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Manual page for FCNTL(2)

fcntl - file control

SYNOPSIS

#include <sys/types.h>


/* POSIX applications #include <unistd.h> */


#include <fcntl.h>

int fcntl(int fd, int cmd, ...);


int fcntl(int fd, int cmd, int arg);


int fcntl(int fd, int cmd, struct flock *arg);

DESCRIPTION

The fcntl function performs a variety of functions on open descriptors. The argument fd is an open descriptor to be operated on by cmd as follows:

F_DUPFD: Return a new descriptor as follows:
: Lowest numbered available descriptor greater than or equal to arg.
: References the same object as the original descriptor.
: New descriptor shares the same file pointer if the object was a file.
: Same access mode (read, write or read/write).
: Same file status flags (that is, both descriptors share the same file status flags).
: The close-on-exec flag associated with the new descriptor is cleared to keep the file open across execve.2 system calls.
F_GETFD: Get the close-on-exec flag associated with the descriptor fd. If the low-order bit of this flag is clear (that is, if it's zero), then the file will remain open across an exec.2 system call. Otherwise the file will be closed upon execution of exec.2 POSIX applications refer to this flag with the constant FD_CLOEXEC.
F_SETFD: Set the close-on-exec flag associated with fd to the low-order bit of the third argument, arg. POSIX applications use the constant FD_CLOEXEC to set this flag and 0 to clear it.
F_GETFL: Get the file status flags and file access modes for the open file descriptor associated with fd.
: POSIX applications can extract the file access modes from the return value using the mask O_ACCMODE (see <fcntl.h>). The file status flags are O_APPEND (set append mode) and O_NONBLOCK (no delay). The file access modes are O_RDONLY (open for reading only), O_RDWR (open for reading and writing) and O_WRONLY (open for writing only).
F_SETFL: Set the file status flags for the open file descriptor associated with fd from the corresponding bits in the third argument, arg. Bits corresponding to the file access modes and the oflag values that are set in arg are ignored.
F_GETOWN: Get the process ID or process group currently receiving SIGIO and SIGURG signals; process groups are returned as negative values.
F_SETOWN: Set the process or process group to receive SIGIO and SIGURG signals; process groups are specified by supplying arg as negative, otherwise arg is interpreted as a process ID.

The SIGIO facilities are enabled by setting the FASYNC flag with F_SETFL.

The following commands are POSIX-specific:

F_GETLK: Get a description of the first lock which would block the lock specified in the flock structure pointed to by arg. The information retrieved overwrites the information in the flock structure. If no lock is found that would prevent this lock from being created, then the structure is passed back unchanged except for the lock type which will be set to F_UNLCK.
F_SETLK: Set or clear an advisory record lock according to the flock structure pointed to by arg. F_SETLK is used to establish shared (F_RDLCK) and exclusive (F_WRLCK) locks, or to remove either type of lock (F_UNLCK). If the specified lock cannot be applied, fcntl() will return with an error value of -1.
F_SETLKW: This cmd is the same as F_SETLK except that if a shared or exclusive lock is blocked by other locks, the requesting process will sleep until the lock may be applied.

NOTES ON FILE-LOCKING FOR POSIX APPLICATIONS

Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee exclusive access (i.e., processes may still access files without using advisory locks, possibly resulting in inconsistencies).

The record locking mechanism allows two types of locks: shared locks (F_RDLCK) and exclusive locks (F_WRLCK). More than one process may hold a shared lock for a particular segment of a file at any given time, but multiple exclusive, or both shared and exclusive, locks may not exist simultaneously on any segment.

In order to claim a shared lock, the descriptor must have been opened with read access. The descriptor on which an exclusive lock is being placed must have been opened with write access.

A shared lock may be upgraded to an exclusive lock, and vice versa, simply by specifying the appropriate lock type with a cmd of F_SETLK or F_SETLKW; the previous lock will be released and the new lock applied (possibly after other processes have gained and released the lock).

If the cmd is F_SETLKW and the requested lock cannot be claimed immediately (e.g., another process holds an exclusive lock that partially or completely overlaps the current request) then the calling process will block until the lock may be acquired. Processes blocked awaiting a lock may be awakened by signals.

Care should be taken to avoid deadlock situations in applications in which multiple processes perform blocking locks on a set of common records.

The record that is to be locked or unlocked is described by the flock structure, which is defined in <fcntl.h> as follows:

struct flock {
	short	l_type;	/* F_RDLCK, F_WRLCK, or F_UNLCK */
	short	l_whence;	/* flag to choose starting offset */
	off_t	l_start;	/* relative offset, in bytes */
	off_t	l_len;	/* length, in bytes;  0 means lock to EOF */
	pid_t	l_pid;	/* returned with F_GETLK */
};

The flock structure describes the type (l_type), starting offset (l_whence), relative offset (l_start), and size (l_len) of the segment of the file to be affected. L_whence must be set to 0, 1, or 2 to indicate that the relative offset will be measured from the start of the file, current position, or end-of-file, respectively. The process id field (l_pid) is only used with the F_GETLK cmd to return the description of a lock held by another process.

Locks may start and extend beyond the current end-of-file, but may not be negative relative to the beginning of the file. A lock may be set to always extend to the end-of-file by setting l_len to zero (0). If such a lock also has l_whence and l_start set to zero (0), the entire file will be locked. Changing or unlocking a segment from the middle of a larger locked segment leaves two smaller segments at either end. Locking a segment that is already locked by the calling process causes the old lock type to be removed and the new lock type to take affect. All locks associated with a file for a given process are removed when the file is closed or the process terminates. Locks are not inherited by the child process in a fork(2) system call.

RETURN VALUE

Upon successful completion, the value returned depends on cmd as follows:

	F_DUPFD	A new descriptor.
	F_GETFD	Value of flag (only the low-order bit is defined).
	F_GETFL	Value of flags.
	F_GETOWN	Value of descriptor owner.
	other	Value other than -1.

Otherwise, a value of -1 is returned and errno is set to indicate the error.

ERRORS

The fcntl function will fail if one or more of the following are true:

[EAGAIN]: cmd is F_SETLK, the lock type (l_type) is F_RDLCK (shared lock), and the segment of the file to be locked already has an exclusive lock held by another process. This error will also be returned if the lock type is F_WRLCK (exclusive lock) and another process already has the segment locked with either a shared or exclusive lock.
[EBADF]: cmd is F_SETLK or F_SETLKW and the process does not have the appropriate read or write permissions on the file.
[EBADF]: fd is not a valid open descriptor.
[EFAULT]: cmd is F_GETLK, F_SETLK, or F_SETLKW and arg points to an invalid address.
[EINTR]: cmd is F_SETLKW and a signal interrupted the process while it was waiting for the lock to be granted.
[EINVAL]: cmd is F_GETLK, F_SETLK, or F_SETLKW and the data arg points to is not valid; or, either cmd is invalid, or cmd is F_DUPFD and arg is negative or greater than the maximum allowable number (OPEN_MAX for POSIX applications, or the return value of getdtablesize.2 for non-POSIX applications).
[EMFILE]: cmd is F_DUPFD and the maximum allowed number of descriptors are currently open.
[ENOLCK]: cmd is F_SETLK or F_SETLKW and there are no more file lock entries available.

BUGS (POSIX-SPECIFIC)

File locks obtained through the fcntl mechanism do not interact in any way with those acquired by using flock.2

F_GETLK returns F_UNLCK if the requesting process holds the specified lock. Thus, there is no way for a process to determine if it is still holding a specific lock after catching a SIGLOST signal.