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lockf(3)
NAME
  lockf -  Locks and unlocks regions of open file descriptors

SYNOPSIS
  #include <unistd.h>

  int lockf(
	  int filedes,
	  int request,
	  off_t size );

STANDARDS
  Interfaces documented on this reference page conform to industry standards
  as follows:

  lockf():  XSH5.0

  Refer to the standards(5) reference page for more information about
  industry standards and associated tags.

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

  request   Specifies one of the following constants for the lockf()
	    function:

	    F_ULOCK   Unlocks a previously locked region in the file.

	    F_LOCK    Locks the region for exclusive use. This request causes
		      the calling process to sleep if the region overlaps a
		      locked region, and to resume when it is granted the
		      lock.

	    F_TLOCK   Same as F_LOCK, except that the request returns an
		      error if the region overlaps a locked region.

	    F_TEST    Tests to see if another process has already locked a
		      region. The lockf() function returns 0 (zero) if the
		      region is unlocked. If the region is locked, then -1 is
		      returned and errno is set to [EACCES].

  size	    The number of bytes to be locked or unlocked for the lockf()
	    function. The region starts at the current location in the open
	    file and extends forward if size is positive and backward if size
	    is negative. If the size parameter is 0 (zero),  the region
	    starts at the current location and extends forward to the maximum
	    possible file size, including the unallocated space after the end
	    of the file.

DESCRIPTION
  The lockf() function locks and unlocks sections of an open file.  Unlike
  the fcntl() function, however, its interface is limited to setting only
  write (exclusive) locks.

  Although the lockf() and fcntl() functions are different, the
  implementations are fully integrated.	 Therefore, locks obtained from one
  function are honored and enforced by the other lock function.

  Each lock is either an enforced lock or an advisory lock, and must also be
  either a read lock or a write lock.

  Locks on a file are advisory or enforced depending on the mode of the file
  (see the chmod() function.) A given file can have advisory or enforced
  locks, but not both.	See the sys/mode.h header file for a description of
  file attributes.

  When a process holds an enforced exclusive lock on a section of a file, no
  other process can access that section of the file with the read() or
  write() functions. In addition, the open(), truncate(), and ftruncate()
  functions cannot truncate the locked section of the file.  If another
  process attempts to read or modify the locked section of the file, it
  sleeps until the section is unlocked or returns with an error indication.

  The file descriptor on which an exclusive lock is being placed must have
  been opened with write access.

  Some general rules about file locking include the following:

    ·  Changing or unlocking part of a file in the middle of a locked section
       leaves two smaller sections locked at each end of the originally
       locked section.

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

    ·  Locks are not inherited by a child process after running a fork()
       function.

  Locks can start and extend beyond the current end of a file, but cannot be
  negative relative to the beginning of the file. A lock can be set to extend
  to the end of the file by setting the l_len field to 0 (zero). If a lock is
  specified with the l_start field set to 0 and the l_whence field set to
  SEEK_SET, the whole file is locked.

  Advisory file region locking is supported over NFS, provided the locking
  daemon (rpc.lockd) and status monitor daemon (rpc.statd) are running.

NOTES
  Buffered I/O does not work properly when used with file locking. Do not use
  the standard I/O package routines on files that will be locked.

  Deadlocks due to file locks in a distributed system are not always
  detected. When such deadlocks are possible, the programs requesting the
  locks should set time-out timers.

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



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

  [EACCES]  The file region is locked and F_TEST was specified; or the file
	    region is locked and F_TLOCK was specified.

  [EBADF]   The filedes parameter is not a valid open file descriptor; or the
	    request parameter is F_LOCK or F_TLOCK and filedes is not a valid
	    file descriptor open for writing.

  [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.

  [EINTR]   The request parameter is F_TLOCK and the lockf() function was
	    interrupted by a signal which was caught.

  [EINVAL]  The request parameter is not valid or size plus the current file
	    offset is less than 0 (zero).

  [ENOLCK]  The request parameter is F_LOCK, F_TLOCK, or F_UNLOCK and
	    satisfying the lock or unlock request would exceed the
	    configurable system limit of NLOCK_RECORD.

	    [Tru64 UNIX]  If using NFS, the server is out of resources or the
	    file handle is stale.

RELATED INFORMATION
  Functions: chmod(2), close(2), exec(2), fcntl(2), flock(2), fork(2),
  open(2), read(2), write(2)

  Commands: rpc.lockd(8), rpc.statd(8)

  Standards: standards(5)

Index for Section 3

Alphabetical listing for L

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