Another way to record the results of tests is to set output
variables, which are shell variables whose values are substituted into
files that configure
outputs. The two macros below create new
output variables. See Preset Output Variables, for a list of output
variables that are always available.
Create an output variable from a shell variable. Make AC_OUTPUT
substitute the variable variable into output files (typically one
or more makefiles). This means that AC_OUTPUT
replaces instances of ‘@variable@’ in input files with the
value that the shell variable variable has when AC_OUTPUT
is called. The value can contain any non-NUL
character, including
newline. If you are using Automake 1.11 or newer, for newlines in values
you might want to consider using AM_SUBST_NOTMAKE
to prevent
automake
from adding a line variable =
@variable@
to the Makefile.in files (see Automake in Other things Automake recognizes).
Variable occurrences should not overlap: e.g., an input file should
not contain ‘@var1@var2@’ if var1 and var2
are variable names.
The substituted value is not rescanned for more output variables;
occurrences of ‘@variable@’ in the value are inserted
literally into the output file. (The algorithm uses the special marker
|#_!!_#|
internally, so neither the substituted value nor the
output file may contain |#_!!_#|
.)
If value is given, in addition assign it to variable.
The string variable is passed to m4_pattern_allow
(see Forbidden Patterns). variable is not further expanded,
even if there is another macro by the same name.
Another way to create an output variable from a shell variable. Make
AC_OUTPUT
insert (without substitutions) the contents of the file
named by shell variable variable into output files. This means
that AC_OUTPUT
replaces instances of
‘@variable@’ in output files (such as Makefile.in)
with the contents of the file that the shell variable variable
names when AC_OUTPUT
is called. Set the variable to
/dev/null for cases that do not have a file to insert.
This substitution occurs only when the ‘@variable@’ is on a
line by itself, optionally surrounded by spaces and tabs. The
substitution replaces the whole line, including the spaces, tabs, and
the terminating newline.
This macro is useful for inserting makefile fragments containing
special dependencies or other make
directives for particular host
or target types into makefiles. For example, configure.ac
could contain:
AC_SUBST_FILE([host_frag]) host_frag=$srcdir/conf/sun4.mh
and then a Makefile.in could contain:
@host_frag@
The string variable is passed to m4_pattern_allow
(see Forbidden Patterns).
Running configure
in varying environments can be extremely
dangerous. If for instance the user runs ‘CC=bizarre-cc
./configure’, then the cache, config.h, and many other output
files depend upon bizarre-cc
being the C compiler. If
for some reason the user runs ./configure
again, or if it is
run via ‘./config.status --recheck’, (See Automatic Remaking,
and see config.status Invocation), then the configuration can be
inconsistent, composed of results depending upon two different
compilers.
Environment variables that affect this situation, such as ‘CC’
above, are called precious variables, and can be declared as such
by AC_ARG_VAR
.
Declare variable is a precious variable, and include its description in the variable section of ‘./configure --help’.
Being precious means that
AC_SUBST
.
configure
was launched is
saved in the cache, including if it was not specified on the command
line but via the environment. Indeed, while configure
can
notice the definition of CC
in ‘./configure CC=bizarre-cc’,
it is impossible to notice it in ‘CC=bizarre-cc ./configure’,
which, unfortunately, is what most users do.
We emphasize that it is the initial value of variable which
is saved, not that found during the execution of configure
.
Indeed, specifying ‘./configure FOO=foo’ and letting
‘./configure’ guess that FOO
is foo
can be two
different things.
configure
runs. For instance:
$ ./configure --silent --config-cache $ CC=cc ./configure --silent --config-cache configure: error: 'CC' was not set in the previous run configure: error: changes in the environment can compromise \ the build configure: error: run 'make distclean' and/or \ 'rm config.cache' and start over
and similarly if the variable is unset, or if its content is changed. If the content has white space changes only, then the error is degraded to a warning only, but the old value is reused.
$ CC=/usr/bin/cc ./configure var=raboof --silent $ ./config.status --recheck running CONFIG_SHELL=/bin/sh /bin/sh ./configure var=raboof \ CC=/usr/bin/cc --no-create --no-recursion