e1f4e80c88
in the main text of various man pages. Thanks to Warner Losh for adding an option to manck to allow it to scan the entire man page looking for bogus xrefs, instead of just checking the SEE ALSO section.
505 lines
13 KiB
Groff
505 lines
13 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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.\"
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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 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" "int arg"
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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 follows:
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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 execv 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
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call, or if a
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.Xr write
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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_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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.ne 7v
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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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.Nm 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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.Dv (F_RDLCK)
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or exclusive (or write) locks,
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.Dv (F_WRLCK) ,
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as well as remove either type of lock
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.Dv (F_UNLCK) .
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If a shared or exclusive lock cannot be set,
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.Nm fcntl
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returns immediately with
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.Er EACCES .
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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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.Nm fcntl
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is waiting for a region, the
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.Nm 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. 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 \fIany\fP 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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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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.Pp
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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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.Va 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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.Va 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 EACCES
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The argument
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.Fa arg
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is
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.Dv F_SETLK ,
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the type of lock
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.Fa (l_type)
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is a shared lock
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.Dv (F_RDLCK)
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or exclusive lock
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.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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.Fa (l_type)
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is a shared lock
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.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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.Fa (l_type)
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is an exclusive lock
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.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 EMFILE
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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 allowed number of file descriptors are currently
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open.
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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 ) .
|
|
.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
|
|
.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
|
|
The argument
|
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.Fa cmd
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is
|
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.Dv F_DUPED
|
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and the maximum number of file descriptors permitted for the
|
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process are already in use,
|
|
or no file descriptors greater than or equal to
|
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.Fa arg
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are available.
|
|
.It Bq Er ENOLCK
|
|
The argument
|
|
.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 ,
|
|
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 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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|
.Sh SEE ALSO
|
|
.Xr close 2 ,
|
|
.Xr execve 2 ,
|
|
.Xr flock 2 ,
|
|
.Xr getdtablesize 2 ,
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.Xr open 2 ,
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.Xr sigvec 2
|
|
.Sh HISTORY
|
|
The
|
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.Nm
|
|
function call appeared in
|
|
.Bx 4.2 .
|