flock(3UCB)					     SunOS/BSD Compatibility Library Functions					       flock(3UCB)

NAME

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

SYNOPSIS

       /usr/ucb/cc[ flag ... ] file ...
       #include <sys/file.h>

       int flock( fd,  operation);
       int fd, operation;

DESCRIPTION

       flock()	applies  or  removes an advisory lock on the file associated with the file descriptor fd. The compatibility version of flock() has
       been implemented on top of fcntl(2) locking.
	It does not provide complete binary compatibility.

       Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee exclusive access (that is,  pro-
       cesses 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. More than one process may hold a shared lock for a file
       at any given time, but multiple exclusive, or both shared and exclusive, locks may not exist simultaneously on a file.

       A lock is applied by specifying an operation parameter LOCK_SH for a shared lock or LOCK_EX for an exclusive lock. The operation  paramerer
       may be ORed with LOCK_NB to make the operation non-blocking. To unlock an existing lock, the operation should be LOCK_UN.

       Read permission is required on a file to obtain a shared lock, and write permission is required to obtain an exclusive lock. Locking a seg-
       ment that is already locked by the calling process causes the old lock type to be removed and the new lock type to take effect.

       Requesting a lock on an object that is already locked normally causes the caller to block 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.

RETURN VALUES

       flock() returns:

       0	on success.

       -1	on failure and sets errno to indicate the error.

ERRORS

       EBADF		       The argument fd is an invalid descriptor.

       EINVAL		       operation is not a valid argument.

       EOPNOTSUPP	       The argument fd refers to an object other than a file.

       EWOULDBLOCK	       The file is locked and the  LOCK_NB option was specified.

SEE ALSO

       lockd(1M), chmod(2), close(2), dup(2), exec(2), fcntl(2), fork(2), open(2), lockf(3C)

NOTES

       Use of these interfaces should be restricted to only applications written on BSD platforms.  Use of these interfaces with any of the system
       libraries or in multi-thread applications is unsupported.

       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. Locks are not inherited by a child process.

       Processes blocked awaiting a lock may be awakened by signals.

       Mandatory locking may occur, depending on the mode bits of the file.  See chmod(2).

       Locks obtained through the flock() mechanism under SunOS 4.1 were known only within the system on which	they  were  placed.   This  is	no
       longer true.

SunOS 5.10							    19 Jul 1994 						       flock(3UCB)