Originální popis anglicky:
flockfile, ftrylockfile, funlockfile - stdio locking functions
Návod, kniha: POSIX Programmer's Manual
#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 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
IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.
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 protecting uses of the
*_unlocked() functions/macros. This moves
the cost/performance tradeoff to the optimal point.
None.
getc_unlocked() ,
putc_unlocked() , the Base Definitions volume of
IEEE Std 1003.1-2001,
<stdio.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- 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
.
