bsd man page for flock

FLOCK(2)							System Calls Manual							  FLOCK(2)

NAME
       flock - apply or remove 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 */

       flock(fd, operation)
       int fd, operation;

DESCRIPTION
       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 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 (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 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.

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.

SEE ALSO
       open(2), close(2), dup(2), execve(2), fork(2)

4.2 Berkeley Distribution					   May 22, 1986 							  FLOCK(2)