file lock

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flock

flock(2) System Calls Manual flock(2)

Name
flock - apply or remove an advisory lock on an open file

Syntax
#include <sys/file.h>

#define LOCK_SH 1 /* shared lock */
#define LOCK_EX 2 /* exclusive lock */
#define LOCK_NB 4 /* don't block when locking */
#define LOCK_UN 8 /* unlock */

flock(fd, operation)
int fd, operation;

Description
The system call applies or removes an advisory lock on the file associated with the file descriptor, fd. A lock is applied by specifying
an operation parameter that is the inclusive OR of LOCK_SH or LOCK_EX and, possibly, LOCK_NB. To unlock an existing lock, operation should
be LOCK_UN.

Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee consistency; that is, processes
might still access files without using advisory locks, possibly resulting in inconsistencies.

The locking mechanism allows two types of locks: shared locks and exclusive locks. At any time, multiple shared locks can be applied to a
file. However, multiple exclusive locks, or shared and exclusive locks cannot be applied simultaneously on a file.

A shared lock can be upgraded to be an exclusive lock, and an exclusive lock can become shared, simply by specifying the appropriate lock
type. This change results in the previous lock being released and the new lock applied. When upgrading, do not include LOCK_NB in opera-
tion, because there is a possibility that other processes have requests for locks, or have gained or released a lock.

Requesting a lock on an object that is already locked normally causes the caller to blocked until the lock can be acquired. If LOCK_NB is
included in operation, the call is not blocked; instead, the call fails and the error EWOULDBLOCK is returned.

Locks are on files, not file descriptors. That is, file descriptors duplicated through or call do not result in multiple instances of a
lock, but rather multiple references to a single lock. If a process holding a lock on a file forks and the child explicitly unlocks the
file, the parent loses its lock.

Processes blocked awaiting a lock may be awakened by signals.

Return Values
Zero is returned if the operation was successful; on an error, a -1 is returned and an error code is stored in the global variable, errno.

Diagnostics
The call fails under the following conditions:

[EWOULDBLOCK] The file is locked and the LOCK_NB option was specified.

[EBADF] The argument fd is an invalid descriptor.

[EINVAL] The argument fd refers to an object other than a file.

[EOPNOTSUPP] Invalid operation is requested. The argument fd refers to a socket.

Restrictions
File region locking is not supported over NFS.

See Also
close(2), dup(2), execve(2), fork(2), open(2)

flock(2)

UNIX for Advanced & Expert Users