I understand that
off_t to be 64bits. So what does
-D_LARGEFILE_SOURCE do that isn’t already done by
-D_FILE_OFFSET_BITS=64? What do these definitions do exactly?
The GLIBC Feature test macros documentation states:
If this macro is defined some extra functions are available which rectify a few shortcomings in all previous standards. Specifically, the functions fseeko and ftello are available. Without these functions the difference between the ISO C interface (fseek, ftell) and the low-level POSIX interface (lseek) would lead to problems.
This macro was introduced as part of the Large File Support extension (LFS).
So that macro specifically makes
_FILE_OFFSET_BITS settings alone don’t make these functions available.
(Note that if you’re using a GNU dialect of C, the default with GCC, you might not need to explicitly define
_LARGEFILE_SOURCE. You do if you use
-std=c99 for instance.)
Answered By – Mat