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 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
protecting 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 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 .