__USE_FILE_OFFSET64 vs. _FILE_OFFSET_BITS=64

Question

I am trying to maintain code that compiles on lots of different systems. I\'ve seen a dozen different ways of asking for lseek that takes 64-bits. Some systems use

Answer 1

In features.h, you see the relation between _FILE_OFFSET_BITS and __USE_FILE_OFFSET64.

#if defined _FILE_OFFSET_BITS && _FILE_OFFSET_BITS == 64
# define __USE_FILE_OFFSET64    1
#endif

So, only _FILE_OFFSET_BITS is meant for the users.