Help with understanding strict aliasing rules

Martin Lucina mato@kotelna.sk
Mon Aug 28 09:13:00 GMT 2006


Hello all,

I'm having trouble understanding the infamous "dereferencing type-punned
pointer will break strict-aliasing rules" warnings from GCC.

I've read both of the following articles:

1) http://www.cellperformance.com/mike_acton/2006/06/understanding_strict_aliasing.html
2) http://mail-index.netbsd.org/tech-kern/2003/08/11/0001.html

but am still no wiser as to why the code I'm having trouble with is
producing these warnings.  I think the code is legitimate and is not
actually violating the strict aliasing rules, and have found a "fix" to
make the warnings go away.  Comparing the generated assembler with and
without the "fix" shows the same code, and it looks OK to me.

However, I'd like to be 100% sure about this, so I'm asking here.

The following short examples demonstrate my problem:

----Exhibit A
#include <stdio.h>
#include <stdlib.h>

typedef struct {
    int foo;
    int bar;
} item_t;

int return_item (void **item);

int return_item (void **item)
{
    void *mem;

    mem = malloc (1);
    if (mem) {
        *item = mem;
        return 0;
    }
    else
        return 1;
}

int main (int argc, char *argv[])
{
    item_t *item;

    if (return_item ((void **)&item) == 0) {
        printf ("%p\n", item);
        free (item);
    }

    return 0;
}

----Exhibit B
#include <stdio.h>
#include <stdlib.h>

typedef struct {
    int foo;
    int bar;
} item_t;

int return_item (void **item);

int return_item (void **item)
{
    void *mem;

    mem = malloc (1);
    if (mem) {
        *item = mem;
        return 0;
    }
    else
        return 1;
}

int main (int argc, char *argv[])
{
    item_t *item;
    void *item_temp;

    if (return_item (&item_temp) == 0) {
        item = item_temp;
        printf ("%p\n", item);
        free (item);
    }

    return 0;
}
----

The code is a generic example of the problem.  The real code that is
producing the problem is a hashing API which hashes (void *) and hence
uses (void **) as an out parameter type.  

Exhibit A produces a warning as follows:

[nodbug:mato]$ gcc -O2 -Wall -o aliasing-test aliasing-test.c
aliasing-test.c: In function `main':
aliasing-test.c:28: warning: dereferencing type-punned pointer will break strict-aliasing rules

I'm using gcc version 3.3.5 (Debian 1:3.3.5-13) but the problem persists
even when tested with GCC 4.x on newer systems.

Exhibit B is my proposed "fix".  Can anyone advise if the code in
Exhibit A is legitimate, i.e. whether or not it's really violating the
strict aliasing rules as defined by the C standard?  If not, then I
guess the warning is spurious and I can safely use the fix.

If the code is in fact violating the standard then feel free to
enlighten me how to fix it, since it seems like a legitimate thing to do
:-)

Thanks very much for any help,

-mato



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