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6.3 Running the Preprocessor

Sometimes one might need to run the preprocessor on some source file. Usually it is a bad idea, as you typically need to compile your project, not merely run the preprocessor on it; therefore you certainly want to run the compiler, not the preprocessor. Resist to the temptation of following the easiest path.

Nevertheless, if you need to run the preprocessor, then use AC_PREPROC_IFELSE.

Macro: AC_PREPROC_IFELSE (input, [action-if-true], [action-if-false])
Run the preprocessor of the current language (see section 6.1 Language Choice) on the input, run the shell commands action-if-true on success, action-if-false otherwise. The input can be made by AC_LANG_PROGRAM and friends.

This macro uses CPPFLAGS, but not CFLAGS, because `-g', `-O', etc. are not valid options to many C preprocessors.

It is customary to report unexpected failures with AC_MSG_FAILURE.

For instance:

 
AC_INIT(Autoconf Documentation, 2.57, bug-autoconf@gnu.org)
AC_DEFINE([HELLO_WORLD], ["Hello, World\n"])
AC_PREPROC_IFELSE(
   [AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]],
                    [[fputs (hw, stdout);]])],
   [AC_MSG_RESULT([OK])],
   [AC_MSG_FAILURE([unexpected preprocessor failure])])

results in:

 
checking for gcc... gcc
checking for C compiler default output... a.out
checking whether the C compiler works... yes
checking whether we are cross compiling... no
checking for suffix of executables...
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ANSI C... none needed
checking how to run the C preprocessor... gcc -E
OK

The macro AC_TRY_CPP (see section 15.4 Obsolete Macros) used to play the role of AC_PREPROC_IFELSE, but double quotes its argument, making it impossible to use it to ellaborate sources. You are encouraged to get rid of your old use of the macro AC_TRY_CPP in favor of AC_PREPROC_IFELSE, but, in the first place, are you sure you need to run the preprocessor and not the compiler?

Macro: AC_EGREP_HEADER (pattern, header-file, action-if-found, [action-if-not-found])
If the output of running the preprocessor on the system header file header-file matches the extended regular expression pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.

Macro: AC_EGREP_CPP (pattern, program, [action-if-found], [action-if-not-found])
program is the text of a C or C++ program, on which shell variable, back quote, and backslash substitutions are performed. If the output of running the preprocessor on program matches the extended regular expression pattern, execute shell commands action-if-found, otherwise execute action-if-not-found.


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