[Bug c/95130] GCC ignoring attribute(format(gnu_printf)) on printf in mingw

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95130

--- Comment #21 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Yong <jyong@gcc.gnu.org>:

https://gcc.gnu.org/g:966f3c134bb4802ac7ba0517de4e8e3f6384cfa3

commit r14-3334-g966f3c134bb4802ac7ba0517de4e8e3f6384cfa3
Author: Tomas Kalibera <tomas.kalibera@gmail.com>
Date:   Sun Aug 20 02:16:16 2023 +0000

    Fix format attribute for printf

    Since a long time (GCC 4.4?) GCC does support annotating functions
    with either the format attribute "gnu_printf" or "ms_printf" to
    distinguish between different format string interpretations.

    However, it seems like the attribute is ignored for the "printf"
    symbol; regardless what the function declaration says, GCC treats
    it as "ms_printf". This has become an issue now that mingw-w64
    supports using the UCRT instead of msvcrt.dll, and in this case
    the stdio functions are declared with the gnu_printf attribute,
    and inttypes.h uses the same format specifiers as in GNU mode.

    A reproducible example of the problem:

    $ cat format.c
    __attribute__((__format__ (gnu_printf, 1, 2))) int printf (const char
*__format, ...);
    __attribute__((__format__ (gnu_printf, 1, 2))) int othername (const char
*__format, ...);

    void function(void) {
        long long unsigned x = 42;
        othername("%llu\n", x);
        printf("%llu\n", x);
    }
    $ x86_64-w64-mingw32-gcc -c -Wformat format.c
    format.c: In function 'function':
    format.c:7:15: warning: unknown conversion type character 'l' in format
[-Wformat=]
        7 |     printf("%llu\n", x);
          |               ^
    format.c:7:12: warning: too many arguments for format [-Wformat-extra-args]
        7 |     printf("%llu\n", x);
          |            ^~~~~~~~

    Note how both functions, printf and othername, are declare with
    identical gnu_printf format attributes - GCC does take this into
    account for "othername" and doesn't produce a warning, but GCC
    seems to disregard the attribute in the printf declaration and
    behave as if it was declared as ms_printf.

    If the printf function declaration is changed into a static inline
    function, the actual attribute used is honored though.

    gcc/c-family/ChangeLog:

            PR c/95130
            * c-format.cc: skip default format for printf symbol if
            explicitly declared by prototype.

    Signed-off-by: Tomas Kalibera <tomas.kalibera@gmail.com>
    Signed-off-by: Jonathan Yong <10walls@gmail.com>