FLOCK(2)
NAME
flock − place or remove an advisory lock on an open file
USAGE
#include <sys/file.h>
#defineLOCK_SH1/* shared lock */
#defineLOCK_EX2/* exclusive lock */
#defineLOCK_NB4/* don’t block when locking */
#defineLOCK_UN8/* unlock */
flock(fd, operation)
int fd, operation;
DESCRIPTION
Flock applies or removes an advisory lock on the file identified by the descriptor fd. A lock is applied by specifying an operation parameter which is the ‘inclusive or’ of LOCK_SH or LOCK_EX and, possibly, LOCK_NB. To unlock an existing lock, operation should be LOCK_UN.
Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee consistency. (Processes may still access files without using advisory locks, and this may result in inconsistencies).
The locking mechanism allows two types of locks: shared locks and exclusive locks. Multiple shared locks may be applied to a file at any time. At no time are multiple exclusive locks, or a combination of shared and exclusive locks, allowed on a file.
A shared lock may be upgraded to an exclusive lock (or an exclusive lock made shared) by specifying the appropriate lock type; this releases the previous lock and applies the new one.
Requesting a lock on an object that is already locked normally causes the caller to 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.
NOTES
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.
Processes blocked awaiting a lock may be awakened by signals.
All processes that use advisory locks on a given file must be running on the same node.
RETURN VALUE
Zero is returned if the operation was successful; on an error, a −1 is returned and an error code is left in the global location errno.
ERRORS
The flock call fails if:
[EWOULDBLOCK] The file is locked and the LOCK_NB option was specified.
[EBADF] The argument fd is an invalid descriptor.
[EINVAL] The argument fd refers to an object other than a file.