apply or remove an advisory lock on an open file
Standard C Library (libc, -lc)
#define LOCK_SH 0x01 /* shared file lock
#define LOCK_EX 0x02 /* exclusive file lock */
#define LOCK_NB 0x04 /* do not block when locking */
#define LOCK_UN 0x08 /* unlock file */
flock() system call applies or removes an
advisory lock on the file associated with the file
descriptor fd. A lock is applied by specifying an
operation argument that is one of
LOCK_EX with the
optional addition of
LOCK_NB. To unlock an existing
operation should be
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
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
Locks are on files, not file descriptors. That is, file descriptors duplicated
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.
LOCK_NB is included in
operation, then this will not happen; instead the call
will fail and the error
EWOULDBLOCK will be
locks are compatible. Processes using different locking interfaces can
cooperate over the same file safely. However, only one of such interfaces
should be used within the same process. If a file is locked by a process
flock(), any record within the file will be
seen as locked from the viewpoint of another process using
and vice versa.
Processes blocked awaiting a lock may be awakened by signals.
flock() function returns the value 0 if
successful; otherwise the value -1 is returned and the global variable
errno is set to indicate the error.
flock() system call fails if:
- The file is locked and the
LOCK_NB option was
- The argument fd is an invalid descriptor.
- The argument fd refers to an object other than a
- The argument fd refers to an object that does not
support file locking.
- A lock was requested, but no locks are available.
flock() system call appeared in