Question

I read about __packed__ from here and, I understood that when __packed__ is used in a struct or union, it means that the member variables are placed in such a way to minimize the memory required to store the struct or union.

Now, consider the structures in the following code. They contain same elements (same type, same variable names and placed in the same order). The difference is, one is __packed__ and the other is not.

#include <stdio.h>

int main(void)
{

    typedef struct unpacked_struct {
        char c;
        int i;
        float f;
        double d;
    }ups;

    typedef struct __attribute__ ((__packed__)) packed_struct {
        char c;
        int i;
        float f;
        double d;
    }ps;

    printf("sizeof(my_unpacked_struct)  : %d \n", sizeof(ups));
    printf("sizeof(my_packed_struct)    : %d \n", sizeof(ps));

    ups ups1 = init_ups();
    ps ps1;

    return 0;
}

Is there a way where we can copy unpacked structure ups1 into packed structure ps1 without doing a member-variable-wise-copy? Is there something like memcpy() that is applicable here?

Answer 1

I’m afraid you’ve just gotta write it out. Nothing in standard C (or any standard I know of) will do this for you. Write it once and never think about it again.

ps ups_to_ps(ups ups) {
    return (ps) {
        .c = ups.c,
        .i = ups.i,
        .f = ups.f,
        .d = ups.d,
    };
}