---
name: "flock"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/flock/2"
---

# flock(2)

apply or remove an advisory lock on an open file

## NAME

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

## SYNOPSIS

**#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 \*/**

*int*\

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

## DESCRIPTION

**flock**() 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 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.

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 **flock**() 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.

- \[ENOTSUP\]

  The referenced descriptor is not of the correct type.

## SEE ALSO

[close(2)](/close/2.md), [dup(2)](/dup/2.md), [execve(2)](/execve/2.md), [fork(2)](/fork/2.md), [open(2)](/open/2.md)

## HISTORY

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