Description

   The reentrant version of the fgetc function is locked against
   multiple threads calling it simultaneously. This incurs overhead
   to ensure integrity of the stream. The unlocked version of this
   call, fgetc_unlocked can be used to avoid the overhead. The
   fgetc_unlocked function is functionally identical to the fgetc
   function, except that fgetc_unlocked 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 fgetc_unlocked is used.

   Compiling with the __UNIX_PUTC macro defined enables an
   optimization that uses a faster, inlined version of this
   function.

   See also getc_unlocked, flockfile, ftrylockfile, and funlockfile.