FLOCK

Section: System Calls (2)
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BSD mandoc
BSD 4.2

NAME

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

SYNOPSIS

Fd #include <sys/file.h> Fd #define LOCK_SH 1 /* shared lock */ Fd #define LOCK_EX 2 /* exclusive lock */ Fd #defineLOCK_NB4/* don't block when locking */ Fd #defineLOCK_UN8/* unlock */ Ft int Fn flock int fd int operation

DESCRIPTION

Fn Flock applies or removes an
advisory lock on the file associated with the file descriptor Fa fd . A lock is applied by specifying an Fa operation parameter that is one of LOCK_SH or LOCK_EX with the optional addition of 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 (i.e., processes may 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 may be applied to a file, but at no time are multiple exclusive, or both shared and exclusive, locks allowed simultaneously on a file.

A shared lock may be upgraded to an exclusive lock, and vice versa, simply by specifying the appropriate lock type; this results in the previous lock being released and the new lock applied (possibly after other processes have gained and released the lock).

Requesting a lock on an object that is already locked normally causes the caller to be blocked until the lock may be acquired. If LOCK_NB is included in Fa operation , then this will not happen; instead the call will fail and the error Er EWOULDBLOCK will be returned.

NOTES

Locks are on files, not file descriptors. That is, file descriptors duplicated through dup(2) or fork(2) 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 will lose 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 left in the global location errno

ERRORS

The Fn flock call fails if:

Bq Er EWOULDBLOCK: The file is locked and the LOCK_NB option was specified.
Bq Er EBADF: The argument Fa fd is an invalid descriptor.
Bq Er EINVAL: The argument Fa fd refers to an object other than a file.

HISTORY

The Fn flock function call appeared in BSD 4.2

This document was created by man2html, using the manual pages.
Time: 16:29:00 GMT, April 18, 2022