How to count the offset of different data type in struct?

April 10, 2022

I know that different data type will allign. But I can’t figure out why the size of this structure is 12.

#include <stdio.h>

struct ABC{
    char a; 
    char b; 
    int k;  
    char c; 
};

int main()
{
    struct ABC a;
    printf("%lu\n", sizeof(a)); //12
    return 0;
}

>Solution :

Ok it works like this

Here are the rules:

chars are aligned to 1 byte

ints are aligned to 4 bytes

The entirety of the struct must be aligned to a multiple of the largest element.

the final struct will be

struct ABC
{
    char a; // Byte 0
    char b; // Byte 1
    char padding0[2]; // to align the int to 4 bytes
    int k;  // Byte 4
    char c; // Byte 5
    char padding1[3]; // align the structure to the multiple of largest element.
    // largest element is int with 4 bytes, the closest multiple is 12, so pad 3 bytes more.
}