| FLOCK(2) | System Calls Manual | FLOCK(2) | 
flock —
#include <fcntl.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
    fd, int
  operation);
flock() applies or removes an advisory
  lock on the file associated with the file descriptor fd.
  A lock is applied by specifying an operation parameter
  that is one of LOCK_SH or
  LOCK_EX with the optional addition of
  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 (i.e., processes may still access files without using advisory locks possibly resulting in inconsistencies).
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 EAGAIN will be returned.
Processes blocked awaiting a lock may be awakened by signals.
flock() call fails if:
EAGAIN]LOCK_NB option was
      specified.EBADF]EINVAL]LOCK_EX, LOCK_SH, or
      LOCK_UN.kern.maxfiles.EOPNOTSUPP]flock() function call appeared in
  4.2BSD.
| October 15, 2011 | NetBSD 9.4 |