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FCNTL(3P) POSIX Programmer's Manual FCNTL(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
fcntl — file control
#include <fcntl.h>
int fcntl(int fildes, int cmd, ...);
The fcntl() function shall perform the operations described below on
open files. The fildes argument is a file descriptor.
The available values for cmd are defined in <fcntl.h> and are as
follows:
F_DUPFD Return a new file descriptor which shall be the lowest
numbered available (that is, not already open) file
descriptor greater than or equal to the third argument,
arg, taken as an integer of type int. The new file
descriptor shall refer to the same open file
description as the original file descriptor, and shall
share any locks. The FD_CLOEXEC flag associated with
the new file descriptor shall be cleared to keep the
file open across calls to one of the exec functions.
F_DUPFD_CLOEXEC
Like F_DUPFD, but the FD_CLOEXEC flag associated with
the new file descriptor shall be set.
F_GETFD Get the file descriptor flags defined in <fcntl.h> that
are associated with the file descriptor fildes. File
descriptor flags are associated with a single file
descriptor and do not affect other file descriptors
that refer to the same file.
F_SETFD Set the file descriptor flags defined in <fcntl.h>,
that are associated with fildes, to the third argument,
arg, taken as type int. If the FD_CLOEXEC flag in the
third argument is 0, the file descriptor shall remain
open across the exec functions; otherwise, the file
descriptor shall be closed upon successful execution of
one of the exec functions.
F_GETFL Get the file status flags and file access modes,
defined in <fcntl.h>, for the file description
associated with fildes. The file access modes can be
extracted from the return value using the mask
O_ACCMODE, which is defined in <fcntl.h>. File status
flags and file access modes are associated with the
file description and do not affect other file
descriptors that refer to the same file with different
open file descriptions. The flags returned may include
non-standard file status flags which the application
did not set, provided that these additional flags do
not alter the behavior of a conforming application.
F_SETFL Set the file status flags, defined in <fcntl.h>, for
the file description associated with fildes from the
corresponding bits in the third argument, arg, taken as
type int. Bits corresponding to the file access mode
and the file creation flags, as defined in <fcntl.h>,
that are set in arg shall be ignored. If any bits in
arg other than those mentioned here are changed by the
application, the result is unspecified. If fildes does
not support non-blocking operations, it is unspecified
whether the O_NONBLOCK flag will be ignored.
F_GETOWN If fildes refers to a socket, get the process or
process group ID specified to receive SIGURG signals
when out-of-band data is available. Positive values
indicate a process ID; negative values, other than −1,
indicate a process group ID. If fildes does not refer
to a socket, the results are unspecified.
F_SETOWN If fildes refers to a socket, set the process or
process group ID specified to receive SIGURG signals
when out-of-band data is available, using the value of
the third argument, arg, taken as type int. Positive
values indicate a process ID; negative values, other
than −1, indicate a process group ID. If fildes does
not refer to a socket, the results are unspecified.
The following values for cmd are available for advisory record
locking. Record locking shall be supported for regular files, and may
be supported for other files.
F_GETLK Get the first lock which blocks the lock description
pointed to by the third argument, arg, taken as a
pointer to type struct flock, defined in <fcntl.h>.
The information retrieved shall overwrite the
information passed to fcntl() in the structure flock.
If no lock is found that would prevent this lock from
being created, then the structure shall be left
unchanged except for the lock type which shall be set
to F_UNLCK.
F_SETLK Set or clear a file segment lock according to the lock
description pointed to by the third argument, arg,
taken as a pointer to type struct flock, defined in
<fcntl.h>. F_SETLK can establish shared (or read)
locks (F_RDLCK) or exclusive (or write) locks
(F_WRLCK), as well as to remove either type of lock
(F_UNLCK). F_RDLCK, F_WRLCK, and F_UNLCK are defined in
<fcntl.h>. If a shared or exclusive lock cannot be
set, fcntl() shall return immediately with a return
value of −1.
