Description

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

   Since putc_unlocked is a macro, a file pointer argument with side
   effects might be evaluated incorrectly. In such a case, use the
   fputc_unlocked function instead.

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

   See also flockfile, ftrylockfile, and funlockfile.