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user-mode lock manager, build a kernel with the NFSLOCKD option and add '-k' to 'rpc_lockd_flags' in rc.conf. Highlights include: * Thread-safe kernel RPC client - many threads can use the same RPC client handle safely with replies being de-multiplexed at the socket upcall (typically driven directly by the NIC interrupt) and handed off to whichever thread matches the reply. For UDP sockets, many RPC clients can share the same socket. This allows the use of a single privileged UDP port number to talk to an arbitrary number of remote hosts. * Single-threaded kernel RPC server. Adding support for multi-threaded server would be relatively straightforward and would follow approximately the Solaris KPI. A single thread should be sufficient for the NLM since it should rarely block in normal operation. * Kernel mode NLM server supporting cancel requests and granted callbacks. I've tested the NLM server reasonably extensively - it passes both my own tests and the NFS Connectathon locking tests running on Solaris, Mac OS X and Ubuntu Linux. * Userland NLM client supported. While the NLM server doesn't have support for the local NFS client's locking needs, it does have to field async replies and granted callbacks from remote NLMs that the local client has contacted. We relay these replies to the userland rpc.lockd over a local domain RPC socket. * Robust deadlock detection for the local lock manager. In particular it will detect deadlocks caused by a lock request that covers more than one blocking request. As required by the NLM protocol, all deadlock detection happens synchronously - a user is guaranteed that if a lock request isn't rejected immediately, the lock will eventually be granted. The old system allowed for a 'deferred deadlock' condition where a blocked lock request could wake up and find that some other deadlock-causing lock owner had beaten them to the lock. * Since both local and remote locks are managed by the same kernel locking code, local and remote processes can safely use file locks for mutual exclusion. Local processes have no fairness advantage compared to remote processes when contending to lock a region that has just been unlocked - the local lock manager enforces a strict first-come first-served model for both local and remote lockers. Sponsored by: Isilon Systems PR: 95247 107555 115524 116679 MFC after: 2 weeks
624 lines
15 KiB
Groff
624 lines
15 KiB
Groff
.\" Copyright (c) 1983, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
|
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
|
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
|
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)fcntl.2 8.2 (Berkeley) 1/12/94
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.\" $FreeBSD$
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.\"
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.Dd March 8, 2008
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.Dt FCNTL 2
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.Os
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.Sh NAME
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.Nm fcntl
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.Nd file control
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In fcntl.h
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.Ft int
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.Fn fcntl "int fd" "int cmd" "..."
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.Sh DESCRIPTION
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The
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.Fn fcntl
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system call provides for control over descriptors.
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The argument
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.Fa fd
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is a descriptor to be operated on by
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.Fa cmd
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as described below.
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Depending on the value of
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.Fa cmd ,
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.Fn fcntl
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can take an additional third argument
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.Fa "int arg" .
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.Bl -tag -width F_GETOWNX
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.It Dv F_DUPFD
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Return a new descriptor as follows:
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.Pp
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.Bl -bullet -compact -offset 4n
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.It
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Lowest numbered available descriptor greater than or equal to
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.Fa arg .
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.It
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Same object references as the original descriptor.
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.It
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New descriptor shares the same file offset if the object
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was a file.
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.It
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Same access mode (read, write or read/write).
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.It
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Same file status flags (i.e., both file descriptors
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share the same file status flags).
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.It
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The close-on-exec flag associated with the new file descriptor
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is set to remain open across
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.Xr execve 2
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system calls.
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.El
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.It Dv F_DUP2FD
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It is functionally equivalent to
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.Bd -literal -offset indent
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dup2(fd, arg)
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.Ed
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.Pp
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The
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.Dv F_DUP2FD
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constant is not portable, so it should not be used if portability is needed.
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Use
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.Fn dup2
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instead.
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.It Dv F_GETFD
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Get the close-on-exec flag associated with the file descriptor
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.Fa fd
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as
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.Dv FD_CLOEXEC .
