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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);
int fcntl(int fd, int cmd, long arg);
int fcntl(int fd, int cmd, struct flock *lock);
DESCRIPTION
fcntl() performs one of the operations described below on the open file descriptor
fd. The operation is determined by cmd.
Duplicating a file descriptor
F_DUPFD
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.
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
Read the file descriptor flags.
F_SETFD
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(2). Duplicated file descriptors (made with
dup(), fcntl(F_DUPFD), fork(), 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
Read the file status flags.
F_SETFL
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 exis-
tence 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. l_start is the starting offset for the lock, and is inter-
preted 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 is a non-negative
integer (but see the NOTES below) specifying the number of bytes to be locked.
Bytes past the end of the file may be locked, but not bytes before the start of the
file. 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 mat-
ter how large the file grows.
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
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
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).
F_GETLK
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 lock-
ing 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 incom-
patible 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 or EWOULDBLOCK.
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)).
Managing signals
F_GETOWN, F_SETOWN, F_GETSIG and F_SETSIG are used to manage I/O availability sig-
nals:
F_GETOWN
Get the process ID or process group currently receiving SIGIO and SIGURG
signals for events on file descriptor fd. Process IDs are returned as posi-
tive values; process group IDs are returned as negative values (but see BUGS
below).
F_SETOWN
Set the process ID or process group ID that will receive SIGIO and SIGURG
signals for events on file descriptor fd. 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()).
If you set the O_ASYNC status flag on a file descriptor (either by providing
this flag with the open(2) call, or 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 sig-
nal 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 sub-
ject 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".)
If a non-zero 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 spe-
cific thread within a process. Consequently, it may be necessary to pass
F_SETOWN the result of gettid() instead of getpid() 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() or getpid() 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. Note also that Linux imposes a limit on the number of
real-time signals 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.
F_GETSIG
Get 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.
F_SETSIG
Sets the signal sent when input or output becomes possible. 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.
Additionally, passing a non-zero value to F_SETSIG changes the signal recip-
ient from a whole process to a specific thread within a process. See the
description of F_SETOWN for more details.
By using F_SETSIG with a non-zero value, and setting SA_SIGINFO for the sig-
nal 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.
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_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
and retrieve the current setting of the calling process's lease on the file
referred to by 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 pro-
cess (the "lease breaker") tries to open(2) or truncate(2) that file.
F_SETLEASE
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 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 no other process cur-
rently has the file open.
F_UNLCK
Remove our lease from the file.
A process may hold only one type of lease on a file.
Leases may only be taken out on regular files. An unprivileged process may only
take out a lease on a file whose UID matches the file system UID of the process. A
process with the CAP_LEASE capability may take out leases on arbitrary files.
F_GETLEASE
Indicates what type of lease we hold on the file referred to by fd by
returning either F_RDLCK, F_WRLCK, or F_UNLCK, indicating, respectively,
that the calling process holds a read, a write, or no lease on the file.
(The third argument to fcntl() is omitted.)
When a process (the "lease breaker") performs an open() or truncate() that con-
flicts 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 what-
ever cleanup is required in preparation for the file to be accessed by another pro-
cess (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 we currently hold a write lease on the file, and the lease breaker is opening
the file for reading, then it is sufficient 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() or truncate() is interrupted by a signal han-
dler, 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() or truncate(), then the other steps still occur as described above. If
the lease breaker specifies the O_NONBLOCK flag when calling open(), 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_SIG-
INFO, 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
(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:
Bit Description (event in directory)
-------------------------------------------------------------
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.)
Directory notifications are normally "one-shot", and the application must
re-register to receive further notifications. Alternatively, if DN_MULTI-
SHOT 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 lat-
ter 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 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 consider using the inotify interface (avail-
able since kernel 2.6.13), which provides a superior interface for obtaining
notifications of file system events. See inotify(7).
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_GETOWN Value of descriptor owner.
F_GETSIG Value of signal sent when read or write becomes possible, or zero for tra-
ditional SIGIO behaviour.
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. 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 proto-
col failed (e.g. locking over NFS).
EPERM Attempted to clear the O_APPEND flag on a file that has the append-only
attribute set.
NOTES
The errors returned by dup2() 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(2).
POSIX.1-2001 allows l_len to be negative. (And if it is, the interval described by
the lock covers bytes l_start+l_len up to and including l_start-1.) This is sup-
ported by Linux since Linux 2.4.21 and 2.5.49.
Several systems have more fields in struct flock such as e.g. 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
x86) 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.
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.
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, F_SETLKW, F_GETOWN, and F_SETOWN are specified
in POSIX.1-2001.
F_GETSIG, F_SETSIG, F_NOTIFY, F_GETLEASE, and F_SETLEASE are Linux specific.
(Define the _GNU_SOURCE macro to obtain these definitions.)
SEE ALSO
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), fea-
ture_test_macros(7)
See also locks.txt, mandatory.txt, and dnotify.txt in /usr/src/linux/Documentation.
Linux 2.6.14 2005-05-20 FCNTL(2)
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