[Bug c/39438] Can't compile a wrapper around strftime with -Werror=format-nonliteral

["hugh at mimosa dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39438

--- Comment #6 from D. Hugh Redelmeier <hugh at mimosa dot com> ---
Responding to Comment 5 by Manuel López-Ibáñez:

Thanks for looking at this.

> Could you produce a complete testcase
===
/* compile with -c -Wformat -Werror=format-nonliteral */
#include <time.h>

extern void prettynow(char *buf, size_t buflen, const char *fmt, struct tm *t)
__attribute__ ((format (__strftime__, 3, 0)));

void prettynow(char *buf, size_t buflen, const char *fmt, struct tm *t)
{
        strftime(buf, buflen, fmt, t);
}
===

> The best would be to include only the definitions of the types that you need > and declare as 'extern' library functions that you use to avoid including headers.

I didn't do this part.  My excuses: the type declarations are not portable but
the example should be; the solution might be in fixing headers; this way is
shorter too.

> If you can produce another similar example that works with printf, that would also help.

I don't think that printf is similar enough.  A strftime format doesn't
interact with varargs in a complex and problematic way.  A string, on its own,
is or is not a valid strftime format; a string is only a valid printf format
when considered with the argument list.

If an arg is marked as a const char * (i.e. unchanging) and has the strftime
format attribute, it should be accepted as if it were a literal strftime
argument.  After all, the necessary checking would have been done at this
routine's points of call.
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Subject: [Bug c/61144] Invalid optimizations for extern vars with local weak definitions
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http://gcc.gnu.org/bugzilla/show_bug.cgi?ida144

--- Comment #11 from Rich Felker <bugdal at aerifal dot cx> ---
Adding __attribute__((__used__)) to the static object suppresses the symptom in
case that helps to isolate what's causing it.