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flock - 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(int fd, int operation);
The 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_SH or LOCK_EX with the optional
addition of LOCK_NB. To unlock an existing lock 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 LOCK_NB is
included in operation, then this will not happen; instead the call will
fail and the error EWOULDBLOCK will be returned.
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.
The flock(), fcntl(2), and lockf(3) 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 through flock(), any
record within the file will be seen as locked from the viewpoint of
another process using fcntl(2) or lockf(3), and vice versa.
Processes blocked awaiting a lock may be awakened by signals.
The flock() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
The flock() system call fails if:
[EWOULDBLOCK] The file is locked and the LOCK_NB option was
[EBADF] The argument fd is an invalid descriptor.
[EINVAL] The argument fd refers to an object other than a file.
[EOPNOTSUPP] The argument fd refers to an object that does not
support file locking.
close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), flopen(3),
The flock() system call appeared in 4.2BSD.