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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FLOCKFILE(3P) POSIX Programmer's Manual FLOCKFILE(3P)
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.
flockfile, ftrylockfile, funlockfile — stdio locking functions
#include <stdio.h>
void flockfile(FILE *file);
int ftrylockfile(FILE *file);
void funlockfile(FILE *file);
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.
None for flockfile() and funlockfile().
The ftrylockfile() function shall return zero for success and
non-zero to indicate that the lock cannot be acquired.
No errors are defined.
The following sections are informative.
None.
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().
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.
None.
exit(3p), getc_unlocked(3p)
The Base Definitions volume of POSIX.1‐2017, Section 3.291,
Priority Inversion, stdio.h(0p)
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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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