ultrix man page for flock

flock(2)							System Calls Manual							  flock(2)

Name
       flock - apply or remove an advisory lock on an open file

Syntax
       #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
       The  system  call 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; that is, processes
       might 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 can be applied to  a
       file.  However, multiple exclusive locks, or shared and exclusive locks cannot be applied simultaneously on a file.

       A  shared  lock can be upgraded to be an exclusive lock, and an exclusive lock can become shared, simply by specifying the appropriate lock
       type. This change results in the previous lock being released and the new lock applied. When upgrading, do not include  LOCK_NB	in  opera-
       tion, because there is a possibility that other processes have requests for locks, or have gained or released a lock.

       Requesting  a lock on an object that is already locked normally causes the caller to blocked until the lock can be acquired.  If LOCK_NB is
       included in operation, the call is not blocked; instead, the call fails and the error EWOULDBLOCK is returned.

       Locks are on files, not file descriptors.  That is, file descriptors duplicated through or call 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 loses its lock.

       Processes blocked awaiting a lock may be awakened by signals.

Return Values
       Zero is returned if the operation was successful; on an error, a -1 is returned and an error code is stored in the global variable, errno.

Diagnostics
       The call fails under the following conditions:

       [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.

       [EOPNOTSUPP]   Invalid operation is requested.  The argument fd refers to a socket.

Restrictions
       File region locking is not supported over NFS.

See Also
       close(2), dup(2), execve(2), fork(2), open(2)

																	  flock(2)