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PROLOG
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.
NAME
fcntl — file control
SYNOPSIS
#include <fcntl.h>
int fcntl(int fildes, int cmd, ...);
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
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.
EXAMPLES
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 */;"
APPLICATION USAGE
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.
RATIONALE
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.
FUTURE DIRECTIONS
None.
SEE ALSO
alarm(), close(), exec, open(), sigaction()
The Base Definitions volume of POSIX.1‐2008, <fcntl.h>, <signal.h>
COPYRIGHT
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 .