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fcntl() - Unix, Linux System Call

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fcntl - manipulate file descriptor


#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);


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.


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 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. 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 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 matter 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 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 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 signals:

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 positive 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 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".)

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 specific 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 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.

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 recipient 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 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.

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).


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 process (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 currently 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 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 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 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() 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_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 (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:

BitDescription (event in directory)
DN_MODIFYA file was modified (write, pwrite,
 writev, truncate, ftruncate)
DN_CREATEA file was created (open, creat, mknod,
 mkdir, link, symlink, rename)
DN_DELETEA file was unlinked (unlink, rename to
 another directory, rmdir)
DN_RENAMEA file was renamed within this
 directory (rename)
DN_ATTRIBThe 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_MULTISHOT is included in arg, then notification will remain in effect until explicitly removed.v

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 consider using the inotify interface (available since kernel 2.6.13), which provides a superior interface for obtaining notifications of file system events. See inotify(7).



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 traditional SIGIO behaviour.
All other commands



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 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.


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 supported 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.


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.


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.)


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