clinuxlinux-from-scratch

What is the difference between _LARGEFILE_SOURCE and _FILE_OFFSET_BITS=64?

I understand that -D_FILE_OFFSET_BITS=64 causes 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?

Solution

  • The GLIBC Feature test macros documentation states:

    _LARGEFILE_SOURCE
    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 fseeko and ftello available. _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.)