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Index for Section 2 |
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Alphabetical listing for F |
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flock(2)
NAME
flock - Applies or removes an advisory lock on an open file
SYNOPSIS
#include <sys/file.h>
#define LOCK_SH 1 /* shared lock */
#define LOCK_EX 2 /* exclusive lock */
#define LOCK_NB 4 /* don't block when locking */
#define LOCK_UN 8 /* unlock */
int flock(
int filedes,
int operation );
PARAMETERS
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.
DESCRIPTION
The flock() function applies or removes an advisory lock on the file
associated 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).
The flock() function operates on the local system only. It does not make
any attempt to coordinate a file's lock status with other systems. In a
distributed environment, use the fcntl() or lockf() interfaces to place
advisory locks on files, as they provide a superset of flock() features.
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].
NOTES
Locks are on files, not file descriptors. That is, file descriptors
duplicated 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 file locks set by the flock()function do not interact in any way with
the file locks set by the fcntl() and lockf() functions. If a process sets
an exclusive lock on a file using the flock() function, the lock will not
affect any process that is setting or clearing locks on the same file using
the fcntl() or lockf() functions. It is therefore possible for an
inconsistency to arise if a file is locked by different processes using
flock() and fcntl(). (The fcntl() and lockf() functions use the same
mechanism for record locking.)
RETURN VALUES
Upon successful completion, 0 (zero) is returned. Otherwise, -1 is
returned and errno is set to indicate the error.
ERRORS
If the flock() function fails, errno may be set to one of the following
values:
[EWOULDBLOCK]
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 interuppted 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.
RELATED INFORMATION
Functions: open(2), close(2), exec(2), fcntl(2), fork(2), lockf(3)