FLOCKFILE(3P)						     POSIX Programmer's Manual						     FLOCKFILE(3P)

PROLOG

       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME

       flockfile, ftrylockfile, funlockfile - stdio locking functions

SYNOPSIS

       #include <stdio.h>

       void flockfile(FILE *file);
       int ftrylockfile(FILE *file);
       void funlockfile(FILE *file);

DESCRIPTION

       These functions shall provide for explicit application-level locking of stdio ( FILE *) objects. These functions can be used by a thread to
       delineate a sequence of I/O statements that are executed as a unit.

       The flockfile() function shall acquire for a thread ownership of a ( FILE *) object.

       The ftrylockfile() function shall acquire for a thread ownership of a ( FILE *) object if the object is available; ftrylockfile() is a non-
       blocking version of flockfile().

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

       The  functions shall behave as if there is a lock count associated with each ( FILE *) object. This count is implicitly initialized 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 shall be incremented.  Otherwise, the calling thread shall be suspended, waiting for the count to return to
       zero.  Each  call  to  funlockfile() shall decrement the count. This allows matching calls to flockfile() (or successful calls to ftrylock-
       file()) and funlockfile() to be nested.

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

RETURN VALUE

       None for flockfile() and funlockfile().

       The ftrylockfile() function shall return zero for success and non-zero to indicate that the lock cannot be acquired.

ERRORS

       No errors are defined.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       Applications   using   these   functions  may  be  subject  to  priority  inversion,  as  discussed  in	the  Base  Definitions	volume	of
       IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.

RATIONALE

       The flockfile() and funlockfile() functions provide an orthogonal mutual-exclusion lock for each FILE. The ftrylockfile() function provides
       a non-blocking attempt to acquire a file lock, analogous to pthread_mutex_trylock().

       These  locks  behave as if they are the same as those used internally by stdio for thread-safety. This both provides thread-safety of these
       functions without requiring a second level of internal locking and allows functions in stdio to be implemented  in  terms  of  other  stdio
       functions.

       Application  writers  and  implementors	should be aware that there are potential deadlock problems on FILE objects. For example, the line-
       buffered flushing semantics of stdio (requested via {_IOLBF}) require that certain input operations sometimes cause the	buffered  contents
       of  implementation-defined  line-buffered  output  streams  to  be flushed. If two threads each hold the lock on the other's FILE, deadlock
       ensues. This type of deadlock can be avoided by acquiring FILE locks in a consistent order. In particular, the line-buffered output  stream
       deadlock can typically be avoided by acquiring locks on input streams before locks on output streams if a thread would be acquiring both.

       In summary, threads sharing stdio streams with other threads can use flockfile() and funlockfile() to cause sequences of I/O performed by a
       single thread to be kept bundled.  The only case where the use of flockfile() and funlockfile() is required is to provide a scope  protect-
       ing uses of the *_unlocked() functions/macros. This moves the cost/performance tradeoff to the optimal point.

FUTURE DIRECTIONS

       None.

SEE ALSO

       getc_unlocked(), putc_unlocked(), the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
       Electrical  and	Electronics  Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE
       and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can  be  obtained
       online at http://www.opengroup.org/unix/online.html .

IEEE
/The Open Group						       2003							     FLOCKFILE(3P)