Why do Windows builds of perl currently specify -std=c99 in ccflags ?

[sisyphus]

On MS Windows, the GNUmakefile (as of late in the 5.39 dev cycle) began prepending -std=c99 to $Config{ccflags}.
This ensures that, with gcc-13 and gcc-14, the standard C level is wound back from C17 to C99.
And with gcc-15, the level is wound back from C23 to C99.

With Ubuntu (and, I suspect, many other nix-type systems) no such "winding back" takes place, and the C level remains at gcc's default level.
Does anyone know of a good reason that Windows builds of perl should be treated this way ?

I've just built current blead (at commit ea0856c) on Windows 11 using gcc-15.1.0, such that there is no "winding back" of the C level, which stays at C23.
And I can find no issue - apart from the known Perl/perl5#23192, for which a simple interim workaround exists while we wait for the perl source to be fixed.
Why should this -std=c99 spec be regarded as desirable ?

I'm thinking that we should be aiming to have perl's source compliant with C23, and protecting ourselves under the C99 cloak is not the way to meet that aim.

I note that the winding back was not deemed significant enough to backport to the perl-5.38.x and 5.36.x patchlevel releases, for which the level reamins at gcc's default - which is C17 on Strawberry Perl and on my own builds.

Note: The simplest way to avoid having the -std=c99 switch preprended to $Config{ccflags} is to simply comment out the following 2 lines in win32/GNUmakefile prior to building perl:

else
EXTRACFLAGS	+= -std=c99

[shawnlaffan]

We take the GNUMakefile from perl.

The -std=cc99 setting was added in Perl/perl5@527e765

Commit message is below to save some clicks.

win32/GNUmakefile: enable warnings for gcc
Warnings are already enabled for MSVC, so enable them for GCC too.

-Wformat is disabled since the default "__printf__" format checker
doesn't understand the C99-isms like "%zd", resulting in many spurious
warnings.  The __gnu_printf__ checker does understand them, but it
doesn't understand the Windows specific "%I64d" and similar formats.

[sisyphus]

Thanks for digging up the commit. I spent some time looking for it .... but failed :-(

We take the GNUMakefile from perl.

Yes, I understand that.
And I understand that the GNUmakefile adds those warnings for compatibility with MSVC,
But I don't see why the GNUmakefile had to add the -std=c99 switch, and I don't see why we should accept that.
What useful purpose does it serve ?

Without that switch, SP-5.40.x would be at the (higher) level of C17 - which is the same level as SP-5.38.x and SP-5.36.x.

There might be reason to specify C99 if C17 disables something that C99 enables. (Does that happen ?)
The only other reason I can think of is that there is some feature(s) of C17 that we want to disable by winding back to C99 - and if that's the case then what is this feature(s) ?

I'm unaware of any issues with 5.38 and 5.36 being at C17.
And I know only of the one issue (that I linked to in the opening post) when blead is built against gcc-15.1.0 at the default level of C23.
That one issue can be resolved by specifying -std=c17. AFAICT, it does not require winding back to C99.

I've been using the following Inline::C script to determine the C level at which perl has been built:

use strict; use warnings;
use Inline C =><<'EOC';

int foo() {
  return __STDC_VERSION__;
}

EOC

my $level = foo();
# print "$level\n";
$level = "C" . substr($level, 2, 2);
print "$level\n";

The question is not directed at SP speciifically.
It's directed at all users/builders of gcc-built perls on Windows, irrespective of who the builder is.
Perhaps it's a question for the p5p list, but I'll see what I can learn here before posting there.

[shawnlaffan]

Thanks for digging up the commit. I spent some time looking for it .... but failed :-(

I used git blame, which in the GH web interface can be accessed by clicking on the line number and then the ellipsis that appears to its left.

I'm not aware of the reasoning to set the standard to C99 instead of something later. A google search for GNUMakefile C99 site:https://www.nntp.perl.org/group/perl.perl5.porters/ finds nothing recent. Same for win32 instead of GNUMakefile.

This might be something for p5p.

[sisyphus]

I used git blame

Thanks for the tip - it works well.

I think I'll wait until 5.42 has been released and then make a PR that removes the -std=c99 setting from win32/GNUmakefile.

And I'll certainly remove that setting from all perls that I build from now on (be they perl-5.42 or devel releases or blead source from github.)
If there is some issue with the removal of that switch (in addition to the known issue with the CommonMark module) then I'd like to find out about it - and that's not going to happen if that setting remains in place.

[sisyphus]

I've found that I can replace the -std=c99 spec with -std=c23 without actually modifying the GNUmakefile.
It just requires the passing of the command line argument EXTRACFLAGS="-std=c23" to gmake (and to gmake test and to gmake install).