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).
