flockfile(3p) — Linux manual page


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

PROLOG         top

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

NAME         top

       flockfile, ftrylockfile, funlockfile — stdio locking functions

SYNOPSIS         top

       #include <stdio.h>

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

DESCRIPTION         top

       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         top

       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         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top



       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         top

       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

       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.



SEE ALSO         top

       exit(3p), getc_unlocked(3p)

       The Base Definitions volume of POSIX.1‐2017, Section 3.291,
       Priority Inversion, stdio.h(0p)

COPYRIGHT         top

       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(3P)

Pages that refer to this page: stdio.h(0p)ftrylockfile(3p)funlockfile(3p)getc_unlocked(3p)