FCNTL(2) Linux Programmer's Manual FCNTL(2)
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 argu-
ment 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 long, 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 (long)
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 (long; 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 pro-
gram 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 (long)
Set the file descriptor flags to the value specified by arg.
File status flags
Each open file description has certain associated status flags, ini-
tialized 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)
Read the file status flags; arg is ignored.
F_SETFL (long)
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 only change 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 rela-
tive 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 posi-
tive, 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 sig-
nal 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 auto-
matically 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 manda-
tory, 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 descrip-
tion. 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 com-
patible 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 file system that contains the file to be locked, and on the file
itself. Mandatory locking is enabled on a file system using the "-o
mand" option to mount(8), or the MS_MANDLOCK flag for mount(2). Manda-
tory 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 (long)
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 com-
monly, 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 ker-
nel 2.6.11:
If a nonzero value is given to F_SETSIG in a multi-
threaded 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 sin-
gle-threaded program can equally use gettid(2) or get-
pid(2) in this scenario.) Note, however, that the state-
ments 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 onwards, 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 inte-
ger 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 speci-
fied 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 han-
dler if installed with SA_SIGINFO. arg is ignored.
F_SETSIG (long)
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 struc-
ture. 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. (Queu-
ing 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 sig-
nals that may be queued to a process (see getrlimit(2) and sig-
nal(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 onwards) 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 (long)
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 writ-
ing or is truncated. A read lease can only be placed 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 modi-
fied 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 only be taken out on regular files. An unprivileged process
may only take out a lease on a file whose UID (owner) matches the file
system 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 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 spec-
ifies the O_NONBLOCK flag when calling open(2), then the call immedi-
ately 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 han-
dler 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 (long)
(Linux 2.4 onwards) 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, trun-
cate, 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 appli-
cation must reregister to receive further notifications. Alter-
natively, 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 noti-
fication of all events, make an F_NOTIFY call specifying arg as
0.
Notification occurs via delivery of a signal. The default sig-
nal 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 struc-
ture contains the file descriptor which generated the notifica-
tion (useful when establishing notification on multiple directo-
ries).
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 (avail-
able since kernel 2.6.13), which provides a much superior inter-
face for obtaining notifications of file system events. See
inotify(7).
Changing the capacity of a pipe
F_SETPIPE_SZ (long; 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-size-max (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-size-max
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 capac-
ity 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 flags.
F_GETFL Value of 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 sig-
nal(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_SET-
SIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux-specific. (Define
the _GNU_SOURCE macro to obtain these definitions.)
NOTES
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 opera-
tion 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 unprivi-
leged 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 there-
fore inadvisable to rely on mandatory locking.
SEE ALSO
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), fea-
ture_test_macros(7)
See also locks.txt, mandatory-locking.txt, and dnotify.txt in the ker-
nel source directory Documentation/filesystems/. (On older kernels,
these files are directly under the Documentation/ directory, and manda-
tory-locking.txt is called mandatory.txt.)
COLOPHON
This page is part of release 3.27 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/.
Linux 2010-09-10 FCNTL(2)
Generated by dwww version 1.11.3 on Mon May 20 13:21:45 CEST 2013.