F_SETLKW This command shall be equivalent to F_SETLK except that
if a shared or exclusive lock is blocked by other
locks, the thread shall wait until the request can be
satisfied. If a signal that is to be caught is received
while fcntl() is waiting for a region, fcntl() shall be
interrupted. Upon return from the signal handler,
fcntl() shall return −1 with errno set to [EINTR], and
the lock operation shall not be done.
Additional implementation-defined values for cmd may be defined in
<fcntl.h>. Their names shall start with F_.
When a shared lock is set on a segment of a file, other processes
shall be able to set shared locks on that segment or a portion of it.
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 shall fail if the file descriptor was not opened with read
access.
An exclusive lock shall prevent 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 shall fail if the file
descriptor was not opened with write access.
The structure flock describes the type (l_type), starting offset
(l_whence), relative offset (l_start), size (l_len), and process ID
(l_pid) of the segment of the file to be affected.
The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END, to indicate
that the relative offset l_start bytes shall be measured from the
start of the file, current position, or end of the file,
respectively. The value of l_len is the number of consecutive bytes
to be locked. The value of l_len may be negative (where the
definition of off_t permits negative values of l_len). The l_pid
field is only used with F_GETLK to return the process ID of the
process holding a blocking lock. After a successful F_GETLK request,
when a blocking lock is found, the values returned in the flock
structure shall be as follows:
l_type Type of blocking lock found.
l_whence SEEK_SET.
l_start Start of the blocking lock.
l_len Length of the blocking lock.
l_pid Process ID of the process that holds the blocking lock.
If the command is F_SETLKW and the process must wait for another
process to release a lock, then the range of bytes to be locked shall
be determined before the fcntl() function blocks. If the file size or
file descriptor seek offset change while fcntl() is blocked, this
shall not affect the range of bytes locked.
If l_len is positive, the area affected shall start at l_start and
end at l_start+l_len−1. If l_len is negative, the area affected
shall start at l_start+l_len and end at l_start−1. Locks may start
and extend beyond the current end of a file, but shall not extend
before the beginning of the file. A lock shall be set to extend to
the largest possible value of the file offset for that file by
setting l_len to 0. If such a lock also has l_start set to 0 and
l_whence is set to SEEK_SET, the whole file shall be locked.
There shall be at most one type of lock set for each byte in the
file. Before a successful return from an F_SETLK or an F_SETLKW
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 shall be replaced by the new
lock type. As specified above under the descriptions of shared locks
and exclusive locks, an F_SETLK or an F_SETLKW request (respectively)
shall fail or block 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.
All locks associated with a file for a given process shall be removed
when a file descriptor for that file is closed by that process or the
process holding that file descriptor terminates. Locks are not
inherited by a child process.
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. If the system detects that sleeping until a locked
region is unlocked would cause a deadlock, fcntl() shall fail with an
[EDEADLK] error.
An unlock (F_UNLCK) request in which l_len is non-zero and the offset
of the last byte of the requested segment is the maximum value for an
object of type off_t, when the process has an existing lock in which
l_len is 0 and which includes the last byte of the requested segment,
shall be treated as a request to unlock from the start of the
requested segment with an l_len equal to 0. Otherwise, an unlock
(F_UNLCK) request shall attempt to unlock only the requested segment.
When the file descriptor fildes refers to a shared memory object, the
behavior of fcntl() shall be the same as for a regular file except
the effect of the following values for the argument cmd shall be
unspecified: F_SETFL, F_GETLK, F_SETLK, and F_SETLKW.
If fildes refers to a typed memory object, the result of the fcntl()
function is unspecified.
Upon successful completion, the value returned shall depend on cmd as
follows:
F_DUPFD A new file descriptor.
F_DUPFD_CLOEXEC
A new file descriptor.
F_GETFD Value of flags defined in <fcntl.h>. The return value
shall not be negative.
F_SETFD Value other than −1.
F_GETFL Value of file status flags and access modes. The return
value is not negative.
F_SETFL Value other than −1.
F_GETLK Value other than −1.
F_SETLK Value other than −1.
F_SETLKW Value other than −1.
F_GETOWN Value of the socket owner process or process group; this
will not be −1.
F_SETOWN Value other than −1.
Otherwise, −1 shall be returned and errno set to indicate the error.
