.\"
.\" This file and its contents are supplied under the terms of the
.\" Common Development and Distribution License ("CDDL"), version 1.0.
.\" You may only use this file in accordance with the terms of version
.\" 1.0 of the CDDL.
.\"
.\" A full copy of the text of the CDDL should have accompanied this
.\" source.  A copy of the CDDL is also available via the Internet at
.\" http://www.illumos.org/license/CDDL.
.\"
.\"
.\" Copyright 1989 AT&T
.\" Copyright (c) 2002, Sun Microsystems, Inc.  All Rights Reserved
.\" Portions Copyright (c) 1992, X/Open Company Limited  All Rights Reserved
.\" Copyright 2015 Joyent, Inc.
.\"
.TH FLOCK 3C "Feb 16, 2015"
.SH NAME
flock \- OFD(open file description)-style file locking
.SH SYNOPSIS
.LP
.nf
#include <sys/file.h>

\fBint\fR \fBflock\fR(\fBint\fR \fIfildes\fR, \fBint\fR \fIoperation\fR);
.fi

.SH DESCRIPTION
.LP
The \fBflock()\fR function allows advisory locks to be applied to and removed
from a file. Calls to \fBflock()\fR from callers that attempt to lock
the locked file section via a different open file handle will either return an
error value or be put to sleep until the resource becomes unlocked.
See \fBfcntl\fR(2) for more information about record locking. Locks created or
removed via this function will apply to the entire file, including any future
growth in the file's length.
.sp
.LP
The \fIfildes\fR argument is an open file descriptor. A lock can be established
without regard for the mode with which the file was opened.
.sp
.LP
The \fIoperation\fR argument is a control value that specifies the action to be
taken. The permissible values for \fIoperation\fR are defined in
<\fBsys/file.h\fR> as follows:
.sp
.in +2
.nf
#define   LOCK_SH   1   /* shared file lock */
#define   LOCK_EX   2   /* exclusive file lock */
#define   LOCK_NB   4   /* do not block when attempting to create lock */
#define   LOCK_UN   8   /* remove existing file lock */
.fi
.in -2

.sp
.LP
To create a lock, either \fBLOCK_SH\fR or \fBLOCK_EX\fR should be specified,
optionally bitwise-ored with \fBLOCK_NB\fR.  To remove a lock, \fBLOCK_UN\fR
should be specified. All other values of \fIoperation\fR are reserved for
future extensions and will result in an error if not implemented.
.sp
.LP
This function creates, upgrades, downgrades, or removes either shared or
exclusive OFD-style locks. Locks created by this function are owned by open
files, not file descriptors. That is, file descriptors duplicated through
\fBdup\fR(2), \fBfork\fR(2), or \fBfcntl\fR(2) do not result in multiple
instances of a lock, but rather multiple references to the same lock. If a
process holding a lock on a file forks and the child explicitly unlocks the
file, the parent will lose its lock.  See \fBfcntl\fR(2) for more information
about file locking and the interaction between locks created by this function
and those created by other mechanisms. These locks are currently not supported
over remote file systems (e.g. \fBnfs\fR(5)) which use the Network Lock
Manager.
.sp
.LP
Sleeping on a resource is interrupted with any signal. The \fBalarm\fR(2)
function may be used to provide a timeout facility in applications that require
this facility.
.SH RETURN VALUES
.LP
Upon successful completion, \fB0\fR is returned.  Otherwise, \fB\(mi1\fR is
returned and \fBerrno\fR is set to indicate the error.
.SH ERRORS
.LP
The \fBflock()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 20n
The \fIfildes\fR argument is not a valid open file descriptor; or
\fIoperation\fR contains \fBLOCK_SH\fR and \fIfiledes\fR is not open for
reading; or \fIoperation\fR contains \fBLOCK_EX\fR and \fIfiledes\fR is not
open for writing.
.RE

.sp
.ne 2
.na
\fB\fBEWOULDBLOCK\fR\fR
.ad
.RS 20n
The \fIoperation\fR argument contains \fBLOCK_NB\fR and a conflicting lock
exists.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 20n
A signal was caught during execution of the function.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 20n
The \fIoperation\fR argument does not contain one of \fBLOCK_SH\fR,
\fBLOCK_EX\fR, or \fBLOCK_UN\fR; or the \fIoperation\fR argument contains
\fBLOCK_UN\fR and \fBLOCK_NB\fR; or the \fIoperation\fR argument contains
any bits other than those set by \fBLOCK_SH\fR, \fBLOCK_EX\fR, \fBLOCK_NB\fR,
and \fBLOCK_UN\fR.
.RE

.sp
.LP
The \fBflock()\fR function may fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR\fR
.ad
.RS 24n
The \fIoperation\fR argument contains \fBLOCK_SH\fR or \fBLOCK_EX\fR and the
file is mapped with \fBmmap\fR(2).
.RE

.sp
.ne 2
.na
\fB\fBENOLCK\fR\fR
.ad
.RS 20n
The number of locked file regions in the system would exceed a system-imposed
limit.
.RE

.sp
.ne 2
.na
\fB\fBEOPNOTSUPP\fR
.ad
.RS 24n
The locking of files of the type indicated by the \fIfildes\fR argument is not
supported.
.RE

.SH USAGE
.LP
File-locking should not be used in combination with the \fBfopen\fR(3C),
\fBfread\fR(3C), \fBfwrite\fR(3C) and other \fBstdio\fR functions.  Instead,
the more primitive, non-buffered functions (such as \fBopen\fR(2)) should be
used.  Unexpected results may occur in processes that do buffering in the user
address space.  The process may later read/write data which is/was locked.  The
\fBstdio\fR functions are the most common source of unexpected buffering.
.sp
.LP
The \fBalarm\fR(2) function may be used to provide a timeout facility in
applications requiring it.
.sp
.LP
Locks created by this facility conflict with those created by the
\fBlockf\fR(3C) and \fBfcntl\fR(2) facilities.  This facility creates and
removed OFD-style locks; see \fBfcntl\fR(2) for information about the
interaction between OFD-style and POSIX-style file locks.
.SH ATTRIBUTES
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.LP
.BR Intro (2),
.BR alarm (2),
.BR chmod (2),
.BR close (2),
.BR creat (2),
.BR fcntl (2),
.BR mmap (2),
.BR open (2),
.BR read (2),
.BR write (2),
.BR attributes (7),
.BR standards (7)