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NAME
fcntl - manipulate file descriptor
SYNOPSIS
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fd, int cmd, ... /* arg */ );
DESCRIPTION
fcntl() performs one of the operations described below on the open file descriptor fd.
The operation is determined by cmd.
fcntl() can take an optional third argument. Whether or not this argument is required is
determined by cmd. The required argument type is indicated in parentheses after each cmd
name (in most cases, the required type is int, and we identify the argument using the name
arg), or void is specified if the argument is not required.
Duplicating a file descriptor
F_DUPFD (int)
Find the lowest numbered available file descriptor greater than or equal to arg and
make it be a copy of fd. This is different from dup2(2), which uses exactly the
descriptor specified.
On success, the new descriptor is returned.
See dup(2) for further details.
F_DUPFD_CLOEXEC (int; since Linux 2.6.24)
As for F_DUPFD, but additionally set the close-on-exec flag for the duplicate
descriptor. Specifying this flag permits a program to avoid an additional fcntl()
F_SETFD operation to set the FD_CLOEXEC flag. For an explanation of why this flag
is useful, see the description of O_CLOEXEC in open(2).
File descriptor flags
The following commands manipulate the flags associated with a file descriptor. Currently,
only one such flag is defined: FD_CLOEXEC, the close-on-exec flag. If the FD_CLOEXEC bit
is 0, the file descriptor will remain open across an execve(2), otherwise it will be
closed.
F_GETFD (void)
Read the file descriptor flags; arg is ignored.
F_SETFD (int)
Set the file descriptor flags to the value specified by arg.
File status flags
Each open file description has certain associated status flags, initialized by open(2) and
possibly modified by fcntl(). Duplicated file descriptors (made with dup(2),
fcntl(F_DUPFD), fork(2), etc.) refer to the same open file description, and thus share the
same file status flags.
The file status flags and their semantics are described in open(2).
F_GETFL (void)
Get the file access mode and the file status flags; arg is ignored.
F_SETFL (int)
Set the file status flags to the value specified by arg. File access mode
(O_RDONLY, O_WRONLY, O_RDWR) and file creation flags (i.e., O_CREAT, O_EXCL,
O_NOCTTY, O_TRUNC) in arg are ignored. On Linux this command can change only the
O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.
Advisory locking
F_GETLK, F_SETLK and F_SETLKW are used to acquire, release, and test for the existence of
record locks (also known as file-segment or file-region locks). The third argument, lock,
is a pointer to a structure that has at least the following fields (in unspecified order).
struct flock {
...
short l_type; /* Type of lock: F_RDLCK,
F_WRLCK, F_UNLCK */
short l_whence; /* How to interpret l_start:
SEEK_SET, SEEK_CUR, SEEK_END */
off_t l_start; /* Starting offset for lock */
off_t l_len; /* Number of bytes to lock */
pid_t l_pid; /* PID of process blocking our lock
(F_GETLK only) */
...
};
The l_whence, l_start, and l_len fields of this structure specify the range of bytes we
wish to lock. Bytes past the end of the file may be locked, but not bytes before the
start of the file.
l_start is the starting offset for the lock, and is interpreted relative to either: the
start of the file (if l_whence is SEEK_SET); the current file offset (if l_whence is
SEEK_CUR); or the end of the file (if l_whence is SEEK_END). In the final two cases,
l_start can be a negative number provided the offset does not lie before the start of the
file.
l_len specifies the number of bytes to be locked. If l_len is positive, then the range to
be locked covers bytes l_start up to and including l_start+l_len-1. Specifying 0 for
l_len has the special meaning: lock all bytes starting at the location specified by
l_whence and l_start through to the end of file, no matter how large the file grows.
POSIX.1-2001 allows (but does not require) an implementation to support a negative l_len
value; if l_len is negative, the interval described by lock covers bytes l_start+l_len up
to and including l_start-1. This is supported by Linux since kernel versions 2.4.21 and
2.5.49.
