d0080709e7
of fcntl(2), flock(2), and lockf(3) advisory locks. Add such a paragraph to the flock(2) manpage for the sake of consistency. Reviewed by: Cyrille Lefevre and Kirk McKusick on -arch MFC after: 2 weeks
584 lines
14 KiB
Groff
584 lines
14 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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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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 January 12, 1994
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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. 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_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.
|
|
.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).
|
|
.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;
|
|
process groups are specified by supplying
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|
.Fa arg
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|
as negative, otherwise
|
|
.Fa arg
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|
is interpreted as a process ID.
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|
.El
|
|
.Pp
|
|
The flags for the
|
|
.Dv F_GETFL
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|
and
|
|
.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
|
|
.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 .
|
|
.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
|
|
.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. The system
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|
will attempt to avoid caching the data you read or write. 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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|
};
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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,
|
|
.Fn fcntl
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|
returns immediately with
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|
.Er EAGAIN .
|
|
.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
|
|
.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 ) .
|
|
.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
|
|
lock on any portion of the protected area.
|
|
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
|
|
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.
|
|
.Pp
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|
The value of
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|
.Fa l_whence
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|
is
|
|
.Dv SEEK_SET ,
|
|
.Dv SEEK_CUR ,
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|
or
|
|
.Dv SEEK_END
|
|
to indicate that the relative offset,
|
|
.Fa l_start
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|
bytes, will be measured from the start of the file,
|
|
current position, or end of the file, respectively.
|
|
The value of
|
|
.Fa l_len
|
|
is the number of consecutive bytes to be locked.
|
|
If
|
|
.Fa l_len
|
|
is negative,
|
|
.Fa l_start
|
|
means end edge of the region.
|
|
The
|
|
.Fa l_pid
|
|
field is only used with
|
|
.Dv F_GETLK
|
|
to return the process ID of the process holding a blocking lock.
|
|
After a successful
|
|
.Dv F_GETLK
|
|
request, the value of
|
|
.Fa l_whence
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|
is
|
|
.Dv SEEK_SET .
|
|
.Pp
|
|
Locks may start and extend beyond the current end of a file,
|
|
but may not start or extend before the beginning of the file.
|
|
A lock is set to extend to the largest possible value of the
|
|
file offset for that file if
|
|
.Fa l_len
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|
is set to zero.
|
|
If
|
|
.Fa l_whence
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|
and
|
|
.Fa l_start
|
|
point to the beginning of the file, and
|
|
.Fa l_len
|
|
is zero, the entire file is locked.
|
|
If an application wishes only to do entire file locking, the
|
|
.Xr flock 2
|
|
system call is much more efficient.
|
|
.Pp
|
|
There is at most one type of lock set for each byte in the file.
|
|
Before a successful return from an
|
|
.Dv F_SETLK
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|
or an
|
|
.Dv F_SETLKW
|
|
request when the calling process has previously existing locks
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|
on bytes in the region specified by the request,
|
|
the previous lock type for each byte in the specified
|
|
region is replaced by the new lock type.
|
|
As specified above under the descriptions
|
|
of shared locks and exclusive locks, an
|
|
.Dv F_SETLK
|
|
or an
|
|
.Dv F_SETLKW
|
|
request fails or blocks respectively when another process has existing
|
|
locks on bytes in the specified region and the type of any of those
|
|
locks conflicts with the type specified in the request.
|
|
.Pp
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|
This interface follows the completely stupid semantics of System V and
|
|
.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
|
|
file descriptor for that file is closed by that process.
|
|
This semantic means that applications must be aware of any files that
|
|
a subroutine library may access.
|
|
For example if an application for updating the password file locks the
|
|
password file database while making the update, and then calls
|
|
.Xr getpwnam 3
|
|
to retrieve a record,
|
|
the lock will be lost because
|
|
.Xr getpwnam 3
|
|
opens, reads, and closes the password database.
|
|
The database close will release all locks that the process has
|
|
associated with the database, even if the library routine never
|
|
requested a lock on the database.
|
|
Another minor semantic problem with this interface is that
|
|
locks are not inherited by a child process created using the
|
|
.Xr fork 2
|
|
system call.
|
|
The
|
|
.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
|
|
of their locks when using library routines or wish to pass locks
|
|
to their children.
|
|
.Pp
|
|
The
|
|
.Fn fcntl ,
|
|
.Xr flock 2 ,
|
|
and
|
|
.Xr lockf 3
|
|
locks are compatible.
|
|
Processes using different locking interfaces can cooperate
|
|
over the same file safely.
|
|
However, only one of such interfaces should be used within
|
|
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
|
|
from the viewpoint of another process using
|
|
.Fn fcntl
|
|
or
|
|
.Xr lockf 3 ,
|
|
and vice versa.
|
|
Note that
|
|
.Fn fcntl F_GETLK
|
|
returns \-1 in
|
|
.Fa l_pid
|
|
if the process holding a blocking lock previously locked the
|
|
file descriptor by
|
|
.Xr flock 2 .
|
|
.Pp
|
|
All locks associated with a file for a given process are
|
|
removed when the process terminates.
|
|
.Pp
|
|
All locks obtained before a call to
|
|
.Xr execve 2
|
|
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
|
|
A potential for deadlock occurs if a process controlling a locked region
|
|
is put to sleep by attempting to lock the locked region of another process.
|
|
This implementation detects that sleeping until a locked region is unlocked
|
|
would cause a deadlock and fails with an
|
|
.Er EDEADLK
|
|
error.
|
|
.Sh RETURN VALUES
|
|
Upon successful completion, the value returned depends on
|
|
.Fa cmd
|
|
as follows:
|
|
.Bl -tag -width F_GETOWNX -offset indent
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|
.It Dv F_DUPFD
|
|
A new file descriptor.
|
|
.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
|
|
.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
|
|
.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_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
|
|
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 execve 2 ,
|
|
.Xr flock 2 ,
|
|
.Xr getdtablesize 2 ,
|
|
.Xr open 2 ,
|
|
.Xr sigvec 2 ,
|
|
.Xr lockf 3 ,
|
|
.Xr tcgetpgrp 3 ,
|
|
.Xr tcsetpgrp 3
|
|
.Sh HISTORY
|
|
The
|
|
.Fn fcntl
|
|
system call appeared in
|
|
.Bx 4.2 .
|