lockf(3) - NetBSD Manual Pages

LOCKF(3)                NetBSD Library Functions Manual               LOCKF(3)


NAME

     lockf -- record locking on files


LIBRARY

     Standard C Library (libc, -lc)


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 advi-
     sory-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

     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 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 cur-
     rent 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.

     F_LOCK and F_TLOCK requests differ only by the action taken if the sec-
     tion is not available.  F_LOCK blocks the calling process until the sec-
     tion 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) 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 sec-
     tion), 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 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 descrip-
                        tor.

                        The argument function is F_LOCK or F_TLOCK, and
                        filedes is not a valid file descriptor open for writ-
                        ing.

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

     [EINTR]            The argument function is F_LOCK and lockf() was inter-
                        rupted 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 exceed-
                        ing a system-imposed limit.


SEE ALSO

     fcntl(2), flock(2), flockfile(3)


STANDARDS

     The lockf() function conforms to X/Open Portability Guide Issue 4,
     Version 2 (``XPG4.2'') and IEEE Std 1003.1-2008 (``POSIX.1'').


HISTORY

     The lockf() function first appeared in FreeBSD 1.4.

NetBSD 10.99                   October 15, 2011                   NetBSD 10.99