What is the size of the structure in this code below assuming that we have structure padding and size of int is 4 and size of double is 8 bytes

Question

Can anyone please tell me how come the size of the structure shown below is 24 and not 20.

typedef struct
{
    double d;  // this would be 8 bytes
    char c;   // This should be 4 bytes considering 3 bytes padding
    int a;   // This would be 4 bytes
    float b; // This would be 4 bytes
} abc_t;

main()
{
    abc_t temp;
    printf("The size of struct is %d\n",sizeof(temp));
}

My asumption is that the size of structure would be 20 when we consider padding but when i run this code the size is printing as 24.

Answer 1

Size would be 24. It is because the last member is padded with the number of bytes required so that the total size of the structure should be a multiple of the largest alignment of any structure member.

So padding would be like

typedef struct
{
    double d;  // This would be 8 bytes
    char c;    // This should be 4 bytes considering 3 bytes padding
    int a;     // This would be 4 bytes
    float b;   // Last member of structure. Largest alignment is 8.  
               // This would be 8 bytes to make the size multiple of 8 
} abc_t;

Read the wiki article for more detail.