José Bollo writes:
> hi all,
>
> I repost here a problem that I met using GCC 3.4.4 and ABI for AMD64
> (-march=athlon64).
>
> the following C code
>
> typedef struct {
> int32_t a;
> void *b;
> } S;
> void fs() {
> char t[255];
> int i;
> for(i=0;i<255;i++)
> t[i]=(char)(i+1);
> }
> int cmp1(S a1, S a2) {
> return !memcmp(&a1,&a2,sizeof(S));
> }
> int cmp2(S a1, S a2) {
> return a1.a==a2.a && a1.b==a2.b;
> }
> void put(S y) {
> int a,b;
> S x;
> x=y;
> a=cmp1(x,y);
> b=cmp2(x,y);
> printf("a=%d b=%d\n",a,b);
> }
> int main() {
> S a;
> a.a=1;
> a.b=&a;
> fs();
> put(a);
> return 0;
> }
>
> compiled for my linux x86_64 using the amd64 ABI (-march=athlon64) with gcc
> 3.4.4 prints "a=0 b=1"!
>
> i have seen why and it is clear to me how a such result is produced.
> I add below a commented assembly listing of the incriminated part.
>
> i think that it is not a bug and that the structure can not be compared using
> 'memcmp'.
That's right. It's a consequence of 6.7.2.1 Para 13: "There may be
unnamed padding within a structure object, but not at its beginning".
Andrew.