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If the returned value ANDed with
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.Dv FD_CLOEXEC
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is 0,
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the file will remain open across
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.Fn exec ,
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otherwise the file will be closed upon execution of
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.Fn exec
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.Fa ( arg
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is ignored).
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.It Dv F_SETFD
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Set the close-on-exec flag associated with
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.Fa fd
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to
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.Fa arg ,
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where
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.Fa arg
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is either 0 or
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.Dv FD_CLOEXEC ,
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|
as described above.
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.It Dv F_GETFL
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Get descriptor status flags, as described below
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.Fa ( arg
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is ignored).
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.It Dv F_SETFL
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Set descriptor status flags to
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.Fa arg .
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.It Dv F_GETOWN
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Get the process ID or process group
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currently receiving
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.Dv SIGIO
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and
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.Dv SIGURG
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signals; process groups are returned
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as negative values
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.Fa ( arg
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|
is ignored).
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.It Dv F_SETOWN
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|
Set the process or process group
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to receive
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.Dv SIGIO
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and
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.Dv SIGURG
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signals;
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process groups are specified by supplying
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.Fa arg
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as negative, otherwise
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.Fa arg
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is interpreted as a process ID.
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.El
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.Pp
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The flags for the
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.Dv F_GETFL
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and
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.Dv F_SETFL
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flags are as follows:
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|
.Bl -tag -width O_NONBLOCKX
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.It Dv O_NONBLOCK
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|
Non-blocking I/O; if no data is available to a
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.Xr read 2
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|
system call, or if a
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.Xr write 2
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operation would block,
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the read or write call returns -1 with the error
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.Er EAGAIN .
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.It Dv O_APPEND
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|
Force each write to append at the end of file;
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corresponds to the
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.Dv O_APPEND
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flag of
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.Xr open 2 .
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.It Dv O_DIRECT
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|
Minimize or eliminate the cache effects of reading and writing.
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|
The system
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will attempt to avoid caching the data you read or write.
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If it cannot
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avoid caching the data, it will minimize the impact the data has on the cache.
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Use of this flag can drastically reduce performance if not used with care.
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.It Dv O_ASYNC
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Enable the
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.Dv SIGIO
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signal to be sent to the process group
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when I/O is possible, e.g.,
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upon availability of data to be read.
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.El
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.Pp
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Several commands are available for doing advisory file locking;
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they all operate on the following structure:
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.Bd -literal
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struct flock {
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off_t l_start; /* starting offset */
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off_t l_len; /* len = 0 means until end of file */
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pid_t l_pid; /* lock owner */
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short l_type; /* lock type: read/write, etc. */
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short l_whence; /* type of l_start */
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int l_sysid; /* remote system id or zero for local */
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};
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.Ed
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The commands available for advisory record locking are as follows:
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.Bl -tag -width F_SETLKWX
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.It Dv F_GETLK
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Get the first lock that blocks the lock description pointed to by the
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third argument,
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.Fa arg ,
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taken as a pointer to a
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.Fa "struct flock"
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(see above).
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The information retrieved overwrites the information passed to
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.Fn fcntl
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in the
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.Fa flock
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structure.
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If no lock is found that would prevent this lock from being created,
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the structure is left unchanged by this system call except for the
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lock type which is set to
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.Dv F_UNLCK .
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.It Dv F_SETLK
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Set or clear a file segment lock according to the lock description
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pointed to by the third argument,
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.Fa arg ,
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taken as a pointer to a
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.Fa "struct flock"
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(see above).
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.Dv F_SETLK
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is used to establish shared (or read) locks
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.Pq Dv F_RDLCK
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or exclusive (or write) locks,
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.Pq Dv F_WRLCK ,
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as well as remove either type of lock
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.Pq Dv F_UNLCK .
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If a shared or exclusive lock cannot be set,
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.Fn fcntl
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returns immediately with
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.Er EAGAIN .
