flock − apply or remove an advisory lock on an open file
flag ... ] file ...
flock( fd, operation);
int fd, operation;
applies or removes an advisory lock on the file
associated with the file descriptor fd. The
compatibility version of flock() has been implemented
on top of fcntl(2) locking.
It does not provide complete binary compatibility.
Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee exclusive access (that is, processes may still access files without using advisory locks, possibly resulting in inconsistencies).
The locking mechanism allows two types of locks: shared locks and exclusive locks. More than one process may hold a shared lock for a file at any given time, but multiple exclusive, or both shared and exclusive, locks may not exist simultaneously on a file.
A lock is applied by specifying an operation parameter LOCK_SH for a shared lock or LOCK_EX for an exclusive lock. The operation paramerer may be ORed with LOCK_NB to make the operation non-blocking. To unlock an existing lock, the operation should be LOCK_UN.
Read permission is required on a file to obtain a shared lock, and write permission is required to obtain an exclusive lock. Locking a segment that is already locked by the calling process causes the old lock type to be removed and the new lock type to take effect.
Requesting a lock on an object that is already locked normally causes the caller to block until the lock may be acquired. If LOCK_NB is included in operation, then this will not happen; instead, the call will fail and the error EWOULDBLOCK will be returned.
on failure and sets errno to indicate the error.
The argument fd is an invalid descriptor.
operation is not a valid argument.
The argument fd refers to an object other than a file.
The file is locked and the LOCK_NB option was specified.
Use of these interfaces should be restricted to only applications written on BSD platforms. Use of these interfaces with any of the system libraries or in multi-thread applications is unsupported.
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. Locks are not inherited by a child process.
Processes blocked awaiting a lock may be awakened by signals.
Mandatory locking may occur, depending on the mode bits of the file. See chmod(2).
Locks obtained through the flock() mechanism under SunOS 4.1 were known only within the system on which they were placed. This is no longer true.