.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved .TH "FLOCKFILE" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual" .\" flockfile .SH NAME flockfile, ftrylockfile, funlockfile \- stdio locking functions .SH SYNOPSIS .LP \fB#include .br .sp void flockfile(FILE *\fP\fIfile\fP\fB); .br int ftrylockfile(FILE *\fP\fIfile\fP\fB); .br void funlockfile(FILE *\fP\fIfile\fP\fB); \fP \fB .br \fP .SH DESCRIPTION .LP These functions shall provide for explicit application-level locking of stdio ( \fBFILE *\fP) objects. These functions can be used by a thread to delineate a sequence of I/O statements that are executed as a unit. .LP The \fIflockfile\fP() function shall acquire for a thread ownership of a ( \fBFILE *\fP) object. .LP The \fIftrylockfile\fP() function shall acquire for a thread ownership of a ( \fBFILE *\fP) object if the object is available; \fIftrylockfile\fP() is a non-blocking version of \fIflockfile\fP(). .LP The \fIfunlockfile\fP() function shall relinquish the ownership granted to the thread. The behavior is undefined if a thread other than the current owner calls the \fIfunlockfile\fP() function. .LP The functions shall behave as if there is a lock count associated with each ( \fBFILE *\fP) object. This count is implicitly initialized to zero when the ( \fBFILE *\fP) object is created. The ( \fBFILE *\fP) object is unlocked when the count is zero. When the count is positive, a single thread owns the ( \fBFILE *\fP) object. When the \fIflockfile\fP() function is called, if the count is zero or if the count is positive and the caller owns the ( \fBFILE *\fP) object, the count shall be incremented. Otherwise, the calling thread shall be suspended, waiting for the count to return to zero. Each call to \fIfunlockfile\fP() shall decrement the count. This allows matching calls to \fIflockfile\fP() (or successful calls to \fIftrylockfile\fP()) and \fIfunlockfile\fP() to be nested. .LP All functions that reference ( \fBFILE *\fP) objects shall behave as if they use \fIflockfile\fP() and \fIfunlockfile\fP() internally to obtain ownership of these ( \fBFILE *\fP) objects. .SH RETURN VALUE .LP None for \fIflockfile\fP() and \fIfunlockfile\fP(). .LP The \fIftrylockfile\fP() function shall return zero for success and non-zero to indicate that the lock cannot be acquired. .SH ERRORS .LP No errors are defined. .LP \fIThe following sections are informative.\fP .SH EXAMPLES .LP None. .SH APPLICATION USAGE .LP 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. .SH RATIONALE .LP The \fIflockfile\fP() and \fIfunlockfile\fP() functions provide an orthogonal mutual-exclusion lock for each \fBFILE\fP. The \fIftrylockfile\fP() function provides a non-blocking attempt to acquire a file lock, analogous to \fIpthread_mutex_trylock\fP(). .LP These locks behave as if they are the same as those used internally by \fIstdio\fP for thread-safety. This both provides thread-safety of these functions without requiring a second level of internal locking and allows functions in \fIstdio\fP to be implemented in terms of other \fIstdio\fP functions. .LP Application writers and implementors should be aware that there are potential deadlock problems on \fBFILE\fP objects. For example, the line-buffered flushing semantics of \fIstdio\fP (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 \fBFILE\fP, deadlock ensues. This type of deadlock can be avoided by acquiring \fBFILE\fP 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. .LP In summary, threads sharing \fIstdio\fP streams with other threads can use \fIflockfile\fP() and \fIfunlockfile\fP() to cause sequences of I/O performed by a single thread to be kept bundled. The only case where the use of \fIflockfile\fP() and \fIfunlockfile\fP() is required is to provide a scope protecting uses of the \fI*_unlocked\fP() functions/macros. This moves the cost/performance tradeoff to the optimal point. .SH FUTURE DIRECTIONS .LP None. .SH SEE ALSO .LP \fIgetc_unlocked\fP() , \fIputc_unlocked\fP() , the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI\fP .SH 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 .