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+'\" et
+.TH FLOCKFILE "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
+.\"
+.SH PROLOG
+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.
+.\"
+.SH NAME
+flockfile,
+ftrylockfile,
+funlockfile
+\(em stdio locking functions
+.SH SYNOPSIS
+.LP
+.nf
+#include <stdio.h>
+.P
+void flockfile(FILE *\fIfile\fP);
+int ftrylockfile(FILE *\fIfile\fP);
+void funlockfile(FILE *\fIfile\fP);
+.fi
+.SH DESCRIPTION
+These functions shall provide for explicit application-level locking of
+stdio (\c
+.BR "FILE *" )
+objects. These functions can be used by a thread to delineate a
+sequence of I/O statements that are executed as a unit.
+.P
+The
+\fIflockfile\fR()
+function shall acquire for a thread ownership of a (\c
+.BR "FILE *" )
+object.
+.P
+The
+\fIftrylockfile\fR()
+function shall acquire for a thread ownership of a (\c
+.BR "FILE *" )
+object if the object is available;
+\fIftrylockfile\fR()
+is a non-blocking version of
+\fIflockfile\fR().
+.P
+The
+\fIfunlockfile\fR()
+function shall relinquish the ownership granted to the thread.
+The behavior is undefined if a thread other than the current owner
+calls the
+\fIfunlockfile\fR()
+function.
+.P
+The functions shall behave as if there is a lock count associated with
+each (\c
+.BR "FILE *" )
+object. This count is implicitly initialized to zero when the (\c
+.BR "FILE *" )
+object is created. The (\c
+.BR "FILE *" )
+object is unlocked when the count is zero. When the count is positive,
+a single thread owns the (\c
+.BR "FILE *" )
+object. When the
+\fIflockfile\fR()
+function is called, if the count is zero or if the count is positive
+and the caller owns the (\c
+.BR "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
+\fIfunlockfile\fR()
+shall decrement the count. This allows matching calls to
+\fIflockfile\fR()
+(or successful calls to
+\fIftrylockfile\fR())
+and
+\fIfunlockfile\fR()
+to be nested.
+.P
+All functions that reference (\c
+.BR "FILE *" )
+objects, except those with names ending in
+.IR _unlocked ,
+shall behave as if they use
+\fIflockfile\fR()
+and
+\fIfunlockfile\fR()
+internally to obtain ownership of these (\c
+.BR "FILE *" )
+objects.
+.SH "RETURN VALUE"
+None for
+\fIflockfile\fR()
+and
+\fIfunlockfile\fR().
+.P
+The
+\fIftrylockfile\fR()
+function shall return zero for success and non-zero to indicate
+that the lock cannot be acquired.
+.SH ERRORS
+No errors are defined.
+.LP
+.IR "The following sections are informative."
+.SH EXAMPLES
+None.
+.SH "APPLICATION USAGE"
+Applications using these functions may be subject to priority inversion,
+as discussed in the Base Definitions volume of POSIX.1\(hy2017,
+.IR "Section 3.291" ", " "Priority Inversion".
+.P
+A call to
+\fIexit\fR()
+can block until locked streams are unlocked because a thread having
+ownership of a (\c
+.BR FILE *)
+object blocks all function calls that reference that (\c
+.BR FILE *)
+object (except those with names ending in _unlocked) from other threads,
+including calls to
+\fIexit\fR().
+.SH RATIONALE
+The
+\fIflockfile\fR()
+and
+\fIfunlockfile\fR()
+functions provide an orthogonal mutual-exclusion lock for each
+.BR FILE .
+The
+\fIftrylockfile\fR()
+function provides a non-blocking attempt to acquire a file lock,
+analogous to
+\fIpthread_mutex_trylock\fR().
+.P
+These locks behave as if they are the same as those used internally by
+.IR stdio
+for thread-safety.
+This both provides thread-safety of these functions without requiring a
+second level of internal locking and allows functions in
+.IR stdio
+to be implemented in terms of other
+.IR stdio
+functions.
+.P
+Application developers and implementors should be aware that there are
+potential deadlock problems on
+.BR FILE
+objects. For example, the line-buffered flushing semantics of
+.IR 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
+.BR FILE ,
+deadlock ensues. This type of deadlock can be avoided by acquiring
+.BR 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.
+.P
+In summary, threads sharing
+.IR stdio
+streams with other threads can use
+\fIflockfile\fR()
+and
+\fIfunlockfile\fR()
+to cause sequences of I/O performed by a single thread to be kept
+bundled. The only case where the use of
+\fIflockfile\fR()
+and
+\fIfunlockfile\fR()
+is required is to provide a scope protecting uses of the
+.IR *_unlocked
+functions/macros. This moves the cost/performance tradeoff to the
+optimal point.
+.SH "FUTURE DIRECTIONS"
+None.
+.SH "SEE ALSO"
+.IR "\fIexit\fR\^(\|)",
+.IR "\fIgetc_unlocked\fR\^(\|)"
+.P
+The Base Definitions volume of POSIX.1\(hy2017,
+.IR "Section 3.291" ", " "Priority Inversion",
+.IR "\fB<stdio.h>\fP"
+.\"
+.SH COPYRIGHT
+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 .
+.PP
+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 .