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flock(2)
NAME
  flock - Applies or removes an advisory lock on an open file

SYNOPSIS
  #include <sys/fcntl.h>

  int flock(
	  int filedes,
	  int operation );

PARAMETERS
  filedes   Specifies a file descriptor returned by a successful open() or
	    fcntl() function, identifying the file to which the lock is to be
	    applied or removed.

  operation Specifies one of the following constants for flock(), defined in
	    the fcntl.h file:

	    LOCK_SH   Apply a shared lock.

	    LOCK_EX   Apply an exclusive lock.

	    LOCK_NB   Do not block when locking. This value can be logically
		      ORed with either LOCK_SH or LOCK_EX.

	    LOCK_UN   Remove a lock.

DESCRIPTION
  The flock() function applies or removes an advisory lock on the 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).

  You can use the flock() function to coordinate a file's lock status on
  local, CFS, and NFS file systems.

  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 errno will be set to [EWOULDBLOCK].

NOTES
  Locks are on files, not file descriptors.  That is, file descriptors
  duplicated using the dup() or fork() functions 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.

  The flock() 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 fcntl() file locking interface instead of flock().

RETURN VALUES
  Upon successful completion, 0 (zero) is returned.  Otherwise, -1 is
  returned and errno is set to indicate the error.

ERRORS
  If the flock() function fails, errno may be set to one of the following
  values:

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

  [EBADF]   The filedes argument is not a valid open file descriptor.

  [EINTR]   A signal interrupted the flock call.

  [EINVAL]  The operator is not valid.

  [ENOLCK]  The lock table is full.  Too many regions are already locked.

  [EDEADLK] 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.

RELATED INFORMATION
  Functions: close(2), exec(2), fcntl(2), fork(2), open(2), lockf(3)

Index for Section 2

Alphabetical listing for F

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