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From: "roland.illig at gmx dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/110322] New: Be more helpful when a varargs function is
 called in a wrong way
Date: Tue, 20 Jun 2023 15:43:12 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 12.0
X-Bugzilla-Keywords: diagnostic
X-Bugzilla-Severity: normal
X-Bugzilla-Who: roland.illig at gmx dot de
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110322

            Bug ID: 110322
           Summary: Be more helpful when a varargs function is called in a
                    wrong way
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roland.illig at gmx dot de
  Target Milestone: ---

~~~c
#include <stdarg.h>
#include <stdio.h>

static void __attribute__((__format__(__printf__, 1, 2)))
my_printf(const char *fmt, ...)
{
        va_list ap;

        va_start(ap, fmt);
        fprintf(stderr, fmt, ap);
        va_end(ap);
}

int
main(int argc, char **argv)
{
        my_printf("%d", 4);
        my_printf("%.*s\n", 5, "hello, world");
        return 0;
}
~~~

In the above program, I am accidentally trying to call fprintf with a va_li=
st,
instead of the correct vfprintf. GCC warns:

sl.c:10:2: error: format not a string literal, argument types not checked
    [-Werror=3Dformat-nonliteral]
   10 |  fprintf(stderr, fmt, ap);
      |  ^~~~~~~

In this situation, where my only mistake was to forget the 'v' from the
function name, GCC should not complain that the format string is not a stri=
ng
literal, but rather that I'm calling the wrong function.=