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

["martin at martin dot st" <gcc-bugzilla@gcc.gnu.org>]

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

            Bug ID: 95130
           Summary: GCC ignoring attribute(format(gnu_printf)) on printf
                    in mingw
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: martin at martin dot st
  Target Milestone: ---

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.