int flock(int fd, int operation);
A call to flock() may block if an incompatible lock is held by another process. To make a nonblocking 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 description (see open(2)). 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 file descriptors. Furthermore, the lock is released either by an explicit LOCK_UN operation on any of these duplicate file descriptors, or when all such file descriptors have been closed.
If a process uses open(2) (or similar) to obtain more than one file descriptor for the same file, these file 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 file descriptor.
A process may hold only 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.
In Linux kernels up to 2.6.11, flock() does not lock files over NFS (i.e., the scope of locks was limited to the local system). Instead, one could use fcntl(2) byte-range locking, which does work over NFS, given a sufficiently recent version of Linux and a server which supports locking. Since Linux 2.6.12, NFS clients support flock() locks by emulating them as byte-range locks on the entire file. This means that fcntl(2) and flock() locks do interact with one another over NFS. Since Linux 2.6.37, the kernel supports a compatibility mode that allows flock() locks (and also fcntl(2) byte region locks) to be treated as local; see the discussion of the local_lock option in nfs(5).
flock() places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock() and perform I/O on the file.
flock() and fcntl(2) locks have different semantics with respect to forked processes and dup(2). On systems that implement flock() using fcntl(2), 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 behavior, and occurs on many other implementations.)
Documentation/filesystems/locks.txt in the Linux kernel source tree (Documentation/locks.txt in older kernels)