Bit field memory usage in C

Question

Why does it return with 96 and not 64?
If I sum bit of bit field I will get 64.

Edited:

The

var

variable has
0xFFFFFF
and not
0xFFFFFFFF
. -> The

var

variable has

0x3FFFFFFF00FFFFFF

and not

0xFFFFFFFFFFFFFFFF

.*

#include <stdio.h>

#include <stdlib.h>

#include <stdint.h>



typedef struct{

    uint32_t a : 24;

    uint32_t b : 20;

    uint32_t c : 10;

    uint32_t d : 6;

    uint32_t e : 4;

}MyType_t;



int main(){

    MyType_t test;

    test.a = -1;

    test.b = -1;

    test.c = -1;

    test.d = -1;

    test.e = -1;



    uint64_t var = *((uint64_t*)&test);



    printf("MyType_t: %d bit\n", sizeof(MyType_t) * 8);//96 bit

    printf("Var: %#llX\n", var);//0x3FFFFFFF00FFFFFF

    return 0;

}

This code will be worked correctly:

typedef struct{

    uint32_t a : 16;

    uint32_t b : 16;

    uint32_t c : 16;

    uint32_t d : 8;

    uint32_t e : 8;

}MyType_t;

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Answer 1

The fields a and b cannot possibly fit into a single type of uint32_t:

typedef struct{
    uint32_t a : 24;    //first 32 bits
    uint32_t b : 20;    //second 32 bits
    uint32_t c : 10;    //
    uint32_t d : 6;     //third 32 bits
    uint32_t e : 4;     //
}MyType_t;

so the size of the struct is three times the size of uint32_t.

The behavior of the code uint64_t var = *((uint64_t*)&test); is not defined.

Bit field memory usage in C

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