Description

   The flockfile function locks a stdio stream so that a thread can
   have exclusive use of that stream for multiple I/O operations.
   Use the flockfile function for a thread that wants to ensure
   that the output of several printf functions, for example, is not
   garbled by another thread also trying to use printf.

   File pointers passed are assumed to be valid; flockfile will
   perform locking even on invalid file pointers. Also, the
   funlockfile function will not fail if the calling thread does
   not own a lock on the file pointer passed.

   Matching flockfile and funlockfile calls can be nested. If the
   stream has been locked recursively, it will remain locked until
   the last matching funlockfile is called.

   All C RTL file-pointer I/O functions lock their file pointers as
   if calling flockfile and funlockfile.

   See also ftrylockfile and funlockfile.