https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88270
--- Comment #3 from Eric Blake <eblake at redhat dot com> ---
Confirmed that -Wpendantic flags %m:
$ gcc -Wpedantic -Wformat foo.c -o foo
foo.c: In function ‘main’:
foo.c:3:10: warning: ISO C does not support the ‘%m’ gnu_printf format
[-Wformat=]
printf("%m %B\n");
^
foo.c:3:13: warning: unknown conversion type character ‘B’ in format
[-Wformat=]
printf("%m %B\n");
^
Although -Wpedantic is a rather heavy hammer compared to a fine-grained knob
that can warn for just uses of %m (seeing as how we already have other knobs
like -Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral
-Wformat-overflow=N -Wformat-security -Wformat-signedness
-Wformat-truncation=N).
Then, as you say, if 'printf' starts flagging %m on BSD, we'd still need some
way to specifically mark attributes on wrapper functions that accept more than
the system print but less than the full gnu_printf.
Maybe we could have multiple fine-grained attributes that each control one
class of printf format extensions, which can then be combined as a set, where
the existing printf, ms_printf, and gnu_printf are shorthands for specific
members in that set? Then specific wrappers can hand-build the set matching
what they actually support, rather than relying on the shorthands. Something
like:
Mark my own function as C89 + %m (but not C99 %hhd)
__attribute__ (( format( (printf_c89, printf_m), 1, 0 ) ))
Mark another function as supporting %1$'d which is POSIX extensions beyond C99
C:
__attribute__ (( format( (printf_c89, printf_c99, printf_posix), 1, 0 ) ))
Use the system printf:
__attribute__ (( format( printf, 1, 0 ) ))
which on glibc would probably be shorthand for:
__attribute__ (( format( (printf_c89, printf_c99, printf_posix, printf_m,
printf_llg, printf_q, printf_Z), 1, 0 ) ))
and on Windows would probably be shorthand for:
__attribute__ (( format( (printf_c89, printf_I64), 1, 0 ) ))
I don't know what syntax would be best to specify an attribute as a set of
fine-grained format pieces that act together as a whole; so much as just
tossing out the idea of having finer-grained attributes which can be applied as
a set where the existing high-level attributes are then broken into their
fine-grained components.
