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.
