Description

   The reentrant version of the feof function is locked against
   multiple threads calling it simultaneously. This incurs overhead
   to ensure integrity of the stream. The unlocked version of this
   call, feof_unlocked can be used to avoid the overhead. The feof_
   unlocked function is functionally identical to the feof function,
   except that it is not required to be implemented in a thread-
   safe manner. The feof_unlocked function can be safely used only
   within a scope that is protected by the flockfile and funlockfile
   functions used as a pair. The caller must ensure that the stream
   is locked before feof_unlocked is used.

   See also flockfile, ftrylockfile, and funlockfile.