Question

#include <stdio.h>

int main()
{
    struct value
    {
        int bit1:1;
        int bit2:4;
        int bit3:4;
    } bit;

    printf ("%d\n", sizeof(bit));
    return 0;
}

Output on Tc/Tc++:

2

Output under Linux:

4

I know I am missing some concept of bit fields.

Answer 1

The sizeof for the struct is not the same as the sum of the sizes of all the elements – this is especially the case with bitfields.

Typically, the struct needs to be padded to a certain size and alignment. (Which apparently is 2 on Tc/Tc++ and 4 in Linux.)

So although there’s only 9 bits in use, it’s being padded out to the word-size.

EDIT :

Note that the C standard does not specify how much padding is done. And therefore, you are getting different results from two different compilers.