"Felix J. Ogris" <fjo-lists@xxxxxxxx> writes:
> maybe someone can enlighten me? Given a small test.c:
>
> #include <stdio.h>
>
> int chksum(void *ptr, unsigned int len) {
> int sum = 0;
> short *val = ptr;
>
> for (; len >= sizeof(short); len -= sizeof(short)) {
> sum += *val;
> val++;
> }
>
> return sum;
> }
>
> int main() {
> struct s {
> int a;
> int b;
> } s;
>
> s.a = 2;
> s.b = 1;
> s.a = chksum(&s, sizeof(struct s));
>
> printf("%i\n", s.a);
>
> return 0;
> }
>
>
> Expected output: 3
> Compiled using `gcc -W -Wall -O3 -o test test.c` with gcc 4.1.2 on CentOS 5.4 x86_64 or with gcc 4.2.1 on FreeBSD 8.2 i386 gives random rubbish instead, eg. -16897
> Compiled with -O2 or -fno-strict-aliasing or -fno-inline-functions gives expected output.
>
> Is there anything I could do against this (mis-)behaviour - except using one the above flags or using a union as *val which contains two shorts? Is there any -Warning parameter for this?
This code is accessing values of type int using a pointer of type
short*. That is invalid aliasing and your program uses undefined
behaviour. Your suggested workarounds are the right one.
To get a warning, you could try the -Wstrict-aliasing option. I don't
think it exists in gcc 4.2, though.
Ian
