How to get the bit size in struct? [closed]

Question

I want to print a_size in header struct like "a_size = 3" but, it prints "a_size = 1".

How to get the value a_size?

#include <stdio.h>

struct header {
   unsigned int total_size : 6;
   unsigned int a_size : 3;
   unsigned int b_size : 2;
   unsigned int c_size : 3;
   unsigned int d_size : 2;
};

int main() {
   struct header h;
   printf("\n=========== HEADER ===========\n");
   printf("a_size : %d\n", h.a_size);

   return 0;
}

Answer 1

How to get the value a_size?

First thing first, bit-field is used to limit memory usage, by limiting sizeof() of a data type. So, the value of bit-field like unsigned int a_size : 3; is not printed as 3, instead you have assigned a_size bit-width to 3.

If you want to print 3 then you have to assign a_size to 3 instead of defining it's bit-field as 3.

So, first we have initialize it by using some functions which is written below:

#include <stdio.h>
#include <stdbool.h>

struct header {
   unsigned int total_size : 6;
   unsigned int a_size : 3;
   unsigned int b_size : 2;
   unsigned int c_size : 3;
   unsigned int d_size : 2;
};

/*
  This function inits the `struct header`.
  @param h reference of your variable
  @returns false if `h` was NULL, otherwise returns true
*/
int init(struct header *h)
{
        if(!h)
            return false;
        h->a_size = 3;
        h->b_size = 2;
        h->c_size = 3;
        h->d_size = 2;
        h->total_size = 6;
        return true;
}

int main(void) {
   struct header h;
   init(&h);
   printf("\n=========== HEADER ===========\n");
   printf("a_size : %d\n", h.a_size);

   return 0;
}