ consistently.
Next, with single-quoting, capitalize
outputs a single character,
a set of quotes, then the rest of the characters, making it impossible
to invoke Active
after the fact, and allowing the alternate macro
A
to interfere. Here, the solution is to use additional quoting
in the helper macros, then pass the final over-quoted output string
through _arg1
to remove the extra quoting and finally invoke the
concatenated portions as a single string.
Finally, when passed a double-quoted string, the nested macro
_capitalize
is never invoked because it ended up nested inside
quotes. This one is the toughest to fix. In short, we have no idea how
many levels of quotes are in effect on the substring being altered by
patsubst
. If the replacement string cannot be expressed entirely
in terms of literal text and backslash substitutions, then we need a
mechanism to guarantee that the helper macros are invoked outside of
quotes. In other words, this sounds like a job for changequote
(see Changequote). By changing the active quoting characters, we
can guarantee that replacement text injected by patsubst
always
occurs in the middle of a string that has exactly one level of
over-quoting using alternate quotes; so the replacement text closes the
quoted string, invokes the helper macros, then reopens the quoted
string. In turn, that means the replacement text has unbalanced quotes,
necessitating another round of changequote
.
In the fixed version below, (also shipped as
m4-1.4.19/examples/capitalize2.m4), capitalize
uses the alternate quotes of ‘<<[’ and ‘]>>’ (the longer
strings are chosen so as to be less likely to appear in the text being
converted). The helpers _to_alt
and _from_alt
merely
reduce the number of characters required to perform a
changequote
, since the definition changes twice. The outermost
pair means that patsubst
and _capitalize_alt
are invoked
with alternate quoting; the innermost pair is used so that the third
argument to patsubst
can contain an unbalanced
‘]>>’/‘<<[’ pair. Note that upcase
and downcase
must be redefined as _upcase_alt
and _downcase_alt
, since
they contain nested quotes but are invoked with the alternate quoting
scheme in effect.
$ m4 -I examples include(`capitalize2.m4')dnl define(`active', `act1, ive')dnl define(`Active', `Act2, Ive')dnl define(`ACTIVE', `ACT3, IVE')dnl define(`A', `OOPS')dnl capitalize(active; `active'; ``active''; ```actIVE''') ⇒Act1,Ive; Act2, Ive; Active; `Active' undivert(`capitalize2.m4')dnl ⇒divert(`-1') ⇒# upcase(text) ⇒# downcase(text) ⇒# capitalize(text) ⇒# change case of text, improved version ⇒define(`upcase', `translit(`$*', `a-z', `A-Z')') ⇒define(`downcase', `translit(`$*', `A-Z', `a-z')') ⇒define(`_arg1', `$1') ⇒define(`_to_alt', `changequote(`<<[', `]>>')') ⇒define(`_from_alt', `changequote(<<[`]>>, <<[']>>)') ⇒define(`_upcase_alt', `translit(<<[$*]>>, <<[a-z]>>, <<[A-Z]>>)') ⇒define(`_downcase_alt', `translit(<<[$*]>>, <<[A-Z]>>, <<[a-z]>>)') ⇒define(`_capitalize_alt', ⇒ `regexp(<<[$1]>>, <<[^\(\w\)\(\w*\)]>>, ⇒ <<[_upcase_alt(<<[<<[\1]>>]>>)_downcase_alt(<<[<<[\2]>>]>>)]>>)') ⇒define(`capitalize', ⇒ `_arg1(_to_alt()patsubst(<<[<<[$*]>>]>>, <<[\w+]>>, ⇒ _from_alt()`]>>_$0_alt(<<[\&]>>)<<['_to_alt())_from_alt())') ⇒divert`'dnl
Previous: Improved capitalize, Up: Answers [Contents][Index]
fatal_error
The fatal_error
macro (see M4exit) is not robust to versions
of GNU M4 earlier than 1.4.8, where invoking
__file__
(see Location) inside m4wrap
would result
in an empty string, and __line__
resulted in ‘0’ even
though all files start at line 1. Furthermore, versions earlier than
1.4.6 did not support the __program__
macro. If you want
fatal_error
to work across the entire 1.4.x release series, a
better implementation would be:
define(`fatal_error', `errprint(ifdef(`__program__', `__program__', ``m4'')'dnl `:ifelse(__line__, `0', `', `__file__:__line__:')` fatal error: $* ')m4exit(`1')') ⇒ m4wrap(`divnum(`demo of internal message') fatal_error(`inside wrapped text')') ⇒ ^D error→m4:stdin:6: Warning: excess arguments to builtin `divnum' ignored ⇒0 error→m4:stdin:6: fatal error: inside wrapped text
Next: Copying This Manual, Previous: Answers, Up: Top [Contents][Index]
This appendix covers the license for copying the source code of the overall M4 package. This manual is under a different set of restrictions, covered later (see Copying This Manual).
