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

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'.