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.It Dv F_SETLKW
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This command is the same as
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.Dv F_SETLK
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|
except that if a shared or exclusive lock is blocked by other locks,
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the process waits until the request can be satisfied.
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If a signal that is to be caught is received while
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.Fn fcntl
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is waiting for a region, the
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.Fn fcntl
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will be interrupted if the signal handler has not specified the
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.Dv SA_RESTART
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|
(see
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.Xr sigaction 2 ) .
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.El
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.Pp
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When a shared lock has been set on a segment of a file,
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other processes can set shared locks on that segment
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or a portion of it.
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A shared lock prevents any other process from setting an exclusive
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lock on any portion of the protected area.
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A request for a shared lock fails if the file descriptor was not
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opened with read access.
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.Pp
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An exclusive lock prevents any other process from setting a shared lock or
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an exclusive lock on any portion of the protected area.
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A request for an exclusive lock fails if the file was not
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opened with write access.
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.Pp
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The value of
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.Fa l_whence
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is
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.Dv SEEK_SET ,
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.Dv SEEK_CUR ,
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|
or
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|
.Dv SEEK_END
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|
to indicate that the relative offset,
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.Fa l_start
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|
bytes, will be measured from the start of the file,
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|
current position, or end of the file, respectively.
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|
The value of
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|
.Fa l_len
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|
is the number of consecutive bytes to be locked.
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If
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.Fa l_len
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is negative,
|
|
.Fa l_start
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|
means end edge of the region.
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The
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|
.Fa l_pid
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and
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|
.Fa l_sysid
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fields are only used with
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.Dv F_GETLK
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|
to return the process ID of the process holding a blocking lock and
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the system ID of the system that owns that process.
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|
Locks created by the local system will have a system ID of zero.
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|
After a successful
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.Dv F_GETLK
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|
request, the value of
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.Fa l_whence
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is
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.Dv SEEK_SET .
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.Pp
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|
Locks may start and extend beyond the current end of a file,
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|
but may not start or extend before the beginning of the file.
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|
A lock is set to extend to the largest possible value of the
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file offset for that file if
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.Fa l_len
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|
is set to zero.
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|
If
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|
.Fa l_whence
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|
and
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.Fa l_start
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|
point to the beginning of the file, and
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.Fa l_len
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|
is zero, the entire file is locked.
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|
If an application wishes only to do entire file locking, the
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.Xr flock 2
|
|
system call is much more efficient.
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|
.Pp
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There is at most one type of lock set for each byte in the file.
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|
Before a successful return from an
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.Dv F_SETLK
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|
or an
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.Dv F_SETLKW
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|
request when the calling process has previously existing locks
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|
on bytes in the region specified by the request,
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|
the previous lock type for each byte in the specified
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region is replaced by the new lock type.
|
|
As specified above under the descriptions
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|
of shared locks and exclusive locks, an
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|
.Dv F_SETLK
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|
or an
|
|
.Dv F_SETLKW
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|
request fails or blocks respectively when another process has existing
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locks on bytes in the specified region and the type of any of those
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locks conflicts with the type specified in the request.
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.Pp
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|
This interface follows the completely stupid semantics of System V and
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.St -p1003.1-88
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|
that require that all locks associated with a file for a given process are
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removed when
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.Em any
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file descriptor for that file is closed by that process.
|
|
This semantic means that applications must be aware of any files that
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a subroutine library may access.
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|
For example if an application for updating the password file locks the
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password file database while making the update, and then calls
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.Xr getpwnam 3
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|
to retrieve a record,
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|
the lock will be lost because
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|
.Xr getpwnam 3
|
|
opens, reads, and closes the password database.
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The database close will release all locks that the process has
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associated with the database, even if the library routine never
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requested a lock on the database.
