#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 operation );
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. Specifies one of the following constants for flock(), defined in the fcntl.h file: Apply a shared lock. Apply an exclusive lock. Do not block when locking. This value can be logically ORed with either LOCK_SH or LOCK_EX. Remove a lock.
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].
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.)
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 values: The file is locked and the LOCK_NB option was specified. The filedes argument is not a valid open file descriptor. A signal interuppted the flock call. The operator is not valid. The lock table is full. Too many regions are already locked. 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: open(2), close(2), exec(2), fcntl(2), fork(2), lockf(3) delim off