Question

How is the size of a struct with bit fields in C

Answer 1

#include <stdio.h> 

// 32 bit = 4 bytes 

// 33 bits = 8 bytes (32 + 1)=(unsigned int + unsigned int)
typedef struct 
{
    unsigned int a:1;
    unsigned int b:31;
    unsigned int c:1;
} bits1;

// 34 bits = 8 bytes (32 + 2)=(unsigned int + unsigned int)
typedef struct 
{
    unsigned int a:1;
    unsigned int b:31;
    unsigned int c:2;
} bits2;

// 64 bits = 8 bytes (32 + 32)=(unsigned int + unsigned int)
typedef struct 
{
    unsigned int a:1;
    unsigned int b:31;
    unsigned int c:32;
} bits3;

// 65 bits = 12 bytes (32 + 32 + 1)=(unsigned int + unsigned int + unsigned int)
typedef struct 
{
    unsigned int a:2;
    unsigned int b:31;
    unsigned int c:32;
} bits4;

// 3 bits = 4 bytes (32)=(unsigned int)
typedef struct 
{
    unsigned int a:1;
    unsigned int b:1;
    unsigned int c:1;
} bits5;
   
int main(void)
{   
    bits1 b1;
    printf("%zu\n", sizeof(b1));
    
    bits2 b2;
    printf("%zu\n", sizeof(b2));
    
    bits3 b3;
    printf("%zu\n", sizeof(b3));
    
    bits4 b4;
    printf("%zu\n", sizeof(b4));
    
    bits5 b5;
    printf("%zu\n", sizeof(b5));

    return 0;
}
 
 
         
/*
run:
      
8
8
8
12
4
     
*/

 

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