[Bug c/61898] New: Variadic functions accept va_list without warning

[jzwinck at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61898

            Bug ID: 61898
           Summary: Variadic functions accept va_list without warning
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jzwinck at gmail dot com

This is a type of error I've seen multiple times in production code.  It
typically arises when forwarding variadic arguments to another function, like
so:

--------

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

void printfBadly(const char* fmt, ...)
{
  va_list ap;
  va_start(ap, fmt);
  printf(fmt, ap); // oops, we wanted vprintf
  va_end(ap);
}

--------

GCC 4.9.0 (and 4.7.2) with "-Wall -Wextra" compiles this program with no
complaint, but it produces undefined behavior.  There are two types of errors
which GCC could check for:

(1) GCC knows how to check printf when the format string is a literal, but
otherwise doesn't seem to check the arguments at all.  Passing a va_list to
printf (or any function with attribute(format(printf))) is almost certainly an
error.  The only conversion specifier that seems usable with a va_list is "%p",
and even that is non-portable, unlikely to be useful, and probably not what the
programmer intended.  Related to this, there is also no warning when passing a
struct (by value) to printf, though such code is likely incorrect.

(2) Passing a va_list via variadic arguments is almost never correct.  The
example above used printf when it needed vprintf, but this is a more general
point: a va_list passed anywhere within ellipsis varags is very likely to be a
mistake which results in undefined behavior.  This type of error applies not
only to printf-like functions, but to variadic functions in general.

In either case, a warning would be very useful (with -Wextra at least).