• GNU General Public License | License for copying the M4 package |
Up: Copying This Package [Contents][Index]
Copyright © 2007 Free Software Foundation, Inc. https://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
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Nothing in this License shall be construed as excluding or limiting any implied license or other defenses to infringement that may otherwise be available to you under applicable patent law.
If conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot convey a covered work so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not convey it at all. For example, if you agree to terms that obligate you to collect a royalty for further conveying from those to whom you convey the Program, the only way you could satisfy both those terms and this License would be to refrain entirely from conveying the Program.
Notwithstanding any other provision of this License, you have permission to link or combine any covered work with a work licensed under version 3 of the GNU Affero General Public License into a single combined work, and to convey the resulting work. The terms of this License will continue to apply to the part which is the covered work, but the special requirements of the GNU Affero General Public License, section 13, concerning interaction through a network will apply to the combination as such.
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Each version is given a distinguishing version number. If the Program specifies that a certain numbered version of the GNU General Public License “or any later version” applies to it, you have the option of following the terms and conditions either of that numbered version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of the GNU General Public License, you may choose any version ever published by the Free Software Foundation.
If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Program.
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THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee.
If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively state the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does. Copyright (C) year name of author This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.
Also add information on how to contact you by electronic and paper mail.
If the program does terminal interaction, make it output a short notice like this when it starts in an interactive mode:
program Copyright (C) year name of author This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’. This is free software, and you are welcome to redistribute it under certain conditions; type ‘show c’ for details.
The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General Public License. Of course, your program’s commands might be different; for a GUI interface, you would use an “about box”.
You should also get your employer (if you work as a programmer) or school, if any, to sign a “copyright disclaimer” for the program, if necessary. For more information on this, and how to apply and follow the GNU GPL, see https://www.gnu.org/licenses/.
The GNU General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Lesser General Public License instead of this License. But first, please read https://www.gnu.org/licenses/why-not-lgpl.html.
Next: Indices, Previous: Copying This Package, Up: Top [Contents][Index]
This appendix covers the license for copying this manual. Note that some of the longer examples in this manual are also distributed in the directory m4-1.4.19/examples/, where a more permissive license is in effect when copying just the examples.
• GNU Free Documentation License | License for copying this manual |
Up: Copying This Manual [Contents][Index]
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. https://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
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The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.
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The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding the beginning of the body of the text.
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The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
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If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
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You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
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You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.
You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
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If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
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The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See https://www.gnu.org/licenses/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:
with the Invariant Sections being list their titles, with the Front-Cover Texts being list, and with the Back-Cover Texts being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
Previous: Copying This Manual, Up: Top [Contents][Index]
• Macro index | Index for all m4 macros
| |
• Concept index | Index for many concepts |
Next: Concept index, Up: Indices [Contents][Index]
m4
macrosThis index covers all m4
builtins, as well as several useful
composite macros. References are exclusively to the places where a
macro is introduced the first time.
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Derived from a patch in https://lists.gnu.org/archive/html/bug-gnulib/2007-01/msg00389.html, and a followup patch in https://lists.gnu.org/archive/html/bug-gnulib/2007-02/msg00000.html