Linux man pages : flock (2)
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 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 created by flock() are associated with a file, or, more pre-
cisely, 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.
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
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
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).
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/Documenta-
Linux 2002-04-24 FLOCK(2)