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 sw

Answer 1

You have two 16 bit sections there (the first three fields and the last three fields are 16 bits).

That's only 65536 entries. So have a lookup table that holds the bit-reversed version of the fields. Wrap the struct in a union with another struct that has two 16 bit fields to make this easier?

Something like (untested, I'm not near a C compiler):

union u {
    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;
     } bits;
     struct {
        uint16 first;
        uint16 second;
     } words
} ;

unit16 lookup[65536];

/* swap architectures */

void swapbits ( union u *p)
{
   p->words.first = lookup[p->words.first];
   p->words.second = lookup[p->words.second];
}

Population of the lookup table left as an exercise for the reader :)

However, read your compiler doc carefully. I'm not sure if the C standard requires that struct to fit in a word (although I'd expect most compilers to do that).