Is there a way to enforce specific endianness for a C or C++ struct?

A bit late to the party but with current GCC (tested on 6.2.1 where it works and 4.9.2 where it’s not implemented) there is finally a way to declare that a struct should be kept in X-endian byte order.

The following test program:

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

struct __attribute__((packed, scalar_storage_order("big-endian"))) mystruct {
    uint16_t a;
    uint32_t b;
    uint64_t c;
};


int main(int argc, char** argv) {
    struct mystruct bar = {.a = 0xaabb, .b = 0xff0000aa, .c = 0xabcdefaabbccddee};

    FILE *f = fopen("out.bin", "wb");
    size_t written = fwrite(&bar, sizeof(struct mystruct), 1, f);
    fclose(f);
}

creates a file “out.bin” which you can inspect with a hex editor (e.g. hexdump -C out.bin). If the scalar_storage_order attribute is suppported it will contain the expected 0xaabbff0000aaabcdefaabbccddee in this order and without holes. Sadly this is of course very compiler specific.