CODEWITHSUNDEEP
csizeofunions
JookSon
JookSon

Reputation: 3

Size of Union and why it gives back an 24

So here is the problem:

I need to know how big is union (or something like that im new to C, and if i would like to know how to learn more of C i would be greatful)

Union should give me back hmm 20?

It gives me back 24 and i need an explenation.

#include <stdio.h>
#include <stdlib.h>
union Zadanie {
    int calkowita;
    char znak[20];
    double rzeczywista;
}ZadanieU;
typedef struct {
    int calkowita;
    char znak[20];
    double rzeczywista;
}ZadanieS;
int main()
{
    ZadanieS z1  = {2,"Borys",3.5};
    printf("%d,\n %s,\n%0.1f \n\n", z1.calkowita,z1.znak,z1.rzeczywista);

    printf("Rozmiar unii: %d\n", sizeof(ZadanieU));
    printf("Rozmiar Struktury: %d\n\n", sizeof(ZadanieS));

    printf("Rozmiar unii Calkowitej:%d \n", sizeof(ZadanieU.calkowita));
    printf("Rozmiar Struktury Calkowitej: %d\n\n", sizeof(z1.calkowita));

    printf("Rozmiar unii Char:%d \n", sizeof(ZadanieU.znak));
    printf("Rozmiar Struktury Char: %d\n\n", sizeof(z1.znak));

    printf("Rozmiar unii Rzeczywistej:%d \n", sizeof(ZadanieU.rzeczywista));
    printf("Rozmiar Struktury Rzeczywistej: %d\n\n", sizeof(z1.rzeczywista));
    return 0;
}

Upvotes: 0

Views: 71

Answers (1)

Matt Timmermans
Matt Timmermans

Reputation: 59263

sizeof(ZadanieU) will tell you the difference between the addresses of two consecutive instances of that union in an array of that type.

On some architectures, double values are aligned on 8-byte boundaries, and the union contains a double so it will be aligned on an 8-byte boundary too.

The difference between consecutive union addresses would therefore be 24.

Upvotes: 2

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