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|
Another minor semantic problem with this interface is that
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locks are not inherited by a child process created using the
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.Xr fork 2
|
|
system call.
|
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The
|
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.Xr flock 2
|
|
interface has much more rational last close semantics and
|
|
allows locks to be inherited by child processes.
|
|
The
|
|
.Xr flock 2
|
|
system call is recommended for applications that want to ensure the integrity
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of their locks when using library routines or wish to pass locks
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to their children.
|
|
.Pp
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|
The
|
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.Fn fcntl ,
|
|
.Xr flock 2 ,
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and
|
|
.Xr lockf 3
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|
locks are compatible.
|
|
Processes using different locking interfaces can cooperate
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over the same file safely.
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|
However, only one of such interfaces should be used within
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the same process.
|
|
If a file is locked by a process through
|
|
.Xr flock 2 ,
|
|
any record within the file will be seen as locked
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|
from the viewpoint of another process using
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.Fn fcntl
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|
or
|
|
.Xr lockf 3 ,
|
|
and vice versa.
|
|
Note that
|
|
.Fn fcntl F_GETLK
|
|
returns \-1 in
|
|
.Fa l_pid
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|
if the process holding a blocking lock previously locked the
|
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file descriptor by
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|
.Xr flock 2 .
|
|
.Pp
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|
All locks associated with a file for a given process are
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|
removed when the process terminates.
|
|
.Pp
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|
All locks obtained before a call to
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|
.Xr execve 2
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|
remain in effect until the new program releases them.
|
|
If the new program does not know about the locks, they will not be
|
|
released until the program exits.
|
|
.Pp
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|
A potential for deadlock occurs if a process controlling a locked region
|
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is put to sleep by attempting to lock the locked region of another process.
|
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This implementation detects that sleeping until a locked region is unlocked
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would cause a deadlock and fails with an
|
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.Er EDEADLK
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|
error.
|
|
.Sh RETURN VALUES
|
|
Upon successful completion, the value returned depends on
|
|
.Fa cmd
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|
as follows:
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|
.Bl -tag -width F_GETOWNX -offset indent
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|
.It Dv F_DUPFD
|
|
A new file descriptor.
|
|
.It Dv F_DUP2FD
|
|
A file descriptor equal to
|
|
.Fa arg .
|
|
.It Dv F_GETFD
|
|
Value of flag (only the low-order bit is defined).
|
|
.It Dv F_GETFL
|
|
Value of flags.
|
|
.It Dv F_GETOWN
|
|
Value of file descriptor owner.
|
|
.It other
|
|
Value other than -1.
|
|
.El
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|
.Pp
|
|
Otherwise, a value of -1 is returned and
|
|
.Va errno
|
|
is set to indicate the error.
|
|
.Sh ERRORS
|
|
The
|
|
.Fn fcntl
|
|
system call will fail if:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EAGAIN
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_SETLK ,
|
|
the type of lock
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|
.Pq Fa l_type
|
|
is a shared lock
|
|
.Pq Dv F_RDLCK
|
|
or exclusive lock
|
|
.Pq Dv F_WRLCK ,
|
|
and the segment of a file to be locked is already
|
|
exclusive-locked by another process;
|
|
or the type is an exclusive lock and some portion of the
|
|
segment of a file to be locked is already shared-locked or
|
|
exclusive-locked by another process.
|
|
.It Bq Er EBADF
|
|
The
|
|
.Fa fd
|
|
argument
|
|
is not a valid open file descriptor.
|
|
.Pp
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_DUP2FD ,
|
|
and
|
|
.Fa arg
|
|
is not a valid file descriptor.
|
|
.Pp
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_SETLK
|
|
or
|
|
.Dv F_SETLKW ,
|
|
the type of lock
|
|
.Pq Fa l_type
|
|
is a shared lock
|
|
.Pq Dv F_RDLCK ,
|
|
and
|
|
.Fa fd
|
|
is not a valid file descriptor open for reading.
|
|
.Pp
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_SETLK
|
|
or
|
|
.Dv F_SETLKW ,
|
|
the type of lock
|
|
.Pq Fa l_type
|
|
is an exclusive lock
|
|
.Pq Dv F_WRLCK ,
|
|
and
|
|
.Fa fd
|
|
is not a valid file descriptor open for writing.
|
|
.It Bq Er EDEADLK
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_SETLKW ,
|
|
and a deadlock condition was detected.
|
|
.It Bq Er EINTR
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_SETLKW ,
|
|
and the system call was interrupted by a signal.
