table of contents
|FLOCK(2)||System Calls Manual||FLOCK(2)|
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 */
system call applies or removes an
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 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
LOCK_NB is included in
operation, 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.
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
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
flock() system call fails if:
- The file is locked and the
LOCK_NBoption was specified.
- The argument fd is an invalid descriptor.
- The argument fd refers to an object other than a file.
- The argument fd refers to an object that does not support file locking.
- A lock was requested, but no locks are available.
close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), flopen(3), lockf(3)
flock() system call appeared in
|November 9, 2011||Debian|