Description

   The reentrant version of the getchar function is locked against
   multiple threads calling it simultaneously. This incurs overhead
   to ensure integrity of the input stream. The unlocked version of
   this call, getchar_unlocked can be used to avoid the overhead.
   The getchar_unlocked function is functionally identical to
   the getchar function, except that it is not required to be
   implemented in a thread-safe manner. The getchar_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 getchar_
   unlocked is used.

   See also flockfile, ftrylockfile, and funlockfile.