Some output variables are preset by the Autoconf macros. Some of the
Autoconf macros set additional output variables, which are mentioned in
the descriptions for those macros. See Output Variable Index, for a
complete list of output variables. See Installation Directory Variables, for the list of the preset ones related to installation
directories. Below are listed the other preset ones, many of which are
precious variables (see Setting Output Variables,
AC_ARG_VAR).
The preset variables which are available during config.status
(see Performing Configuration Actions) may also be used during
configure tests. For example, it is permissible to reference
‘$srcdir’ when constructing a list of directories to pass via
the -I option during a compiler feature check. When used in this
manner, coupled with the fact that configure is always run
from the top build directory, it is sufficient to use just
‘$srcdir’ instead of ‘$top_srcdir’.
Debugging and optimization options for the C compiler. If it is not set
in the environment when configure runs, the default value is set
when you call AC_PROG_CC (or empty if you don’t). configure
uses this variable when compiling or linking programs to test for C features.
If a compiler option affects only the behavior of the preprocessor
(e.g., -Dname), it should be put into CPPFLAGS
instead. If it affects only the linker (e.g., -Ldirectory),
it should be put into LDFLAGS instead. If it
affects only the compiler proper, CFLAGS is the natural home for
it. If an option affects multiple phases of the compiler, though,
matters get tricky:
CC, e.g., CC='gcc -m64'. This is
necessary for config.guess to work right.
CC. Another is
to put it into both CPPFLAGS and LDFLAGS, but not into
CFLAGS.
However, remember that some Makefile variables are reserved by
the GNU Coding Standards for the use of the “user”—the person
building the package. For instance, CFLAGS is one such variable.
Sometimes package developers are tempted to set user variables such as
CFLAGS because it appears to make their job easier. However, the
package itself should never set a user variable, particularly not to
include switches that are required for proper compilation of the
package. Since these variables are documented as being for the package
builder, that person rightfully expects to be able to override any of
these variables at build time. If the package developer needs to add
switches without interfering with the user, the proper way to do that is
to introduce an additional variable. Automake makes this easy by
introducing AM_CFLAGS (see Flag Variables Ordering in GNU Automake), but the concept is the same even if
Automake is not used.
A comment saying that the file was generated automatically by
configure and giving the name of the input file.
AC_OUTPUT adds a comment line containing this variable to the top
of every makefile it creates. For other files, you should
reference this variable in a comment at the top of each input file. For
example, an input shell script should begin like this:
#!/bin/sh # @configure_input@
The presence of that line also reminds people editing the file that it
needs to be processed by configure in order to be used.
Preprocessor options for the C, C++, Objective C, and Objective C++
preprocessors and compilers. If
it is not set in the environment when configure runs, the default
value is empty. configure uses this variable when preprocessing
or compiling programs to test for C, C++, Objective C, and Objective C++
features.
This variable’s contents should contain options like -I,
-D, and -U that affect only the behavior of the
preprocessor. Please see the explanation of CFLAGS for what you
can do if an option affects other phases of the compiler as well.
Currently, configure always links as part of a single
invocation of the compiler that also preprocesses and compiles, so it
uses this variable also when linking programs. However, it is unwise to
depend on this behavior because the GNU Coding Standards do
not require it and many packages do not use CPPFLAGS when linking
programs.
See Special Characters in Output Variables, for limitations that CPPFLAGS
might run into.
Debugging and optimization options for the C++ compiler. It acts like
CFLAGS, but for C++ instead of C.
-D options to pass to the C compiler. If AC_CONFIG_HEADERS
is called, configure replaces ‘@DEFS@’ with
-DHAVE_CONFIG_H instead (see Configuration Header Files). This
variable is not defined while configure is performing its tests,
only when creating the output files. See Setting Output Variables, for
how to check the results of previous tests.
How does one suppress the trailing newline from echo for
question-answer message pairs? These variables provide a way:
echo $ECHO_N "And the winner is... $ECHO_C"
sleep 100000000000
echo "${ECHO_T}dead."
Some old and uncommon echo implementations offer no means to
achieve this, in which case ECHO_T is set to tab. You might not
want to use it.
Debugging and optimization options for the Erlang compiler. If it is not set
in the environment when configure runs, the default value is empty.
configure uses this variable when compiling
programs to test for Erlang features.
Debugging and optimization options for the Fortran compiler. If it
is not set in the environment when configure runs, the default
value is set when you call AC_PROG_FC (or empty if you don’t).
configure uses this variable when compiling or linking
programs to test for Fortran features.
Debugging and optimization options for the Fortran 77 compiler. If it
is not set in the environment when configure runs, the default
value is set when you call AC_PROG_F77 (or empty if you don’t).
configure uses this variable when compiling or linking
programs to test for Fortran 77 features.
Options for the linker. If it is not set
in the environment when configure runs, the default value is empty.
configure uses this variable when linking programs to test for
C, C++, Objective C, Objective C++, Fortran, and Go features.
This variable’s contents should contain options like -s and
-L that affect only the behavior of the linker. Please see the
explanation of CFLAGS for what you can do if an option also
affects other phases of the compiler.
Don’t use this variable to pass library names
(-l) to the linker; use LIBS instead.
-l options to pass to the linker. The default value is empty,
but some Autoconf macros may prepend extra libraries to this variable if
those libraries are found and provide necessary functions, see
Library Files. configure uses this variable when linking
programs to test for C, C++, Objective C, Objective C++, Fortran, and Go
features.
Debugging and optimization options for the Objective C compiler. It
acts like CFLAGS, but for Objective C instead of C.
Debugging and optimization options for the Objective C++ compiler. It
acts like CXXFLAGS, but for Objective C++ instead of C++.
Debugging and optimization options for the Go compiler. It acts like
CFLAGS, but for Go instead of C.
Rigorously equal to ‘.’. Added for symmetry only.
Absolute name of builddir.
The relative name of the top level of the current build tree. In the
top-level directory, this is the same as builddir.
The relative name of the top level of the current build tree with final
slash if nonempty. This is the same as top_builddir, except that
it contains zero or more runs of ../, so it should not be
appended with a slash for concatenation. This helps for make
implementations that otherwise do not treat ./file and file
as equal in the top-level build directory.
Absolute name of top_builddir.
The name of the directory that contains the source code for that makefile.
Absolute name of srcdir.
The name of the top-level source code directory for the
package. In the top-level directory, this is the same as srcdir.
Absolute name of top_srcdir.