The l_type field can be used to place a read (F_RDLCK) or a write (F_WRLCK) lock on a
file. Any number of processes may hold a read lock (shared lock) on a file region, but
only one process may hold a write lock (exclusive lock). An exclusive lock excludes all
other locks, both shared and exclusive. A single process can hold only one type of lock
on a file region; if a new lock is applied to an already-locked region, then the existing
lock is converted to the new lock type. (Such conversions may involve splitting,
shrinking, or coalescing with an existing lock if the byte range specified by the new lock
does not precisely coincide with the range of the existing lock.)
F_SETLK (struct flock *)
Acquire a lock (when l_type is F_RDLCK or F_WRLCK) or release a lock (when l_type
is F_UNLCK) on the bytes specified by the l_whence, l_start, and l_len fields of
lock. If a conflicting lock is held by another process, this call returns -1 and
sets errno to EACCES or EAGAIN.
F_SETLKW (struct flock *)
As for F_SETLK, but if a conflicting lock is held on the file, then wait for that
lock to be released. If a signal is caught while waiting, then the call is
interrupted and (after the signal handler has returned) returns immediately (with
return value -1 and errno set to EINTR; see signal(7)).
F_GETLK (struct flock *)
On input to this call, lock describes a lock we would like to place on the file.
If the lock could be placed, fcntl() does not actually place it, but returns
F_UNLCK in the l_type field of lock and leaves the other fields of the structure
unchanged. If one or more incompatible locks would prevent this lock being placed,
then fcntl() returns details about one of these locks in the l_type, l_whence,
l_start, and l_len fields of lock and sets l_pid to be the PID of the process
holding that lock.
In order to place a read lock, fd must be open for reading. In order to place a write
lock, fd must be open for writing. To place both types of lock, open a file read-write.
As well as being removed by an explicit F_UNLCK, record locks are automatically released
when the process terminates or if it closes any file descriptor referring to a file on
which locks are held. This is bad: it means that a process can lose the locks on a file
like /etc/passwd or /etc/mtab when for some reason a library function decides to open,
read and close it.
Record locks are not inherited by a child created via fork(2), but are preserved across an
execve(2).
Because of the buffering performed by the stdio(3) library, the use of record locking with
routines in that package should be avoided; use read(2) and write(2) instead.
Mandatory locking
(Non-POSIX.) The above record locks may be either advisory or mandatory, and are advisory
by default.
Advisory locks are not enforced and are useful only between cooperating processes.
Mandatory locks are enforced for all processes. If a process tries to perform an
incompatible access (e.g., read(2) or write(2)) on a file region that has an incompatible
mandatory lock, then the result depends upon whether the O_NONBLOCK flag is enabled for
its open file description. If the O_NONBLOCK flag is not enabled, then system call is
blocked until the lock is removed or converted to a mode that is compatible with the
access. If the O_NONBLOCK flag is enabled, then the system call fails with the error
EAGAIN.
To make use of mandatory locks, mandatory locking must be enabled both on the filesystem
that contains the file to be locked, and on the file itself. Mandatory locking is enabled
on a filesystem using the "-o mand" option to mount(8), or the MS_MANDLOCK flag for
mount(2). Mandatory locking is enabled on a file by disabling group execute permission on
the file and enabling the set-group-ID permission bit (see chmod(1) and chmod(2)).
The Linux implementation of mandatory locking is unreliable. See BUGS below.
Managing signals
F_GETOWN, F_SETOWN, F_GETOWN_EX, F_SETOWN_EX, F_GETSIG and F_SETSIG are used to manage I/O
availability signals:
F_GETOWN (void)
Return (as the function result) the process ID or process group currently receiving
SIGIO and SIGURG signals for events on file descriptor fd. Process IDs are
returned as positive values; process group IDs are returned as negative values (but
see BUGS below). arg is ignored.
F_SETOWN (int)
Set the process ID or process group ID that will receive SIGIO and SIGURG signals
for events on file descriptor fd to the ID given in arg. A process ID is specified
as a positive value; a process group ID is specified as a negative value. Most
commonly, the calling process specifies itself as the owner (that is, arg is
specified as getpid(2)).
If you set the O_ASYNC status flag on a file descriptor by using the F_SETFL
command of fcntl(), a SIGIO signal is sent whenever input or output becomes
possible on that file descriptor. F_SETSIG can be used to obtain delivery of a
signal other than SIGIO. If this permission check fails, then the signal is
silently discarded.
