flock(2)							System Calls Manual							  flock(2)

NAME

       flock() - apply or remove an advisory or enforced lock on an open file

SYNOPSIS


   Parameters
       filedes	   Specifies  a file descriptor returned by a successful or function, identifying the file on which the locking operation is to be
		   performed.

       operation   Specifies one of the following constants for defined in the file:

		   Apply a shared lock.

		   Apply an exclusive lock.

		   Remove a lock.

		   In addition to the preceding constants, the following constant can be bitwise ORed with either or

		   LOCK_NB	 Do not block when locking.

DESCRIPTION

       The function applies or removes an advisory or enforced lock on the whole file associated with the filedes file descriptor.  Advisory locks
       allow  cooperating  processes  to  perform  consistent  operations on files, but do not guarantee consistency (that is, processes may still
       access files without using advisory locks, possibly resulting in inconsistencies).Enforced locks are in effect if the  enforcement  bit	in
       the file mode is set (see chmod(2)).  This might affect and other system calls performing file operations.

       You can use the function to coordinate a file's lock status on local and NFS file systems.

       The  locking mechanism allows two types of locks: and At any time multiple shared locks may be applied to a file, but at no time are multi-
       ple exclusive locks 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 is
       included in operation, this will not happen; instead, the call will fail and will be set to

APPLICATION USAGE

       Locks are on files, not file descriptors.  This means that:

       o  Locks are not inherited by a child process resulting from a call.

       o  All locks associated with a file for a given process are removed when any thread in the process closes  any  file  descriptor  for  that
	  file.

       Processes  that	are  blocked awaiting a lock may be awakened by signals.  An application needs to check the return value to make sure that
       the lock is really acquired.

       The interface is not part of any UNIX standard.	Therefore, if you are designing and writing applications to be portable across	platforms,
       you should use the file locking interface instead of

NETWORKING FEATURES

   NFS
       The  advisory file locking capabilities of are implemented throughout the network by the network lock daemon If the file server crashes and
       is rebooted, the lock daemon attempts to recover all locks associated with the crashed server.  If a lock cannot be reclaimed, the  process
       that held the lock is issued a signal.  See lockd(1M).

       File locking as implemented for NFS files is only advisory.

RETURN VALUES

       flock() returns:

       for successful completion

       for unsuccessful completion;
	    is set to indicate the error

ERRORS

       If the function fails, may be set to one of the following values:
       The filedes parameter is not a valid open file descriptor.

	      A  shared  lock  was requested and filedes was not opened for reading, or an exclusive lock was requested and filedes was not opened
	      for writing.

       The lock is blocked by some lock from another process.
	      Putting the calling process to sleep while waiting for that lock to become free would cause a deadlock.

       A signal interrupted the
	      call.

       The    filedes parameter refers to a file that does not support locking.

	      The operation parameter is not valid.

       The lock table is full.
	      Too many files are already locked.

	      The filedes parameter refers to an NFS file with the access bits set for enforcement mode.

	      The filedes parameter refers to an NFS file, and a system error occurred on the remote node.

       The file is locked and the
	      option was specified.

	      The file is mapped into virtual memory via the system call (see mmap(2)).

SEE ALSO

       lockd(1M) chmod(2), close(2), creat(2), dup(2), dup2(2), exec(2), fcntl(2), fork(2), lockf(2), open(2), read(2), truncate(2), write(2).

STANDARDS CONFORMANCE

																	  flock(2)