lockf



LOCKF(3)                   Library Functions 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
     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 the locks for a process
     are removed 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
     unlocked.  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,
     contain or be contained by a previously 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.

     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
     sections controlled by the process.  Locked sections will be unlocked
     starting 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 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.

     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,
     Version 2 ("XPG4.2").

OpenBSD 6.2                      June 5, 2013                      OpenBSD 6.2

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