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



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

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
       current 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 ftrylockfile()) and
       funlockfile() to be nested.

       All functions that reference (FILE **) objects, except those with names
       ending in __unlocked, 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 POSIX.1-2017,
       Section 3.291, Priority Inversion.

       A call to exit() can block until locked streams are unlocked because a
       thread having ownership of a (FILE*) object blocks all function calls
       that reference that (FILE*) object (except those with names ending in
       _unlocked) from other threads, including calls to exit().

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 developers 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
       protecting uses of the _*__unlocked functions/macros. This moves the
       cost/performance tradeoff to the optimal point.

FUTURE DIRECTIONS
       None.

SEE ALSO
       exit(), getc__unlocked()

       The Base Definitions volume of POSIX.1-2017, Section 3.291, Priority
       Inversion, <stdio.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                         FLOCKFILE(3)
