GCC and attribute((packed)) in Linux & Windows

05/Mar/2023

This program gives different outputs in windows and linux, even when compiled with similar same GCC versions. Fortunately, it is not a bug and just a matter of compiler flags.

#include <stdio.h>
#include <stdint.h>

struct __attribute__((packed)) {
        uint16_t flag:10;
        uint8_t data:6;
} st2Byte;

struct __attribute__((packed)) {
        uint8_t flag:2;
        uint8_t data:6;
} st1Byte;

struct __attribute__((packed)) {
        uint32_t data:16;
        uint16_t flag:8;
        uint16_t view:8;
} st4Byte;


int main(void)
{
    int expected = 2;
    int size = sizeof(st2Byte);
    printf("Expected: %d, got: %d - %s\n", expected, size, (size == expected ? "PASS" : "FAIL"));

    expected = 1;
    size = sizeof(st1Byte);
    printf("Expected: %d, got: %d - %s\n", expected, size, (size == expected ? "PASS" : "FAIL"));

    expected = 4;
    size = sizeof(st4Byte);
    printf("Expected: %d, got: %d - %s\n", expected, size, (size == expected ? "PASS" : "FAIL"));
    return 0;
}

In order to get the same results, we must specify if the struct attribute will be ms_struct or gcc_struct.

They resulted in different sizes because in Windows the default is ms_struct. See x86 Type Attributes. We can achieve the same results using the following flag:

gcc test.c --mno-ms-bitfields