Bitfield endianness in gcc

Question

The endianness of bitfields is implementation defined. Is there a way to check, at compile time, whether via some macro or other compiler flag, what gcc's bitfield endianness actually is?

In other words, given something like:

struct X {
    uint32_t a : 8;
    uint32_t b : 24;
};

Is there a way for me to know at compile time whether or not a is the first or last byte in X?

Answer 1

On Linux systems, you can check the __BYTE_ORDER macro to see if it is __LITTLE_ENDIAN or __BIG_ENDIAN. While this is not authoritative, in practice it should work.

A hint that this is the right way to do it is in the definition of struct iphdr in netinet/ip.h, which is for an IP header. The first byte contains two 4-bit fields which are implemented as bitfields, so the order is important:

struct iphdr
  {
#if __BYTE_ORDER == __LITTLE_ENDIAN
    unsigned int ihl:4;
    unsigned int version:4;
#elif __BYTE_ORDER == __BIG_ENDIAN
    unsigned int version:4;
    unsigned int ihl:4;
#else
# error "Please fix <bits/endian.h>"
#endif
    u_int8_t tos;
    u_int16_t tot_len;
    u_int16_t id;
    u_int16_t frag_off;
    u_int8_t ttl;
    u_int8_t protocol;
    u_int16_t check;
    u_int32_t saddr;
    u_int32_t daddr;
    /*The options start here. */
  };