apply or remove an advisory lock on an
applies or removes an
lock on the file associated with the file descriptor
fd. The operation argument is
LOCK_EX may be combined with the optional
LOCK_NB for nonblocking mode.
Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee consistency (i.e., processes may still access files without using advisory locks possibly resulting in inconsistencies).
A shared lock may be upgraded to an exclusive lock, and vice versa, simply by specifying the appropriate lock type; this results in the previous lock being released and the new lock applied (possibly after other processes have gained and released the lock).
Requesting a lock on an object that is already locked normally
causes the caller to be blocked until the lock may be acquired. If
operation is the bitwise OR of
LOCK_EX, then this will not happen; instead the call
will fail and the error
EWOULDBLOCK will be
Locks are on files, not file descriptors. That is, file descriptors duplicated through dup(2) or fork(2) do not result in multiple instances of a lock, but rather multiple references to a single lock. If a process holding a lock on a file forks and the child explicitly unlocks the file, the parent will lose its lock.
Processes blocked awaiting a lock may be awakened by signals.
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
flock() call fails if:
- The file is locked and the
LOCK_NBoption was specified.
- The argument fd is an invalid descriptor.
- The argument operation has an invalid value.
- The argument fd refers to a file that does not support locking.
close(2), dup(2), execve(2), fcntl(2), fork(2), open(2)
flock() system call first appeared in