Where to add a CFLAG, such as -std=gnu99, into an autotools project

Question

I have a simple Autotools C project (not C++).

CFLAGs (by inspection) seem to be -g -O2.

I want all of the generated make files to also have -std=gnu99 appended to the CFLAGs, because I use for (int i = 0; i < MAX; i++) and similar.

I can obviously hack the Makefile, but this gets overwritten on ./configure.

Where is the correct place to add (or change) CFLAGs which are required by the code (as opposed to those CFLAGs which the user might want to change)?

(Note this is partial duplicate of Where to add a CFLAG, such as -std=gnu99, into an (Eclipse CDT) autotools project as I was getting Eclipse-specific answers which I didn't want.)

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@DevSolar's answer has not helped yet. A configure.ac file (below) generates the configure script (also below).

configure.ac:

dnl Process this file with autoconf to produce a configure script.  CFLAGS="$CFLAGS -std=gnu99" AC_PREREQ(2.59) AC_INIT(tuntest, 1.0)   AC_CANONICAL_SYSTEM AM_INIT_AUTOMAKE()  AC_PROG_CC  AC_CONFIG_FILES(Makefile src/Makefile) AC_OUTPUT 

$ grep CFLAGS configure

CFLAGS CFLAGS To assign environment variables (e.g., CC, CFLAGS...), specify them as   CFLAGS      C compiler flags ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5' ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5' ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5' ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5' ac_test_CFLAGS=${CFLAGS+set} ac_save_CFLAGS=$CFLAGS    CFLAGS="-g"   CFLAGS=""      CFLAGS="-g" if test "$ac_test_CFLAGS" = set; then   CFLAGS=$ac_save_CFLAGS     CFLAGS="-g -O2"     CFLAGS="-g"     CFLAGS="-O2"     CFLAGS= ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5' ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5' 

Answer 1

autoconf has a macro for this:

Just put:

AC_PROG_CC_STDC 

after your AC_PROG_CC and everything will be right.

Especially when you use other compilers that do not have -std=gnu99 but operate in C99 mode by default (or have a different option hpcc's -AC99 springs to mind).

I would NOT use CFLAGS for that kind of thing.

From the docs:

-- Macro: AC_PROG_CC_STDC If the C compiler cannot compile ISO Standard C (currently C99), try to add an option to output variable `CC' to make it work.  If the compiler does not support C99, fall back to supporting ANSI C89 (ISO C90).  After calling this macro you can check whether the C compiler has been set to accept Standard C; if not, the shell variable `ac_cv_prog_cc_stdc' is set to `no'.