flock() - Unix, Linux System Call
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flock() - Unix, Linux System Call

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


#include <sys/file.h> 

int flock(int fd, int operation);


Apply or remove an advisory lock on the open file specified by fd. The parameter operation is one of the following:

LOCK_SH Place a shared lock. More than one process may hold a shared lock for a given file at a given time.
LOCK_EX Place an exclusive lock. Only one process may hold an exclusive lock for a given file at a given time.
LOCK_UN Remove an existing lock held by this process.

A call to flock() may block if an incompatible lock is held by another process. To make a non-blocking request, include LOCK_NB (by ORing) with any of the above operations.

A single file may not simultaneously have both shared and exclusive locks.

Locks created by flock() are associated with an open file table entry. This means that duplicate file descriptors (created by, for example, fork(2) or dup(2)) refer to the same lock, and this lock may be modified or released using any of these descriptors. Furthermore, the lock is released either by an explicit LOCK_UN operation on any of these duplicate descriptors, or when all such descriptors have been closed.

If a process uses open(2) (or similar) to obtain more than one descriptor for the same file, these descriptors are treated independently by flock(). An attempt to lock the file using one of these file descriptors may be denied by a lock that the calling process has already placed via another descriptor.

A process may only hold one type of lock (shared or exclusive) on a file. Subsequent flock() calls on an already locked file will convert an existing lock to the new lock mode.

Locks created by flock() are preserved across an execve(2).

A shared or exclusive lock can be placed on a file regardless of the mode in which the file was opened.


On success, zero is returned. On error, -1 is returned, and errno is set appropriately.


Error CodeDescription
EBADF fd is not a not an open file descriptor.
EINTR While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler.
EINVAL operation is invalid.
ENOLCK The kernel ran out of memory for allocating lock records.
EWOULDBLOCK The file is locked and the LOCK_NB flag was selected.


4.4BSD (the flock(2) call first appeared in 4.2BSD). A version of flock(2), possibly implemented in terms of fcntl(2), appears on most Unices.


flock(2) does not lock files over NFS. Use fcntl(2) instead: that does work over NFS, given a sufficiently recent version of Linux and a server which supports locking.

Since kernel 2.0, flock(2) is implemented as a system call in its own right rather than being emulated in the GNU C library as a call to fcntl(2). This yields true BSD semantics: there is no interaction between the types of lock placed by flock(2) and fcntl(2), and flock(2) does not detect deadlock.

flock(2) places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock(2) and perform I/O on the file.

flock(2) and fcntl(2) locks have different semantics with respect to forked processes and dup(2). On systems that implement flock() using fcntl(), the semantics of flock() will be different from those described in this manual page.

Converting a lock (shared to exclusive, or vice versa) is not guaranteed to be atomic: the existing lock is first removed, and then a new lock is established. Between these two steps, a pending lock request by another process may be granted, with the result that the conversion either blocks, or fails if LOCK_NB was specified. (This is the original BSD behaviour, and occurs on many other implementations.)


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