Linux / Unix Command: fcntl

Command Library

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 various miscellaneous operations on fd. The operation in question is determined by cmd.  

Handling close-on-exec

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 form dup2(2) which uses exactly the descriptor specified.

The old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using lseek on one of the descriptors, the position is also changed for the other.

The two descriptors do not share the close-on-exec flag, however. The close-on-exec flag of the copy is off, meaning that it will not be closed on exec.

On success, the new descriptor is returned.

F_GETFD

Read the close-on-exec flag. If the FD_CLOEXEC bit is 0, the file will remain open across exec, otherwise it will be closed.

F_SETFD

Set the close-on-exec flag to the value specified by the FD_CLOEXEC bit of arg.

 

The file status flags

A file descriptor has certain associated flags, initialized by open(2) and possibly modified by fcntl(2). The flags are shared between copies (made with dup(2), fork(2), etc.) of the same file descriptor.

The flags and their semantics are described in open(2).

F_GETFL

Read the file descriptor's flags.

F_SETFL

Set the file status flags part of the descriptor's flags to the value specified by arg. Remaining bits (access mode, file creation flags) in arg are ignored. On Linux this command can only change the O_APPEND, O_NONBLOCK, O_ASYNC, and O_DIRECT 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_WDLCK) 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 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. To make use of mandatory locks, mandatory locking must be enabled (using the "-o mand" option to mount(8)) for the file system containing the file to be locked and enabled on the file itself (by disabling group execute permission on the file and enabling the set-GID permission bit).

Advisory locks are not enforced and are useful only between cooperating processes. Mandatory locks are enforced for all processes.  

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 groups are returned as negative values.

F_SETOWN

Set the process ID or process group that will receive SIGIO and SIGURG signals for events on file descriptor fd. Process groups are specified using negative values. (F_SETSIG can be used to specify a different signal instead of SIGIO).

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.

The process or process group to receive the signal can be selected by using the F_SETOWN command to the fcntl function. If the file descriptor is a socket, this 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 the file descriptor corresponds to a terminal device, then SIGIO signals are sent to the foreground process group of the terminal.

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.

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 POSIX.1b 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 another process (the "contestant") 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 us to be notified when another process opens the file for writing or truncates it.

F_WRLCK

Take out a write lease. This will cause us to be notified when another process opens the file (for reading or writing) or truncates it. 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.

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 the contestant performs an open() or truncate() that conflicts with a lease established via F_SETLEASE, the system call is blocked by the kernel (unless the O_NONBLOCK flag was specified to open(), in which case it returns immediately with the error EWOULDBLOCK). The kernel then 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 remove its lease by performing an F_SETLEASE command specifying arg as F_UNLCK.

If the lease holder fails to release the lease within the number of seconds specified in /proc/sys/fs/lease-break-time and the contestant's system call remains blocked (i.e., the contestant did not specify O_NONBLOCK on its open() call, and the system call was not interrupted by a signal handler) then the kerrnel forcibly breaks the lease holder's lease.

Once the lease has been voluntarily or forcibly removed, and assuming the contestant has not unblocked its system call, the kernel permits the contestant's system call to proceed.

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 sructure 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

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
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 macro must be defined before including <fcntl.h>.)

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. A series of calls specifying DN_MULTISHOT 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 POSIX.1b real time signal should be used for notication, so that multiple notifications can be queued.

 

RETURN VALUE

For a successful call, the return value depends on the operation:

F_DUPFD

The new descriptor.

F_GETFD

Value of flag.

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

Zero.

On error, -1 is returned, and errno is set appropriately.  

ERRORS

EACCES or EAGAIN

Operation is prohibited by locks held by other processes. Or, 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.

 

SEE ALSO

dup2(2), flock(2), lockf(3), open(2), socket(2) See also locks.txt, mandatory.txt, and dnotify.txt in /usr/src/linux/Documentation.

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Important: Use the man command (% man) to see how a command is used on your particular computer.

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