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4.8.1 Preset Output Variables

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 section Output Variable Index, for a complete list of output variables. See section 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 section Setting Output Variables, AC_ARG_VAR).

The preset variables which are available during ‘config.status’ (see section Performing Configuration Actions) may also be used during configure tests. For example, it is permissible to reference ‘$srcdir’ when constructing a list of directories to pass via option ‘-I’ during a compiler feature check. When used in this manner, coupled with the fact that configure is always run from the top build directory, it is sufficient to use just ‘$srcdir’ instead of ‘$top_srcdir’.

Variable: CFLAGS

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. One approach to put such options directly into CC, e.g., CC='gcc -m64'. Another is to put them 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.

Variable: configure_input

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.

Variable: CPPFLAGS

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 section Special Characters in Output Variables, for limitations that CPPFLAGS might run into.

Variable: CXXFLAGS

Debugging and optimization options for the C++ compiler. It acts like CFLAGS, but for C++ instead of C.

Variable: DEFS

-D’ options to pass to the C compiler. If AC_CONFIG_HEADERS is called, configure replaces ‘@DEFS@’ with ‘-DHAVE_CONFIG_H’ instead (see section Configuration Header Files). This variable is not defined while configure is performing its tests, only when creating the output files. See section Setting Output Variables, for how to check the results of previous tests.

Variable: ECHO_C
Variable: ECHO_N
Variable: ECHO_T

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.

Variable: ERLCFLAGS

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.

Variable: FCFLAGS

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.

Variable: FFLAGS

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.

Variable: LDFLAGS

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.

Variable: LIBS

-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.

Variable: OBJCFLAGS

Debugging and optimization options for the Objective C compiler. It acts like CFLAGS, but for Objective C instead of C.

Variable: OBJCXXFLAGS

Debugging and optimization options for the Objective C++ compiler. It acts like CXXFLAGS, but for Objective C++ instead of C++.

Variable: GOFLAGS

Debugging and optimization options for the Go compiler. It acts like CFLAGS, but for Go instead of C.

Variable: builddir

Rigorously equal to ‘.’. Added for symmetry only.

Variable: abs_builddir

Absolute name of builddir.

Variable: top_builddir

The relative name of the top level of the current build tree. In the top-level directory, this is the same as builddir.

Variable: top_build_prefix

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 toplevel build directory.

Variable: abs_top_builddir

Absolute name of top_builddir.

Variable: srcdir

The name of the directory that contains the source code for that makefile.

Variable: abs_srcdir

Absolute name of srcdir.

Variable: top_srcdir

The name of the top-level source code directory for the package. In the top-level directory, this is the same as srcdir.

Variable: abs_top_srcdir

Absolute name of top_srcdir.


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