Hello,
I was recently bitten by a piece of code that gcc (3.3.3 and 3.4.1)
compiled without warnings and which ran just fine, but only on i386,
on ppc it failed. However on ppc with 3.4.6 the code failed. Reduced
test case is presented bellow.
<safp.c>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <dlfcn.h>
struct {
void (*fummy) (void);
void (*dummy) (void);
void (*tummy) (void);
} funcs;
void dummy (void)
{
}
int main (void)
{
printf ("dummy is %p\n", __extension__ (void *) &dummy);
*(void **) (char **) (void *) &funcs.dummy = dlsym (NULL, "dummy");
if (!funcs.dummy) {
printf ("failed %p\n", *(void **) (char **) (void *) &funcs.dummy);
}
else {
printf ("success %p\n", __extension__ (void *) funcs.dummy);
}
exit (EXIT_SUCCESS);
}
</safp.c>
Addmitedly this is somewhat dodgy, but gcc does not complain even
with `-Wall', `-W|-Wextra' and `-pedantic'.
<transcript-ppc>
safp$ uname -m
ppc
safp$ gcc --version | head -1
gcc (GCC) 3.4.6
safp$ gcc -O2 -Wall -W -pedantic safp.c -o safp -ldl -g -Wl,-E
safp$ ./safp
dummy is 0x10001778
failed 0x10001778
</transcript-ppc>
<transcript-i386>
safp$ uname -m
i686
safp$ gcc --version|head -1
gcc (GCC) 3.3.3 (SuSE Linux)
safp$ gcc -O2 -Wall -Werror -W -pedantic -o safp ./safp.c -ldl -Wl,-E
safp$ ./safp
dummy is 0x8048700
success 0x8048700
safp$ gcc-4.1.0 --version|head -1
gcc-4.1.0 (GCC) 4.1.0
safp$ gcc-4.1.0 -O2 -Wall -Werror -Wextra -W -pedantic -o safp ./safp.c -ldl -Wl,-E
safp$ ./safp
dummy is 0x80486f0
success 0x80486f0
</transcript-i386>
If the code is indeed in error w.r.t. to standard aliasing rules then
perhaps someone could explain how to fill those pointers to functions
in some other manner and without resorting to removing -W... flags from
GCCs command line. Thanks in advance.
P.S. It seems like i can use the tripple casting trick once again
instead of straightforward `struct.field', but this again begs
the question of portability/conformance.
--
vale
