Apply or remove an advisory lock on the open file specified by fd. The
parameter operation is one of the following:
Place a shared lock. More than one process may hold a
shared lock for a given file at a given time.
Place an exclusive lock. Only one process may hold an
exclusive lock for a given file at a given time.
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 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.
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).