flock - Applies or removes an advisory lock on an open file
int operation );
filedes Specifies a file descriptor returned by a successful open() or
fcntl() function, identifying the file to which the lock is to be
applied or removed.
operation Specifies one of the following constants for flock(), defined in
the fcntl.h file:
LOCK_SH Apply a shared lock.
LOCK_EX Apply an exclusive lock.
LOCK_NB Do not block when locking. This value can be logically
ORed with either LOCK_SH or LOCK_EX.
LOCK_UN Remove a lock.
The flock() function applies or removes an advisory lock on the file asso-
ciated with the filedes file descriptor. Advisory locks allow cooperating
processes to perform consistent operations on files, but do not guarantee
consistency (that is, processes may still access files without using
advisory locks, possibly resulting in inconsistencies).
You can use the flock() function to coordinate a file's lock status on
local, CFS, and NFS file systems.
The locking mechanism allows two types of locks: shared locks and exclusive
locks. At any time multiple shared locks may be applied to a file, but at
no time are multiple exclusive, or both shared and exclusive, locks allowed
simultaneously on a file.
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 LOCK_NB is
included in operation, then this will not happen; instead, the call will
fail and errno will be set to [EWOULDBLOCK].
Locks are on files, not file descriptors. That is, file descriptors dupli-
cated using the dup() or fork() functions 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.
The flock() interface is not part of any UNIX standard. Therefore, if you
are designing and writing applications to be portable across platforms, you
should use the fcntl() file locking interface instead of flock().
Upon successful completion, 0 (zero) is returned. Otherwise, -1 is
returned and errno is set to indicate the error.
If the flock() function fails, errno may be set to one of the following
The file is locked and the LOCK_NB option was specified.
[EBADF] The filedes argument is not a valid open file descriptor.
[EINTR] A signal interrupted the flock call.
[EINVAL] The operator is not valid.
[ENOLCK] The lock table is full. Too many regions are already locked.
[EDEADLK] The lock is blocked by some lock from another process. Putting
the calling process to sleep while waiting for that lock to
become free would cause a deadlock.
Functions: close(2), exec(2), fcntl(2), fork(2), open(2), lockf(3)