Sending a signal to the owner process (group) specified by F_SETOWN is subject to
the same permissions checks as are described for kill(2), where the sending process
is the one that employs F_SETOWN (but see BUGS below).
If the file descriptor fd refers to a socket, F_SETOWN also selects the recipient
of SIGURG signals that are delivered when out-of-band data arrives on that socket.
(SIGURG is sent in any situation where select(2) would report the socket as having
an "exceptional condition".)
The following was true in 2.6.x kernels up to and including kernel 2.6.11:
If a nonzero value is given to F_SETSIG in a multithreaded process running
with a threading library that supports thread groups (e.g., NPTL), then a
positive value given to F_SETOWN has a different meaning: instead of being a
process ID identifying a whole process, it is a thread ID identifying a
specific thread within a process. Consequently, it may be necessary to pass
F_SETOWN the result of gettid(2) instead of getpid(2) to get sensible
results when F_SETSIG is used. (In current Linux threading implementations,
a main thread's thread ID is the same as its process ID. This means that a
single-threaded program can equally use gettid(2) or getpid(2) in this
scenario.) Note, however, that the statements in this paragraph do not
apply to the SIGURG signal generated for out-of-band data on a socket: this
signal is always sent to either a process or a process group, depending on
the value given to F_SETOWN.
The above behavior was accidentally dropped in Linux 2.6.12, and won't be restored.
From Linux 2.6.32 onward, use F_SETOWN_EX to target SIGIO and SIGURG signals at a
particular thread.
F_GETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
Return the current file descriptor owner settings as defined by a previous
F_SETOWN_EX operation. The information is returned in the structure pointed to by
arg, which has the following form:
struct f_owner_ex {
int type;
pid_t pid;
};
The type field will have one of the values F_OWNER_TID, F_OWNER_PID, or
F_OWNER_PGRP. The pid field is a positive integer representing a thread ID,
process ID, or process group ID. See F_SETOWN_EX for more details.
F_SETOWN_EX (struct f_owner_ex *) (since Linux 2.6.32)
This operation performs a similar task to F_SETOWN. It allows the caller to direct
I/O availability signals to a specific thread, process, or process group. The
caller specifies the target of signals via arg, which is a pointer to a f_owner_ex
structure. The type field has one of the following values, which define how pid is
interpreted:
F_OWNER_TID
Send the signal to the thread whose thread ID (the value returned by a call
to clone(2) or gettid(2)) is specified in pid.
F_OWNER_PID
Send the signal to the process whose ID is specified in pid.
F_OWNER_PGRP
Send the signal to the process group whose ID is specified in pid. (Note
that, unlike with F_SETOWN, a process group ID is specified as a positive
value here.)
F_GETSIG (void)
Return (as the function result) the signal sent when input or output becomes
possible. A value of zero means SIGIO is sent. Any other value (including SIGIO)
is the signal sent instead, and in this case additional info is available to the
signal handler if installed with SA_SIGINFO. arg is ignored.
F_SETSIG (int)
Set the signal sent when input or output becomes possible to the value given in
arg. A value of zero means to send the default SIGIO signal. Any other value
(including SIGIO) is the signal to send instead, and in this case additional info
is available to the signal handler if installed with SA_SIGINFO.
By using F_SETSIG with a nonzero value, and setting SA_SIGINFO for the signal
handler (see sigaction(2)), extra information about I/O events is passed to the
handler in a siginfo_t structure. If the si_code field indicates the source is
SI_SIGIO, the si_fd field gives the file descriptor associated with the event.
Otherwise, there is no indication which file descriptors are pending, and you
should use the usual mechanisms (select(2), poll(2), read(2) with O_NONBLOCK set
etc.) to determine which file descriptors are available for I/O.
By selecting a real time signal (value >= SIGRTMIN), multiple I/O events may be
queued using the same signal numbers. (Queuing is dependent on available memory).
Extra information is available if SA_SIGINFO is set for the signal handler, as
above.
