Re: WARNS > 3 then -Wformat=2: pb with va_list

Le Tue, Jun 20, 2023 at 05:34:19PM +0200, Roland Illig a écrit :
> Am 20.06.2023 um 15:45 schrieb [email protected]:
> > Is there a way to please the compiler when using a va_list and calling
> > syslog(3) or fprintf(3)?
> 
> When you forward to syslog, you have to call vsyslog(3), not syslog(3).
> 
> ~~~c
> #include <stdarg.h>
> #include <stdio.h>
> #include <syslog.h>
> 
> static void __sysloglike(2, 3)
> my_syslog(int prio, const char *fmt, ...)
> {
> 	va_list ap;
> 
> 	va_start(ap, fmt);
> 	vfprintf(stderr, fmt, ap);
> 	va_end(ap);
> 	fprintf(stderr, "\n");
> 
> 	va_start(ap, fmt);
> 	vsyslog(prio, fmt, ap);
> 	va_end(ap);
> }
> 
> int
> main(int argc, char **argv)
> {
> 	my_syslog(1, "%d", 4);
> 	my_syslog(2, "%.*s", 5, "hello, world");
> 	return 0;
> }

OK, thanks!

> ~~~
> 
> When you change the 'vsyslog' to plain 'syslog', you get:
> 
> > sl.c:16:2: error: format not a string literal,
> >     argument types not checked [-Werror=format-nonliteral]
> 
> This error message is highly confusing, it should rather say that
> 'syslog' takes varargs, not a 'va_list'.
> 
> Curiously, GCC 10 doesn't complain when I pass a __sysloglike string to
> a printf-like function, and vice versa.

I think this is due to the definition of __sysloglike in cdefs.h: it is
in fact an alias to __printflike if __syslog_attribute__ is not defined.
-- 
        Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
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