FLOCK(2) Linux Programmer's Manual FLOCK(2)
flock - apply or remove an advisory lock on an open file
int flock(int fd, int operation);
Apply or remove an advisory lock on the open file specified by fd. The parameter opera-
tion 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 a file, or, more precisely, an open file ta-
ble 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 opera-
tion on any of these duplicate descriptors, or when all such descriptors have been closed.
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
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.
The file is locked and the LOCK_NB flag was selected.
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.
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 seman-
tics: 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
open(2), close(2), dup(2), execve(2), fcntl(2), fork(2), lockf(3)
There are also locks.txt and mandatory.txt in /usr/src/linux/Documentation.
Linux 2002-04-24 FLOCK(2)