Note that Linux imposes a limit on the number of real-time signals that may be
queued to a process (see getrlimit(2) and signal(7)) and if this limit is reached,
then the kernel reverts to delivering SIGIO, and this signal is delivered to the
entire process rather than to a specific thread.
Using these mechanisms, a program can implement fully asynchronous I/O without using
select(2) or poll(2) most of the time.
The use of O_ASYNC, F_GETOWN, F_SETOWN is specific to BSD and Linux. F_GETOWN_EX,
F_SETOWN_EX, F_GETSIG, and F_SETSIG are Linux-specific. POSIX has asynchronous I/O and
the aio_sigevent structure to achieve similar things; these are also available in Linux as
part of the GNU C Library (Glibc).
Leases
F_SETLEASE and F_GETLEASE (Linux 2.4 onward) are used (respectively) to establish a new
lease, and retrieve the current lease, on the open file description referred to by the
file descriptor fd. A file lease provides a mechanism whereby the process holding the
lease (the "lease holder") is notified (via delivery of a signal) when a process (the
"lease breaker") tries to open(2) or truncate(2) the file referred to by that file
descriptor.
F_SETLEASE (int)
Set or remove a file lease according to which of the following values is specified
in the integer arg:
F_RDLCK
Take out a read lease. This will cause the calling process to be notified
when the file is opened for writing or is truncated. A read lease can be
placed only on a file descriptor that is opened read-only.
F_WRLCK
Take out a write lease. This will cause the caller to be notified when the
file is opened for reading or writing or is truncated. A write lease may be
placed on a file only if there are no other open file descriptors for the
file.
F_UNLCK
Remove our lease from the file.
Leases are associated with an open file description (see open(2)). This means that
duplicate file descriptors (created by, for example, fork(2) or dup(2)) refer to the same
lease, and this lease may be modified or released using any of these descriptors.
Furthermore, the lease is released by either an explicit F_UNLCK operation on any of these
duplicate descriptors, or when all such descriptors have been closed.
Leases may be taken out only on regular files. An unprivileged process may take out a
lease only on a file whose UID (owner) matches the filesystem UID of the process. A
process with the CAP_LEASE capability may take out leases on arbitrary files.
F_GETLEASE (void)
Indicates what type of lease is associated with the file descriptor fd by returning
either F_RDLCK, F_WRLCK, or F_UNLCK, indicating, respectively, a read lease , a
write lease, or no lease. arg is ignored.
When a process (the "lease breaker") performs an open(2) or truncate(2) that conflicts
with a lease established via F_SETLEASE, the system call is blocked by the kernel and the
kernel notifies the lease holder by sending it a signal (SIGIO by default). The lease
holder should respond to receipt of this signal by doing whatever cleanup is required in
preparation for the file to be accessed by another process (e.g., flushing cached buffers)
and then either remove or downgrade its lease. A lease is removed by performing an
F_SETLEASE command specifying arg as F_UNLCK. If the lease holder currently holds a write
lease on the file, and the lease breaker is opening the file for reading, then it is
sufficient for the lease holder to downgrade the lease to a read lease. This is done by
performing an F_SETLEASE command specifying arg as F_RDLCK.
If the lease holder fails to downgrade or remove the lease within the number of seconds
specified in /proc/sys/fs/lease-break-time then the kernel forcibly removes or downgrades
the lease holder's lease.
Once a lease break has been initiated, F_GETLEASE returns the target lease type (either
F_RDLCK or F_UNLCK, depending on what would be compatible with the lease breaker) until
the lease holder voluntarily downgrades or removes the lease or the kernel forcibly does
so after the lease break timer expires.
Once the lease has been voluntarily or forcibly removed or downgraded, and assuming the
lease breaker has not unblocked its system call, the kernel permits the lease breaker's
system call to proceed.
If the lease breaker's blocked open(2) or truncate(2) is interrupted by a signal handler,
then the system call fails with the error EINTR, but the other steps still occur as
described above. If the lease breaker is killed by a signal while blocked in open(2) or
truncate(2), then the other steps still occur as described above. If the lease breaker
specifies the O_NONBLOCK flag when calling open(2), then the call immediately fails with
the error EWOULDBLOCK, but the other steps still occur as described above.