The fcntl() function shall fail if:
EACCES or EAGAIN
The cmd argument is F_SETLK; the type of lock (l_type) is a
shared (F_RDLCK) or exclusive (F_WRLCK) lock 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.
EBADF The fildes argument is not a valid open file descriptor, or
the argument cmd is F_SETLK or F_SETLKW, the type of lock,
l_type, is a shared lock (F_RDLCK), and fildes is not a valid
file descriptor open for reading, or the type of lock, l_type,
is an exclusive lock (F_WRLCK), and fildes is not a valid file
descriptor open for writing.
EINTR The cmd argument is F_SETLKW and the function was interrupted
by a signal.
EINVAL The cmd argument is invalid, or the cmd argument is F_DUPFD or
F_DUPFD_CLOEXEC and arg is negative or greater than or equal
to {OPEN_MAX}, or the cmd argument is F_GETLK, F_SETLK, or
F_SETLKW and the data pointed to by arg is not valid, or
fildes refers to a file that does not support locking.
EMFILE The argument cmd is F_DUPFD or F_DUPFD_CLOEXEC and all file
descriptors available to the process are currently open, or no
file descriptors greater than or equal to arg are available.
ENOLCK The argument cmd is F_SETLK or 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.
EOVERFLOW
One of the values to be returned cannot be represented
correctly.
EOVERFLOW
The cmd argument is F_GETLK, F_SETLK, or F_SETLKW and the
smallest or, if l_len is non-zero, the largest offset of any
byte in the requested segment cannot be represented correctly
in an object of type off_t.
The fcntl() function may fail if:
EDEADLK
The cmd argument is F_SETLKW, the lock is blocked by a lock
from another process, and putting the calling process to sleep
to wait for that lock to become free would cause a deadlock.
The following sections are informative.
Locking and Unlocking a File
The following example demonstrates how to place a lock on bytes 100
to 109 of a file and then later remove it. F_SETLK is used to perform
a non-blocking lock request so that the process does not have to wait
if an incompatible lock is held by another process; instead the
process can take some other action.
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
int fd;
struct flock fl;
fd = open("testfile", O_RDWR);
if (fd == -1)
/* Handle error */;
/* Make a non-blocking request to place a write lock
on bytes 100-109 of testfile */
fl.l_type = F_WRLCK;
fl.l_whence = SEEK_SET;
fl.l_start = 100;
fl.l_len = 10;
if (fcntl(fd, F_SETLK, &fl) == −1) {
if (errno == EACCES || errno == EAGAIN) {
printf("Already locked by another process\n");
/* We can't get the lock at the moment */
} else {
/* Handle unexpected error */;
}
} else { /* Lock was granted... */
/* Perform I/O on bytes 100 to 109 of file */
/* Unlock the locked bytes */
fl.l_type = F_UNLCK;
fl.l_whence = SEEK_SET;
fl.l_start = 100;
fl.l_len = 10;
if (fcntl(fd, F_SETLK, &fl) == −1)
/* Handle error */;
}
exit(EXIT_SUCCESS);
} /* main */
Setting the Close-on-Exec Flag
The following example demonstrates how to set the close-on-exec flag
for the file descriptor fd.
#include <unistd.h>
#include <fcntl.h>
...
int flags;
flags = fcntl(fd, F_GETFD);
if (flags == −1)
/* Handle error */;
flags |= FD_CLOEXEC;
if (fcntl(fd, F_SETFD, flags) == −1)
/* Handle error */;"
The arg values to F_GETFD, F_SETFD, F_GETFL, and F_SETFL all
represent flag values to allow for future growth. Applications using
these functions should do a read-modify-write operation on them,
rather than assuming that only the values defined by this volume of
POSIX.1‐2008 are valid. It is a common error to forget this,
particularly in the case of F_SETFD. Some implementations set
additional file status flags to advise the application of default
behavior, even though the application did not request these flags.
The ellipsis in the SYNOPSIS is the syntax specified by the ISO C
standard for a variable number of arguments. It is used because
System V uses pointers for the implementation of file locking
functions.
