Description

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

   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.