printf("%m") doesn't generate a warning -- shouldn't it?

[Dimitry Andric]

On 21 May 2018, at 04:38, Thomas Munro <munro at ip9.org> wrote:
> 
> As discussed on the PostgreSQL[1] and NetBSD mailing lists[2][3],
> syslog-like printf("%m") is a GNU extension that doesn't generate a
> warning from Clang or GCC on other operating systems even though when
> it doesn't actually work.  That's because the __printf__ attribute
> that our __printflike macro in /usr/include/stdio.h expands to
> effectively means "like printf in glibc, allowing %m", not like POSIX
> or our actual libc which just prints out "m" when it sees it.
> 
> It'd sure be nice to get a compiler warning on FreeBSD when porting
> software that uses that if it doesn't actually work (or ... to support
> it).

Please submit upstream bug(s) for clang and gcc.  It turns out clang has
a -Wformat-non-iso warning flag which should warn about this, but I
can't get it to emit it:

$ cat printf-m.c
#include <stdio.h>

int main(void)
{
        printf("error: %m\n");
        return 0;
}

$ clang -std=c99 -Wformat-non-iso -c printf-m.c
<nothing>

-Dimitry

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