Re: C23 support in Autoconf

On 2024-05-01 05:33, Nick Bowler wrote:
> Just configure with CFLAGS=-std=c99 or whatever.

I was thinking along the same line. We should keep things simple. Using 
CFLAGS is a documented way to specify the compiler flags, and ideally 
there would be no need for a new feature in this area.

However, I have some qualms. './configure CFLAGS=-std=c99' works 
somewhat by accident, because it tries things in the following order:

  echo ' ... C23 test ... ' >conftest.c
  gcc -std=c99 conftest.c
  gcc -std=gnu23 -std=c99  conftest.c
  echo ' ... C11 test ... ' >conftest.c
  gcc -std=c99 conftest.c
  gcc -std=gnu11 -std=c99  conftest.c
  gcc -std:c11 -std=c99 conftest.c
  echo ' ... C99 test ... ' >conftest.c
  gcc -std=c99 conftest.c

The last GCC invocation is the first one that works, so 'configure' 
announces that it has enabled C99 features.

This approach works because GCC treats a later -std= option as 
overriding an earlier one. Does the same thing happen with other compilers?

If not, there should be a way for the builder to say "Don't try to mess 
with the C standard version; just leave $CC alone". With current 
Autoconf on Savannah there's a hacky way to do that:

   ./configure CC='-std=c99' ac_cv_prog_cc_c23=

but that sort of thing is not something we want to document, as it 
depends on the Autoconf version. So perhaps there should be a better way 
to disable the C standard version testing that AC_PROG_CC normally does.