[Bug c/54454] New: gcc violates c99 specification w.r.t. flexible arrays

[Bug c/54454] New: gcc violates c99 specification w.r.t. flexible arrays

[mikulas at artax dot karlin.mff.cuni.cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54454

             Bug #: 54454
           Summary: gcc violates c99 specification w.r.t. flexible arrays
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mikulas@artax.karlin.mff.cuni.cz
              Host: hppa-linux-gnu
            Target: hppa-linux-gnu
             Build: hppa-linux-gnu


The c99 specification says (section 6.7.2.1, paragraph 16): "The size of the
structure [with flexible array member] shall be equal to the offset of the
last element of an otherwise identical structure that replaces the flexible
array member with an array of unspecified length". This is not true in gcc.
Try to complile and run this program: the size of the structure with the
flexible array member is 8. However, offset of the flexible array member is 6.
According to the specification, sizeof(struct flexible) should be 6.

#include <stdio.h>
#include <stddef.h>

struct flexible {
        unsigned a;
        unsigned short b;
        unsigned char array[];
};

struct not_flexible {
        unsigned a;
        unsigned short b;
        unsigned char array[1];
};

int main(void)
{
        printf("sizeof(struct flexible) = %d\n", (int)sizeof(struct flexible));
        printf("offsetof(struct not_flexible, array)) = %d\n",
(int)offsetof(struct not_flexible, array));
        printf("offsetof(struct flexible, array)) = %d\n", (int)offsetof(struct
flexible, array));
        return 0;
}

[Bug c/54454] gcc violates c99 specification w.r.t. flexible arrays

[mikulas at artax dot karlin.mff.cuni.cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54454

--- Comment #1 from mikulas at artax dot karlin.mff.cuni.cz 2012-09-02 01:52:54 UTC ---
Another specification violation:

section 6.7.2.1 paragraph 20 says that assignment of a structure with flexible
array member doesn't copy any of the array elements. In gcc it is buggy because
it copies some array elements.

Try this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct flexible {
        unsigned a;
        unsigned short b;
        unsigned char array[];
};

int main(void)
{
        struct flexible *f1, *f2;
        f1 = malloc(sizeof(struct flexible) + 3);
        f1->array[0] = 0;
        f1->array[1] = 0;
        f1->array[2] = 0;
        f2 = malloc(sizeof(struct flexible) + 3);
        f2->array[0] = 1;
        f2->array[1] = 1;
        f2->array[2] = 1;
        *f2 = *f1;
        printf("f2->array[0] == %d (should be 1)\n", f2->array[0]);
        printf("f2->array[1] == %d (should be 1)\n", f2->array[1]);
        printf("f2->array[2] == %d (should be 1)\n", f2->array[2]);
        return 0;
}

[Bug c/54454] gcc violates c99 specification w.r.t. flexible arrays

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54454

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-09-02 02:05:47 UTC ---
I think this comes down to alignment.

[Bug c/54454] gcc violates c99 specification w.r.t. flexible arrays

[jsm28 at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54454

Joseph S. Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #3 from Joseph S. Myers <jsm28 at gcc dot gnu.org> 2012-09-02 12:20:39 UTC ---
All the text you are quoting changed in TC2 following DR 282.

[Bug c/54454] gcc violates c99 specification w.r.t. flexible arrays

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54454

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |DUPLICATE

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup.

*** This bug has been marked as a duplicate of bug 9058 ***