scalar_storage_order

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

#define __sso_be __attribute__((scalar_storage_order("big-endian")))

struct __sso_be ipv6hdr {
uint32_t
    version:4,
    tos:8,
    flow_lbl:20;
};

int main()
{
    struct ipv6hdr hdr;

    memcpy(&hdr, "\x60\x01\x23\x45", 4);

    printf("size:   %ld\n\n", sizeof(hdr));

    printf("version:    %x\n", hdr.version);
    printf("tos:        %x\n", hdr.tos);
    printf("flow_lbl:   %x\n\n", hdr.flow_lbl);

    return 0;
}