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LOCKF(3)                 BSD Library Functions Manual                 LOCKF(3)

NAME
     lockf -- record locking on files

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     lockf(int fildes, int function, off_t size);

DESCRIPTION
     The lockf() function allows sections of a file to be locked with advisory-mode locks.  Calls to lockf()
     from other processes which attempt to lock the locked file section will either return an error value or
     block until the section becomes unlocked.  All of the locks for a process are removed when the process
     terminates.

     The argument fildes is an open file descriptor.  The file descriptor must have been opened either for
     write-only (O_WRONLY) or read/write (O_RDWR) operation.

     The function argument is a control value which specifies the action to be taken.  The permissible val-ues values
     ues for function are as follows:
           Function   Description
           F_ULOCK    unlock locked sections
           F_LOCK     lock a section for exclusive use
           F_TLOCK    test and lock a section for exclusive use
           F_TEST     test a section for locks by other processes

     F_ULOCK removes locks from a section of the file; F_LOCK and F_TLOCK both lock a section of a file if
     the section is available; F_TEST detects if a lock by another process is present on the specified sec-tion. section.
     tion.

     The size argument is the number of contiguous bytes to be locked or unlocked.  The section to be locked
     or unlocked starts at the current offset in the file and extends forward for a positive size or back-ward backward
     ward for a negative size (the preceding bytes up to but not including the current offset).  However, it
     is not permitted to lock a section that starts or extends before the beginning of the file.  If size is
     0, the section from the current offset through the largest possible file offset is locked (that is,
     from the current offset through the present or any future end-of-file).

     The sections locked with F_LOCK or F_TLOCK may, in whole or in part, contain or be contained by a pre-viously previously
     viously locked section for the same process.  When this occurs, or if adjacent locked sections would
     occur, the sections are combined into a single locked section.  If the request would cause the number
     of locks to exceed a system-imposed limit, the request will fail.

     F_LOCK and F_TLOCK requests differ only by the action taken if the section is not available.  F_LOCK
     blocks the calling process until the section is available.  F_TLOCK makes the function fail if the sec-tion section
     tion is already locked by another process.

     File locks are released on first close by the locking process of any file descriptor for the file.

     F_ULOCK requests release (wholly or in part) one or more locked sections controlled by the process.
     Locked sections will be unlocked starting at the current file offset through size bytes or to the end
     of file if size is 0.  When all of a locked section is not released (that is, when the beginning or end
     of the area to be unlocked falls within a locked section), the remaining portions of that section are
     still locked by the process.  Releasing the center portion of a locked section will cause the remaining
     locked beginning and end portions to become two separate locked sections.  If the request would cause
     the number of locks in the system to exceed a system-imposed limit, the request will fail.

     An F_ULOCK request in which size is non-zero and the offset of the last byte of the requested section
     is the maximum value for an object of type off_t, when the process has an existing lock in which size
     is 0 and which includes the last byte of the requested section, will be treated as a request to unlock
     from the start of the requested section with a size equal to 0.  Otherwise an F_ULOCK request will
     attempt to unlock only the requested section.

     A potential for deadlock occurs if a process controlling a locked region is put to sleep by attempting
     to lock the locked region of another process.  This implementation detects that sleeping until a locked
     region is unlocked would cause a deadlock and fails with an EDEADLK error.

     The lockf(), fcntl(2), and flock(2) locks are compatible.  Processes using different locking interfaces
     can cooperate over the same file safely.  However, only one of such interfaces should be used within
     the same process.  If a file is locked by a process through flock(2), any record within the file will
     be seen as locked from the viewpoint of another process using fcntl(2) or lockf(), and vice versa.

     Blocking on a section is interrupted by any signal.

RETURN VALUES
     The lockf() function returns the value 0 if successful; otherwise the value -1 is returned and the
     global variable errno is set to indicate the error.  In the case of a failure, existing locks are not
     changed.

ERRORS
     The lockf() function will fail if:

     [EAGAIN]           The argument function is F_TLOCK or F_TEST and the section is already locked by
                        another process.

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

                        The argument function is F_LOCK or F_TLOCK, and fildes is not a valid file descrip-tor descriptor
                        tor open for writing.

     [EDEADLK]          The argument function is F_LOCK and a deadlock is detected.

     [EINTR]            The argument function is F_LOCK and lockf() was interrupted by the delivery of a
                        signal.

     [EINVAL]           The argument function is not one of F_LOCK, F_TEST, F_TLOCK, or F_ULOCK.

                        The argument fildes refers to a file that does not support advisory locking.

     [ENOLCK]           The argument function is F_LOCK, F_TLOCK, or F_ULOCK and satisfying the lock or
                        unlock request would result in the number of locked regions in the system exceeding
                        a system-imposed limit.

     [EOPNOTSUPP]       The argument fildes refers to a socket; these do not support advisory locking.

SEE ALSO
     fcntl(2), flock(2)

STANDARDS
     The lockf() function conforms to X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'').

BSD                            December 19, 1997                           BSD