Use gnu_printf format attribute to avoid warnings on MinGW

We use %zu format specifiers to log size_t values.
However, __attribute__((format(printf, ...))) in modern GCC means
"system's default" printf, which on MinGW means MSVCRT.DLL printf,
which does not support the z length modifier (even on Windows 10),
so GCC warns that our format strings are invalid.

Older GCC is unable to check Microsoft's format strings at all and
has only format(printf), which is equivalent to the newer gnu_printf.
Clang also lacks gnu_printf, but it is covered by the same macro
check because its __GNUC...__ macros always report version 4.2.1.

On non-Microsoft platforms, printf and gnu_printf are currently
aliases, so the __MINGW32__ check is redundant. However, with some
luck, GCC may start to check other platforms' printf formats more
carefully in the future; and we would like to receive warnings
if our format strings don't work on some platform, although we
intend to stick to standard C99 format strings. Indeed, if we use
an extension by accident, this might help us catch it. And even if
we make no mistake but there is another platform that fails to
support C99 format strings, this might warn some poor soul building
on that platform that their system printf doesn't understand our
log format strings, so they will know they need to work around it
in their log callback or to patch libass.

1 files changed, 3 insertions, 1 deletions

diff --git a/libass/ass_utils.h b/libass/ass_utils.h
index 0743ff2..4789179 100644
--- a/libass/ass_utils.h
+++ b/libass/ass_utils.h
@@ -104,7 +104,9 @@ int numpad2align(int val);
 unsigned ass_utf8_get_char(char **str);
 unsigned ass_utf8_put_char(char *dest, uint32_t ch);
 void ass_utf16be_to_utf8(char *dst, size_t dst_size, uint8_t *src, size_t src_size);
-#ifdef __GNUC__
+#if defined(__MINGW32__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 4))
+    __attribute__ ((format (gnu_printf, 3, 4)))
+#elif defined(__GNUC__)
     __attribute__ ((format (printf, 3, 4)))
 #endif
 void ass_msg(ASS_Library *priv, int lvl, const char *fmt, ...);