> > I'm hacking on the Autoconf scripts for the OCaml compiler [1]. I'm
> > trying to make them more useful when compiling with the dreaded MSVC.
>
> Thanks for tackling this!
Thanks for your reply and thorough comments.
> (Can you check if there's maybe some way to make the UCRT declare
> POSIX functions like open(), without a leading underscore, *even if*
> __STDC__ is defined? Another macro that you define, maybe?)
Indeed, there's one! I missed it. It's _CRT_DECLARE_NONSTDC_NAMES.
The following program links without warnings with this invocation:
cl -W4 -std:c11 -Zc:__STDC__ -D_CRT_DECLARE_NONSTDC_NAMES test.c
#include <io.h>
int open(const char *, int, ...);
int main(void) {}
> Because the C standard doesn't allow non-standard names in header
> files, by default /std:c11 and /std:c17 don't expose the default
> names for POSIX functions, types, and macros. If these names are
> necessary, define _CRT_DECLARE_NONSTDC_NAMES to expose them.
https://learn.microsoft.com/en-us/cpp/c-runtime-library/compatibility?view=msvc-170
The documentation is sometimes a bit confused between C, C++, names
being exposed or not, or generating warnings, or the actual relations
between all that.
> > However the VLA test is included as-is in the C11 main program, but
> > C11 made that feature optional. MSVC correctly defines the
> > `__STDC_NO_VLA__` macro, but fails the Autoconf test. This is clearly
> > an Autoconf bug, but the fix isn't clear.
>
> I think what we should do here is fold AC_C_VARARRAYS into AC_PROG_CC.
> Take the test for VLAs completely out of _AC_C_C99_TEST_MAIN, but
> unconditionally *run* a test for VLAs as part of AC_PROG_CC. If that
> test fails, and __STDC_NO_VLA__ was not defined by the compiler,
> then AC_DEFINE([__STDC_NO_VLA__], 1) and set a cache variable.
> Mark AC_C_VARARRAYS as obsolete (i.e. AU_DEFUN it) and have it
> just check the cache variable and AC_DEFINE(HAVE_C_VARARRAYS),
> with post-autoupdate diagnostics telling the reader to replace
> HAVE_C_VARARRAYS with !__STDC_NO_VLA__ and then remove the
> compatibility code. Do you know how to do all of that? I can
> do it (not for another couple of weeks) if it sounds too daunting,
It does seem a bit daunting... I can try, you've well explained the
path to follow.
> but I will need you to test it for me as I don't have convenient
> access to a compiler that doesn't support VLAs.
GCC and clang have a -Wvla flag that warns if VLAs are encountered.
Coupled with -Werror, you cannot miss one.
-- Antonin
