---
name: "flock"
section: "2freebsd"
platform: "linux"
package: "freebsd-manpages"
version: "12.2-2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/flock/2freebsd"
---

# flock(2freebsd)

apply or remove an advisory lock on an open file

## NAME

**flock** – apply or remove an advisory lock on an open file

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<sys/file.h\>**

**#define   LOCK\_SH        0x01      /\* shared file lock \*/**

**#define   LOCK\_EX        0x02      /\* exclusive file lock \*/**

**#define   LOCK\_NB        0x04      /\* do not block when locking \*/**

**#define   LOCK\_UN        0x08      /\* unlock file \*/**

*int*\

**flock**(*int fd*, *int operation*);

## DESCRIPTION

The **flock**() 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* argument 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 *operation*, then this will not happen; instead the call will fail and the error EWOULDBLOCK will be returned.

## NOTES

Locks are on files, not file descriptors. That is, file descriptors duplicated through [dup(2)](/dup/2.md) or [fork(2)](/fork/2.md) 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.

The **flock**(), [fcntl(2)](/fcntl/2.md), and [lockf(3)](/lockf/3.md) locks are compatible. Processes using different locking interfaces can cooperate over the same file safely. However, only one of such interfaces should be used within the same process. If a file is locked by a process through **flock**(), any record within the file will be seen as locked from the viewpoint of another process using [fcntl(2)](/fcntl/2.md) or [lockf(3)](/lockf/3.md), and vice versa.

Processes blocked awaiting a lock may be awakened by signals.

## RETURN VALUES

The **flock()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

## ERRORS

The **flock**() system call fails if:

- \[EWOULDBLOCK\]

  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.

- \[EOPNOTSUPP\]

  The argument *fd* refers to an object that does not support file locking.

- \[ENOLCK\]

  A lock was requested, but no locks are available.

## SEE ALSO

[close(2)](/close/2.md), [dup(2)](/dup/2.md), [execve(2)](/execve/2.md), [fcntl(2)](/fcntl/2.md), [fork(2)](/fork/2.md), [open(2)](/open/2.md), [flopen(3)](/flopen/3.md), [lockf(3)](/lockf/3.md)

## HISTORY

The **flock**() system call appeared in 4.2BSD.