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa cmd
|
|
argument
|
|
is
|
|
.Dv F_DUPFD
|
|
and
|
|
.Fa arg
|
|
is negative or greater than the maximum allowable number
|
|
(see
|
|
.Xr getdtablesize 2 ) .
|
|
.Pp
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_GETLK ,
|
|
.Dv F_SETLK
|
|
or
|
|
.Dv F_SETLKW
|
|
and the data to which
|
|
.Fa arg
|
|
points is not valid.
|
|
.It Bq Er EMFILE
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_DUPFD
|
|
or
|
|
.Dv F_DUP2FD
|
|
and the maximum number of file descriptors permitted for the
|
|
process are already in use,
|
|
or no file descriptors greater than or equal to
|
|
.Fa arg
|
|
are available.
|
|
.It Bq Er ENOLCK
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_SETLK
|
|
or
|
|
.Dv F_SETLKW ,
|
|
and satisfying the lock or unlock request would result in the
|
|
number of locked regions in the system exceeding a system-imposed limit.
|
|
.It Bq Er EOPNOTSUPP
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_GETLK ,
|
|
.Dv F_SETLK
|
|
or
|
|
.Dv F_SETLKW
|
|
and
|
|
.Fa fd
|
|
refers to a file for which locking is not supported.
|
|
.It Bq Er EOVERFLOW
|
|
The argument
|
|
.Fa cmd
|
|
is
|
|
.Dv F_GETLK ,
|
|
.Dv F_SETLK
|
|
or
|
|
.Dv F_SETLKW
|
|
and an
|
|
.Fa off_t
|
|
calculation overflowed.
|
|
.It Bq Er EPERM
|
|
The
|
|
.Fa cmd
|
|
argument
|
|
is
|
|
.Dv F_SETOWN
|
|
and
|
|
the process ID or process group given as an argument is in a
|
|
different session than the caller.
|
|
.It Bq Er ESRCH
|
|
The
|
|
.Fa cmd
|
|
argument
|
|
is
|
|
.Dv F_SETOWN
|
|
and
|
|
the process ID given as argument is not in use.
|
|
.El
|
|
.Pp
|
|
In addition, if
|
|
.Fa fd
|
|
refers to a descriptor open on a terminal device (as opposed to a
|
|
descriptor open on a socket), a
|
|
.Fa cmd
|
|
of
|
|
.Dv F_SETOWN
|
|
can fail for the same reasons as in
|
|
.Xr tcsetpgrp 3 ,
|
|
and a
|
|
.Fa cmd
|
|
of
|
|
.Dv F_GETOWN
|
|
for the reasons as stated in
|
|
.Xr tcgetpgrp 3 .
|
|
.Sh SEE ALSO
|
|
.Xr close 2 ,
|
|
.Xr dup2 2 ,
|
|
.Xr execve 2 ,
|
|
.Xr flock 2 ,
|
|
.Xr getdtablesize 2 ,
|
|
.Xr open 2 ,
|
|
.Xr sigvec 2 ,
|
|
.Xr lockf 3 ,
|
|
.Xr tcgetpgrp 3 ,
|
|
.Xr tcsetpgrp 3
|
|
.Sh STANDARDS
|
|
The
|
|
.Dv F_DUP2FD
|
|
constant is non portable.
|
|
It is provided for compatibility with AIX and Solaris.
|
|
.Sh HISTORY
|
|
The
|
|
.Fn fcntl
|
|
system call appeared in
|
|
.Bx 4.2 .
|
|
.Pp
|
|
The
|
|
.Dv F_DUP2FD
|
|
constant first appeared in
|
|
.Fx 7.1 .
|