.\" Copyright (c) 1983, 1993
.\" The Regents of the University of California. All rights reserved.
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" @(#)fcntl.2 8.1 (Berkeley) 7/11/93
.Fn fcntl "int fd" "int cmd" "int arg"
provides for control over descriptors.
is a descriptor to be operated on by
.Bl -tag -width F_GETOWNX
Return a new descriptor as follows:
.Bl -bullet -compact -offset 4n
Lowest numbered available descriptor greater than or equal to
Same object references as the original descriptor.
New descriptor shares the same file offset if the object
Same access mode (read, write or read/write).
Same file status flags (i.e., both file descriptors
share the same file status flags).
The close-on-exec flag associated with the new file descriptor
is set to remain open across
Get the close-on-exec flag associated with the file descriptor
If the low-order bit of the returned value is 0,
the file will remain open across
otherwise the file will be closed upon execution of
Set the close-on-exec flag associated with
Get descriptor status flags, as described below
Set descriptor status flags to
Get the process ID or process group
signals; process groups are returned
Set the process or process group
process groups are specified by supplying
is interpreted as a process ID.
.Bl -tag -width O_NONBLOCKX
Non-blocking I/O; if no data is available to a
the read or write call returns -1 with the error
Force each write to append at the end of file;
signal to be sent to the process group
when I/O is possible, e.g.,
upon availability of data to be read.
Several commands are available for doing advisory file locking;
they all operate on the following structure:
off_t l_start; /* starting offset */
off_t l_len; /* len = 0 means until end of file */
pid_t l_pid; /* lock owner */
short l_type; /* lock type: read/write, etc. */
short l_whence; /* type of l_start */
The commands available for advisory record locking are as follows:
.Bl -tag -width F_SETLKWX
Get the first lock that blocks the lock description pointed to by the
The information retrieved overwrites the information passed to
If no lock is found that would prevent this lock from being created,
the structure is left unchanged by this function call except for the
lock type which is set to
Set or clear a file segment lock according to the lock description
pointed to by the third argument,
is used to establish shared (or read) locks
or exclusive (or write) locks,
as well as remove either type of lock
If a shared or exclusive lock cannot be set,
This command is the same as
except that if a shared or exclusive lock is blocked by other locks,
the process waits until the request can be satisfied.
If a signal that is to be caught is received while
is waiting for a region, the
will be interrupted if the signal handler has not specified the
When a shared lock has been set on a segment of a file,
other processes can set shared locks on that segment
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
An exclusive lock prevents any other process from setting a shared lock or
an exclusive lock on any portion of the protected area.
A request for an exclusive lock fails if the file was not
opened with write access.
to indicate that the relative offset,
bytes, will be measured from the start of the file,
current position, or end of the file, respectively.
is the number of consecutive bytes to be locked.
is negative, the result is undefined.
to return the process ID of the process holding a blocking lock.
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
point to the beginning of the file, and
is zero, the entire file is locked.
If an application wishes only to do entire file locking, the
system call is much more efficient.
There is at most one type of lock set for each byte in the file.
Before a successful return from an
request when the calling process has previously existing locks
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
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.
This interface follows the completely stupid semantics of System V and
that require that all locks associated with a file for a given process are
removed when \fIany\fP 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
the lock will be lost because
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
interface has much more rational last close semantics and
allows locks to be inherited by child processes.
is recommended for applications that want to ensure the integrity
of their locks when using library routines or wish to pass locks
locks may be safely used concurrently.
All locks associated with a file for a given process are
removed when the process terminates.
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
Upon successful completion, the value returned depends on
.Bl -tag -width F_GETOWNX -offset indent
Value of flag (only the low-order bit is defined).
Value of file descriptor owner.
Otherwise, a value of -1 is returned and
is set to indicate the error.
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.
is not a valid open file descriptor.
is not a valid file descriptor open for reading.
is not a valid file descriptor open for writing.
and the maximum allowed number of file descriptors are currently
and a deadlock condition was detected.
and the function was interrupted by a signal.
is negative or greater than the maximum allowable number
refers to a file that does not support locking.
and the maximum number of file descriptors permitted for the
process are already in use,
or no file descriptors greater than or equal to
and satisfying the lock or unlock request would result in the
number of locked regions in the system exceeding a system-imposed limit.
the process ID given as argument is not in use.
function call appeared in