The default signal used to notify the lease holder is SIGIO, but this can be changed using
the F_SETSIG command to fcntl(). If a F_SETSIG command is performed (even one specifying
SIGIO), and the signal handler is established using SA_SIGINFO, then the handler will
receive a siginfo_t structure as its second argument, and the si_fd field of this argument
will hold the descriptor of the leased file that has been accessed by another process.
(This is useful if the caller holds leases against multiple files).
File and directory change notification (dnotify)
F_NOTIFY (int)
(Linux 2.4 onward) Provide notification when the directory referred to by fd or any
of the files that it contains is changed. The events to be notified are specified
in arg, which is a bit mask specified by ORing together zero or more of the
following bits:
DN_ACCESS A file was accessed (read, pread, readv)
DN_MODIFY A file was modified (write, pwrite, writev, truncate, ftruncate).
DN_CREATE A file was created (open, creat, mknod, mkdir, link, symlink, rename).
DN_DELETE A file was unlinked (unlink, rename to another directory, rmdir).
DN_RENAME A file was renamed within this directory (rename).
DN_ATTRIB The attributes of a file were changed (chown, chmod, utime[s]).
(In order to obtain these definitions, the _GNU_SOURCE feature test macro must be
defined before including any header files.)
Directory notifications are normally "one-shot", and the application must
reregister to receive further notifications. Alternatively, if DN_MULTISHOT is
included in arg, then notification will remain in effect until explicitly removed.
A series of F_NOTIFY requests is cumulative, with the events in arg being added to
the set already monitored. To disable notification of all events, make an F_NOTIFY
call specifying arg as 0.
Notification occurs via delivery of a signal. The default signal is SIGIO, but
this can be changed using the F_SETSIG command to fcntl(). In the latter case, the
signal handler receives a siginfo_t structure as its second argument (if the
handler was established using SA_SIGINFO) and the si_fd field of this structure
contains the file descriptor which generated the notification (useful when
establishing notification on multiple directories).
Especially when using DN_MULTISHOT, a real time signal should be used for
notification, so that multiple notifications can be queued.
NOTE: New applications should use the inotify interface (available since kernel
2.6.13), which provides a much superior interface for obtaining notifications of
filesystem events. See inotify(7).
Changing the capacity of a pipe
F_SETPIPE_SZ (int; since Linux 2.6.35)
Change the capacity of the pipe referred to by fd to be at least arg bytes. An
unprivileged process can adjust the pipe capacity to any value between the system
page size and the limit defined in /proc/sys/fs/pipe-max-size (see proc(5)).
Attempts to set the pipe capacity below the page size are silently rounded up to
the page size. Attempts by an unprivileged process to set the pipe capacity above
the limit in /proc/sys/fs/pipe-max-size yield the error EPERM; a privileged process
(CAP_SYS_RESOURCE) can override the limit. When allocating the buffer for the
pipe, the kernel may use a capacity larger than arg, if that is convenient for the
implementation. The F_GETPIPE_SZ operation returns the actual size used.
Attempting to set the pipe capacity smaller than the amount of buffer space
currently used to store data produces the error EBUSY.
F_GETPIPE_SZ (void; since Linux 2.6.35)
Return (as the function result) the capacity of the pipe referred to by fd.
RETURN VALUE
For a successful call, the return value depends on the operation:
F_DUPFD The new descriptor.
F_GETFD Value of file descriptor flags.
F_GETFL Value of file status flags.
F_GETLEASE
Type of lease held on file descriptor.
F_GETOWN Value of descriptor owner.
F_GETSIG Value of signal sent when read or write becomes possible, or zero for traditional
SIGIO behavior.
F_GETPIPE_SZ
The pipe capacity.
All other commands
Zero.
On error, -1 is returned, and errno is set appropriately.
ERRORS
EACCES or EAGAIN
Operation is prohibited by locks held by other processes.
EAGAIN The operation is prohibited because the file has been memory-mapped by another
process.
EBADF fd is not an open file descriptor, or the command was F_SETLK or F_SETLKW and the
file descriptor open mode doesn't match with the type of lock requested.
EDEADLK
It was detected that the specified F_SETLKW command would cause a deadlock.
