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FCNTL(2) Linux Programmer's Manual FCNTL(2)
fcntl - manipulate file descriptor
#include <unistd.h>
#include <fcntl.h>
int fcntl(int fd, int cmd, ... /* arg */ );
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.
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).
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.
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.
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.
(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 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). 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.
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).
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 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 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).
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 file system
events. See inotify(7).
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.
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.
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.
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.)
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.
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.
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)
This page is part of release 3.51 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 2012-04-15 FCNTL(2)
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