543 lines
14 KiB
Groff
543 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 BSD 4.2
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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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.Fd #include <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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.Fn Fcntl
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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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.Nm
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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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If the low-order bit of the returned value 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 the low order bit of
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.Fa arg
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(0 or 1 as 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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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. 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 function 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, the result is undefined.
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The
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.Fa l_pid
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field is 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.
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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
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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.
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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
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.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.
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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
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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
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function.
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The
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.Xr flock 2
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interface has much more rational last close semantics and
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allows locks to be inherited by child processes.
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.Xr Flock 2
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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.
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Note that
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.Xr flock 2
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and
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.Xr fcntl 2
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locks may be safely used concurrently.
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.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.
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.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.
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If the new program does not know about the locks, they will not be
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released until the program exits.
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.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.
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.Sh IMPLEMENTATION NOTES
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In the non-threaded library
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.Fn fcntl
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is implemented as the
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.Va fcntl
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syscall.
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.Pp
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In the threaded library, the
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.Va fcntl
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syscall is assembled to
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.Fn _thread_sys_fcntl
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and
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.Fn fcntl
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is implemented as a function which disables thread rescheduling, locks
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.Fa fd
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for read and write, then calls
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.Fn _thread_sys_fcntl .
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Before returning,
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.Fn fcntl
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unlocks
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.Fa fd
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and enables thread rescheduling.
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.Sh RETURN VALUES
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Upon successful completion, the value returned depends on
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.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
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A new file descriptor.
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.It Dv F_GETFD
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Value of flag (only the low-order bit is defined).
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.It Dv F_GETFL
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Value of flags.
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.It Dv F_GETOWN
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Value of file descriptor owner.
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.It other
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Value other than -1.
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.El
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.Pp
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Otherwise, a value of -1 is returned and
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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.Fn Fcntl
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EAGAIN
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The argument
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.Fa cmd
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is
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.Dv F_SETLK ,
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the type of lock
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.Pq Fa l_type
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is a shared lock
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.Pq Dv F_RDLCK
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or exclusive lock
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.Pq Dv F_WRLCK ,
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and the segment of a file to be locked is already
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exclusive-locked by another process;
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or the type is an exclusive lock and some portion of the
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segment of a file to be locked is already shared-locked or
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exclusive-locked by another process.
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.It Bq Er EBADF
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.Fa Fildes
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is not a valid open file descriptor.
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.Pp
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The argument
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.Fa cmd
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is
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.Dv F_SETLK
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or
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.Dv F_SETLKW ,
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the type of lock
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.Pq Fa l_type
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is a shared lock
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.Pq Dv F_RDLCK ,
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and
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.Fa fildes
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is not a valid file descriptor open for reading.
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.Pp
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The argument
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.Fa cmd
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is
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.Dv F_SETLK
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or
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.Dv F_SETLKW ,
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the type of lock
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.Pq Fa l_type
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is an exclusive lock
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.Pq Dv F_WRLCK ,
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and
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.Fa fildes
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is not a valid file descriptor open for writing.
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.It Bq Er EDEADLK
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The argument
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.Fa cmd
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is
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.Dv F_SETLKW ,
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and a deadlock condition was detected.
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.It Bq Er EINTR
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The argument
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.Fa cmd
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is
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.Dv F_SETLKW ,
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and the function was interrupted by a signal.
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.It Bq Er EINVAL
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.Fa Cmd
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is
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.Dv F_DUPFD
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and
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.Fa arg
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is negative or greater than the maximum allowable number
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(see
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.Xr getdtablesize 2 ) .
|
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.Pp
|
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The argument
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.Fa cmd
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is
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.Dv F_GETLK ,
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.Dv F_SETLK ,
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or
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.Dv F_SETLKW
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and the data to which
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.Fa arg
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points is not valid, or
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.Fa fildes
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refers to a file that does not support locking.
|
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.It Bq Er EMFILE
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The argument
|
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.Fa cmd
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is
|
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.Dv F_DUPFD
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and the maximum number of file descriptors permitted for the
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process are already in use,
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or no file descriptors greater than or equal to
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.Fa arg
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are available.
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.It Bq Er ENOLCK
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The argument
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.Fa cmd
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is
|
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.Dv F_SETLK
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or
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.Dv F_SETLKW ,
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and satisfying the lock or unlock request would result in the
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number of locked regions in the system exceeding a system-imposed limit.
|
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.It Bq Er EPERM
|
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.Fa Cmd
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is
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.Dv F_SETOWN
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and
|
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the process ID or process group given as an argument is in a
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different session than the caller.
|
|
.It Bq Er ESRCH
|
|
.Fa Cmd
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|
is
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.Dv F_SETOWN
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|
and
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the process ID given as argument is not in use.
|
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.El
|
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.Pp
|
|
In addition, if
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.Fa fd
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|
refers to a descriptor open on a terminal device (as opposed to a
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descriptor open on a socket), a
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|
.Fa cmd
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|
of
|
|
.Dv F_SETOWN
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|
can fail for the same reasons as in
|
|
.Xr tcsetpgrp 3 ,
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|
and a
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.Fa cmd
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|
of
|
|
.Dv F_GETOWN
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|
for the reasons as stated in
|
|
.Xr tcgetpgrp 3 .
|
|
.Sh SEE ALSO
|
|
.Xr close 2 ,
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|
.Xr execve 2 ,
|
|
.Xr flock 2 ,
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.Xr getdtablesize 2 ,
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|
.Xr open 2 ,
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.Xr sigvec 2 ,
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.Xr tcgetpgrp 3 ,
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.Xr tcsetpgrp 3
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|
.Sh HISTORY
|
|
The
|
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.Fn fcntl
|
|
function call appeared in
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.Bx 4.2 .
|