This volume of POSIX.1‐2008 permits concurrent read and write access
to file data using the fcntl() function; this is a change from the
1984 /usr/group standard and early proposals. Without concurrency
controls, this feature may not be fully utilized without occasional
loss of data.
Data losses occur in several ways. One case occurs when several
processes try to update the same record, without sequencing controls;
several updates may occur in parallel and the last writer ``wins''.
Another case is a bit-tree or other internal list-based database that
is undergoing reorganization. Without exclusive use to the tree
segment by the updating process, other reading processes chance
getting lost in the database when the index blocks are split,
condensed, inserted, or deleted. While fcntl() is useful for many
applications, it is not intended to be overly general and does not
handle the bit-tree example well.
This facility is only required for regular files because it is not
appropriate for many devices such as terminals and network
connections.
Since fcntl() works with ``any file descriptor associated with that
file, however it is obtained'', the file descriptor may have been
inherited through a fork() or exec operation and thus may affect a
file that another process also has open.
The use of the open file description to identify what to lock
requires extra calls and presents problems if several processes are
sharing an open file description, but there are too many
implementations of the existing mechanism for this volume of
POSIX.1‐2008 to use different specifications.
Another consequence of this model is that closing any file descriptor
for a given file (whether or not it is the same open file description
that created the lock) causes the locks on that file to be
relinquished for that process. Equivalently, any close for any
file/process pair relinquishes the locks owned on that file for that
process. But note that while an open file description may be shared
through fork(), locks are not inherited through fork(). Yet locks
may be inherited through one of the exec functions.
The identification of a machine in a network environment is outside
the scope of this volume of POSIX.1‐2008. Thus, an l_sysid member,
such as found in System V, is not included in the locking structure.
Changing of lock types can result in a previously locked region being
split into smaller regions.
Mandatory locking was a major feature of the 1984 /usr/group
standard.
For advisory file record locking to be effective, all processes that
have access to a file must cooperate and use the advisory mechanism
before doing I/O on the file. Enforcement-mode record locking is
important when it cannot be assumed that all processes are
cooperating. For example, if one user uses an editor to update a
file at the same time that a second user executes another process
that updates the same file and if only one of the two processes is
using advisory locking, the processes are not cooperating.
Enforcement-mode record locking would protect against accidental
collisions.
Secondly, advisory record locking requires a process using locking to
bracket each I/O operation with lock (or test) and unlock operations.
With enforcement-mode file and record locking, a process can lock the
file once and unlock when all I/O operations have been completed.
Enforcement-mode record locking provides a base that can be enhanced;
for example, with sharable locks. That is, the mechanism could be
enhanced to allow a process to lock a file so other processes could
read it, but none of them could write it.
Mandatory locks were omitted for several reasons:
1. Mandatory lock setting was done by multiplexing the set-group-ID
bit in most implementations; this was confusing, at best.
2. The relationship to file truncation as supported in 4.2 BSD was
not well specified.
3. Any publicly readable file could be locked by anyone. Many
historical implementations keep the password database in a
publicly readable file. A malicious user could thus prohibit
logins. Another possibility would be to hold open a long-distance
telephone line.
4. Some demand-paged historical implementations offer memory mapped
files, and enforcement cannot be done on that type of file.
Since sleeping on a region is interrupted with any signal, alarm()
may be used to provide a timeout facility in applications requiring
it. This is useful in deadlock detection. Since implementation of
full deadlock detection is not always feasible, the [EDEADLK] error
was made optional.
None.
alarm(3p), close(3p), exec(1p), open(3p), sigaction(3p)
The Base Definitions volume of POSIX.1‐2008, fcntl.h(0p),
signal.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 FCNTL(3P)
Pages that refer to this page: fcntl.h(0p), stropts.h(0p), aio_fsync(3p), dup(3p), exec(3p), fchmod(3p), fdatasync(3p), fork(3p), fstatvfs(3p), ioctl(3p), lockf(3p), mmap(3p), open(3p), pipe(3p), posix_spawn(3p), posix_typed_mem_open(3p), pselect(3p), read(3p), shm_open(3p), write(3p)
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