c - sizeof(struct) different for different compilers

Question

Supposing I have a code like this:

#include <stdio.h>
#include <stdint.h>
int main(int argc, char *argv[]) {
    typedef struct{
        uint16_t x : 9;
        uint8_t y : 7;
    } z;
    printf("sizeof(z) = %lu
",sizeof(z));
}

I have different results for clang on Mac (2) and someone told me on Windows it returned (3). Not sure if I understand it well, but I see that while first compiler compresses the struct to 9+7 = 16 bits, the other uses 16 bits of uint16_t and 8 of uint8_t. Could you advise?

Answer 1

Not sure if I understand it well, but I see that while first compiler compresses the struct to 9+7 = 16 bits, the other uses 16 bits of uint16_t and 8 of uint8_t. Could you advise?

The first thing to remember about bit-field is this phrase from K&R, 2nd:

(6.9 Bit-fields) "Almost everything about fields is implementation-dependent."

It includes padding, alignment and bit endianness.