CODEWITHSUNDEEP
cstruct
Sangeeth Saravanaraj
Sangeeth Saravanaraj

Reputation: 16627

How to copy the contents of an unpacked struct to a __packed__ struct?

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?

Upvotes: 3

Views: 1931

Answers (2)

alk
alk

Reputation: 70979

Without detailed knowlegde of the differences of the memory layout of the two structures: No.

Upvotes: 1

Matt Joiner
Matt Joiner

Reputation: 118720

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,
    };
}

Upvotes: 7

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