comparison of structures

comparison of structures

[José Bollo]

hi,

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 think that it is not a bug and that the structure can not be compared using 
'memcmp'.

do you agree or it a bug ?

regards
josé

Re: comparison of structures

[Fabian Cenedese]

>the following C code 
>
>typedef struct {
>    int32_t a;
>    void *b;
>} S;
>
>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 think that it is not a bug and that the structure can not be compared using 
>'memcmp'.
>
>do you agree or it a bug ?

You can use memcmp if you fill up the struct with a dummy value that you
can set explicitely. Or use a memset(&a, 0, sizeof(S)) before doing any
work. Both are workarounds but I wouldn't say that it's a bug.

bye  Fabi

Re: comparison of structures

[cheng long]

In computer, the result is a=1, b=1.
My box isFC5 on IA-32.