Question

I wrote a piece of c code :

struct Res {
    int a;
    float b;
    double c;
};

struct Res ModRes(struct Res rrr)
{
    rrr.a=222;
    return rrr;
}

int main()
{
    struct Res r[10]={1,2,3};
    ModRes(r[0]);
    return 0;
}

why r[0] is not 222 after ModRes ?

Answer 1

In C, arguments are passed by value. You could make the function accept a struct Res * rather than a struct Res:

struct Res *ModRes(struct Res *rrr)
{
    rrr->a=222;
    return rrr;
}

int main()
{
    struct Res r[10]={1,2,3};
    ModRes(&r[0]);
    return 0;
}