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4.7.1 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 B.2 Output Variable Index, for a complete list of output variables. See section 4.7.2 Installation Directory Variables, for the list of the preset ones related to installation directories. Below are listed the other preset ones. They all are precious variables (see section 7.2 Setting Output Variables, AC_ARG_VAR).

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 programs to test for C features.

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
Header file search directory (`-Idir') and any other miscellaneous options for the C and 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 compiling or preprocessing programs to test for C and C++ features.

Variable: CXXFLAGS
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_CXX (or empty if you don't). configure uses this variable when compiling programs to test for C++ features.

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 4.8 Configuration Header Files). This variable is not defined while configure is performing its tests, only when creating the output files. See section 7.2 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: 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 programs to test for Fortran 77 features.

Variable: LDFLAGS
Stripping (`-s'), path (`-L'), and any other miscellaneous options for the linker. Don't use this variable to pass library names (`-l') to the linker, use LIBS instead. 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++ and Fortran 77 features.

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 5.4 Library Files. configure uses this variable when linking programs to test for C, C++ and Fortran 77 features.

Variable: builddir
Rigorously equal to `.'. Added for symmetry only.

Variable: abs_builddir
Absolute path of builddir.

Variable: top_builddir
The relative path to the top-level of the current build tree. In the top-level directory, this is the same as builddir.

Variable: abs_top_builddir
Absolute path of top_builddir.

Variable: srcdir
The relative path to the directory that contains the source code for that `Makefile'.

Variable: abs_srcdir
Absolute path of srcdir.

Variable: top_srcdir
The relative path to 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 path of top_srcdir.


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