[Bug c/53391] New: Slightly misleading warning on printf format mismatch

[Bug c/53391] New: Slightly misleading warning on printf format mismatch

[Keith.S.Thompson at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53391

             Bug #: 53391
           Summary: Slightly misleading warning on printf format mismatch
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: Keith.S.Thompson@gmail.com


Given the following program (c.c):

#include <stdio.h>
int main(void) {
    typedef long my_long;
    my_long x = 42;
    printf("x = %d\n", (long)x);
    return 0;
}

"gcc -c c.c" produces the following warning:

c.c: In function ‘main’:
c.c:5:5: warning: format ‘%d’ expects argument of type ‘int’, but argument 2
has type ‘my_long’ [-Wformat]

The argument is actually of type long, not my_long; the warning should say
"..., but argument 2 has type ‘long’".

Now because of the way typedefs work, long and my_long are really different
names for the same type. gcc is obviously trying to be clever about which name
makes for a clearer warning message. In this case, since x is explicitly
converted to long, referring to "long" would be less confusing. *Some* warning
is certainly appropriate, since "%d" requires an int argument.

(Speculation: gcc sees that the cast is a no-op, so it ignores it, which is
fine except that it results in this misleading warning message.)

Ubuntu x86 12.04
gcc (Ubuntu/Linaro 4.7.0-7ubuntu3) 4.7.0

[Bug c/53391] Slightly misleading warning on printf format mismatch

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53391

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-05-18 08:31:59 UTC ---
This is not about trying to be clever, the useless conversion is long gone
before the warning is emitted.

[Bug c/53391] Slightly misleading warning on printf format mismatch

[Keith.S.Thompson at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53391

--- Comment #2 from Keith Thompson <Keith.S.Thompson at gmail dot com> 2012-05-18 18:26:31 UTC ---
The cleverness I was referring to was having diagnostics refer to an
appropriate typedef rather than to the original type (e.g., "size_t" rather
than "unsigned int").  I find this clever in a *good* way.

The conversion is a no-op if my_long happens to be the same type as long, as it
is in this example -- but it's very useful from the programmer's point of view
if my_long (or something with a more meaningful name) *could* be either long or
int.

I suppose solving this would require saving some information about the cast so
that the name of the type of the expression ("my_long" rather than "long") can
be referred to in diagnostic messages.

I should note that this is not production code.  The original program was
intended to use "%ld" and convert the argument to long; I ran into the
misleading warning due to a logic error.