Converting Endianess on a bit field structure

Question

I need to convert a bit-field structure from little-endian to big-endia architecture. What is the best way to do that, as there will be issues in byte boundaries, if I simply swap the structure elements.

Ex Structure is:

struct {
    unsigned int    b1:1;
    unsigned int    b2:8;
    unsigned int    b3:7;
    unsigned int    b4:8;
    unsigned int    b5:7;
    unsigned int    b6:1;
}; 

Answer 1

You could use a 32 bit integer, and extract information out of it using and- and bitshift operators. With that in place, you could simply use htonl (host-to-network, long). Network byte order is big endian.

This won't be as elegant as a bit-field, but at least you'll know what you have and won't have to worry about the compiler padding your structures.

Answer 2

Processor endianness is unrelated to bit field ordering. It's quite possible to have two compilers on the same computer use opposite ordering for bitfields. So, given this:

union {
    unsigned char x;
    struct {
        unsigned char b1 : 1;
        unsigned char b2 : 7;
    };
} abc;
abc.x = 0;
abc.b1 = 1;
printf( "%02x\n", abc.x );

Unless you happen to have detailed documentation, the only way to know whether that will print out 01 or 80 is to try it.