[Bug c/29227] New: gcc sometimes doesn't recognize uninitialized variables

["debian-gcc at lists dot debian dot org" <gcc-bugzilla@gcc.gnu.org>]

[forwarded from http://bugs.debian.org/386174]

bug submitter writes (4.1 and trunk):

In the following program, gcc fails to recognize that "n" is
uninitialized, unless the "sscanf" line is commented out.  I expected
no change at all, since sscanf() is called after foo().  Since the
compiler is responsible for guaranteeing that the program runs as if
this order is true (even if it is not), I expect it to be aware of the
initialized state of the variables.

gcc-4.1 -std=gnu99 -W -Wall -O3 -g    ss.c   -o ss
ss.c: In function 'main':
ss.c:6: warning: 'n' is used uninitialized in this function

gcc-4.1 -DSS -std=gnu99 -W -Wall -O3 -g    ss.c   -o ss
[no warning]


#define _GNU_SOURCE
#include <stdio.h>

static void foo(size_t *n)
{
        printf("%d\n", *n);
}

int main()
{
        size_t n;
        foo(&n);
#ifdef  SS
        sscanf("1234", "%zd", &n);
#endif

        return 0;
}


-- 
           Summary: gcc sometimes doesn't recognize uninitialized variables
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: debian-gcc at lists dot debian dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29227