LOCKF(3)                  OpenBSD Programmer's Manual                 LOCKF(3)

NAME
     lockf - record locking on files



SYNOPSIS
     #include <unistd.h>

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

DESCRIPTION
     The lockf() function allows sections of a file to be locked with adviso-
     ry-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 the locks for a process are re-
     moved when the process terminates.

     The argument filedes 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 values 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.

     The F_ULOCK function 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 section.

     The size argument is the number of contiguous bytes to be locked or un-
     locked. The section to be locked or unlocked starts at the current offset
     in the file and extends forward for a positive size or backward 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, con-
     tain or be contained by a previously locked section for the same process.
     When this occurs, or if adjacent locked sections would occur, the sec-
     tions 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.

     The 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 section 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) of one or more locked sec-
     tions controlled by the process. Locked sections will be unlocked start-
     ing at the current file offset through size bytes or to the end of the
     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 sepa-
     rate 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 re-
     quest 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 re-
     quested 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 pro-
     cess.  This implementation detects that sleeping until a locked region is
     unlocked would cause a deadlock and fails with an EDEADLK error.

     lockf(), fcntl(2) and flock(2) locks may be safely used concurrently.

     Blocking on a section is interrupted by any signal.

RETURN VALUES
     If successful, the lockf() function returns 0.  Otherwise, it returns -1,
     sets the global variable errno to indicate an error, and existing locks
     are not changed.

ERRORS
     lockf() 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 filedes is not a valid open file descriptor.

                   The argument function is F_LOCK or F_TLOCK, and filedes is
                   not a valid file descriptor 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_ULOCK, F_LOCK,
                   F_TLOCK or F_TEST.

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

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

SEE ALSO
     fcntl(2),  flock(2)

STANDARDS
     The lockf() function conforms to X/Open Portability Guide Issue 4.2
     (``XPG4.2'').

OpenBSD 2.6                    December 19, 1997                             2