FLOCKFILE(3)              OpenBSD Programmer's Manual             FLOCKFILE(3)

NAME
     flockfile, ftrylockfile, funlockfile - application level locking of stdio
     files

SYNOPSIS
     #include <stdio.h>

     void
     flockfile(FILE *file);

     int
     ftrylockfile(FILE *file);

     void
     funlockfile(FILE *file);

DESCRIPTION
     The flockfile(), ftrylockfile(), and funlockfile() functions provide for
     explicit application-level locking of stdio FILE * objects. These func-
     tions can be used by a thread to delineate a sequence of I/O statements
     that are to be executed as a unit.

     The flockfile() function is used by a thread to acquire ownership of a
     FILE * object.

     The ftrylockfile() function is used by a thread to acquire ownership of a
     FILE * object if the object is available; ftrylockfile() is a non-block-
     ing version of flockfile().

     The funlockfile() function is used to relinquish the ownership granted to
     the thread.  The behaviour is undefined if a thread other than the cur-
     rent owner calls the funlockfile() function.

     Logically, there is a lock count associated with each FILE * object. This
     count is implicitly intialized to zero when the FILE * object is created.
     The FILE * object is unlocked when the count is zero.  When the count is
     positive, a single thread owns the FILE * object. When the flockfile()
     function is called, if the count is zero or if the count is positive and
     the caller owns the FILE * object, the count is incremented.  Otherwise,
     the calling thread is suspended, waiting for the count to return to zero.
     Each call to funlockfile() decrements the count. This allows matching
     calls to flockfile() (or successful calls to ftrylockfile()) and
     funlockfile() to be nested.

     Library functions that reference FILE * behave as if they use flockfile()
     and funlockfile() internally to obtain ownership of these FILE * objects.

RETURN VALUES
     None for flockfile() and funlockfile().  The function ftrylock() returns
     zero for success and non-zero to indicate that the lock cannot be ac-
     quired.

ERRORS
     None.

SEE ALSO
     getc_unlocked(3),  getchar_unlocked(3),  pthreads(3),  putc_unlocked(3),
     putchar_unlocked(3)

STANDARDS
     flockfile(), ftrylockfile() and funlockfile() conform to ISO/IEC 9945-1
     ANSI/IEEE (``POSIX'') Std 1003.1c/D10.

OpenBSD 2.6                     August 20, 1998                              1