EFAULT lock is outside your accessible address space.
EINTR For F_SETLKW, the command was interrupted by a signal; see signal(7). For F_GETLK
and F_SETLK, the command was interrupted by a signal before the lock was checked or
acquired. Most likely when locking a remote file (e.g., locking over NFS), but can
sometimes happen locally.
EINVAL For F_DUPFD, arg is negative or is greater than the maximum allowable value. For
F_SETSIG, arg is not an allowable signal number.
EMFILE For F_DUPFD, the process already has the maximum number of file descriptors open.
ENOLCK Too many segment locks open, lock table is full, or a remote locking protocol
failed (e.g., locking over NFS).
EPERM Attempted to clear the O_APPEND flag on a file that has the append-only attribute
set.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. Only the operations F_DUPFD, F_GETFD, F_SETFD, F_GETFL,
F_SETFL, F_GETLK, F_SETLK and F_SETLKW, are specified in POSIX.1-2001.
F_GETOWN and F_SETOWN are specified in POSIX.1-2001. (To get their definitions, define
BSD_SOURCE, or _XOPEN_SOURCE with the value 500 or greater, or define _POSIX_C_SOURCE with
the value 200809L or greater.)
F_DUPFD_CLOEXEC is specified in POSIX.1-2008. (To get this definition, define
_POSIX_C_SOURCE with the value 200809L or greater, or _XOPEN_SOURCE with the value 700 or
greater.)
F_GETOWN_EX, F_SETOWN_EX, F_SETPIPE_SZ, F_GETPIPE_SZ, F_GETSIG, F_SETSIG, F_NOTIFY,
F_GETLEASE, and F_SETLEASE are Linux-specific. (Define the _GNU_SOURCE macro to obtain
these definitions.)
NOTES
The original Linux fcntl() system call was not designed to handle large file offsets (in
the flock structure). Consequently, an fcntl64() system call was added in Linux 2.4. The
newer system call employs a different structure for file locking, flock64, and
corresponding commands, F_GETLK64, F_SETLK64, and F_SETLKW64. However, these details can
be ignored by applications using glibc, whose fcntl() wrapper function transparently
employs the more recent system call where it is available.
The errors returned by dup2(2) are different from those returned by F_DUPFD.
Since kernel 2.0, there is no interaction between the types of lock placed by flock(2) and
fcntl().
Several systems have more fields in struct flock such as, for example, l_sysid. Clearly,
l_pid alone is not going to be very useful if the process holding the lock may live on a
different machine.
BUGS
A limitation of the Linux system call conventions on some architectures (notably i386)
means that if a (negative) process group ID to be returned by F_GETOWN falls in the range
-1 to -4095, then the return value is wrongly interpreted by glibc as an error in the
system call; that is, the return value of fcntl() will be -1, and errno will contain the
(positive) process group ID. The Linux-specific F_GETOWN_EX operation avoids this
problem. Since glibc version 2.11, glibc makes the kernel F_GETOWN problem invisible by
implementing F_GETOWN using F_GETOWN_EX.
In Linux 2.4 and earlier, there is bug that can occur when an unprivileged process uses
F_SETOWN to specify the owner of a socket file descriptor as a process (group) other than
the caller. In this case, fcntl() can return -1 with errno set to EPERM, even when the
owner process (group) is one that the caller has permission to send signals to. Despite
this error return, the file descriptor owner is set, and signals will be sent to the
owner.
The implementation of mandatory locking in all known versions of Linux is subject to race
conditions which render it unreliable: a write(2) call that overlaps with a lock may
modify data after the mandatory lock is acquired; a read(2) call that overlaps with a lock
may detect changes to data that were made only after a write lock was acquired. Similar
races exist between mandatory locks and mmap(2). It is therefore inadvisable to rely on
mandatory locking.
SEE ALSO
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), feature_test_macros(7)
locks.txt, mandatory-locking.txt, and dnotify.txt in the Linux kernel source directory
Documentation/filesystems/ (on older kernels, these files are directly under the
Documentation/ directory, and mandatory-locking.txt is called mandatory.txt)
COLOPHON
This page is part of release 3.54 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
http://www.kernel.org/doc/man-pages/.