


      LOCKF(3C)                   (UNIX 5.0)                  LOCKF(3C)



      NAME
           lockf - record locking on files

      SYNTAX
           # include <unistd.h>

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

      DESCRIPTION
           The lockf command will allow sections of a file to be
           locked; advisory or mandatory write locks depending on the
           mode bits of the file (see chmod(2)). Locking calls from
           other processes which attempt to lock the locked file
           section will either return an error value or be put to sleep
           until the resource becomes unlocked.  All the locks for a
           process are removed when the process terminates.  [See
           fcntl(2) for more information about record locking.]

           Fildes is an open file descriptor.  The file descriptor must
           have O_WRONLY or O_RDWR permission in order to establish
           lock with this function call.

           Function is a control value which specifies the action to be
           taken.  The permissible values for function are defined in
           <unistd.h> as follows:

           #define   F_ULOCK   0    /* Unlock a previously locked section */
           #define   F_LOCK    1    /* Lock a section for exclusive use */
           #define   F_TLOCK   2    /* Test and lock a section for exclusive use */
           #define   F_TEST    3    /* Test section for other processes locks */


           All other values of function are reserved for future
           extensions and will result in an error return if not
           implemented.

           F_TEST is used to detect if a lock by another process is
           present on the specified section.  F_LOCK and F_TLOCK both
           lock a section of a file if the section is available.
           F_UNLOCK removes locks from a section of the file.

           Size is the number of contiguous bytes to be locked or
           unlocked.  The resource to be locked starts at the current
           offset in the file and extends forward for a positive size
           and backward for a negative size (the preceding bytes up to,
           but not including the current offset).  If size is zero, the
           section from the current offset through the largest file
           offset is locked (i.e., from the current offset through the
           present or any future end-of-file).  An area need not be
           allocated to the file in order to be locked, as such locks



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           may exist past the 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 sections occur, the sections are combined into a
           single section.  If the request requires that a new element
           be added to the table of active locks and this table is
           already full, an error is returned, and the new section is
           not locked.

           F_LOCK and F_TLOCK requests differ only by the action taken
           if the resource is not available.  F_LOCK will cause the
           calling process to sleep until the resource is available.
           F_TLOCK will cause the function to return a -1 and set errno
           to [EAGAIN] error if the section is already locked by
           another process.

           F_ULOCK requests may, in whole or in part, release one or
           more locked sections controlled by the process.  When
           sections are not fully released, the remaining sections are
           still locked by the process.  Releasing the center section
           of a locked section requires an additional element in the
           table of active locks.  If this table is full, an [ENOLCK]
           error is returned and the requested section is not released.

           A potential for deadlock occurs if a process controlling a
           locked resource is put to sleep by accessing another
           process's locked resource.  Thus calls to lock or fcntl scan
           for a deadlock prior to sleeping on a locked resource.  An
           error return is made if sleeping on the locked resource
           would cause a deadlock.

           Sleeping on a resource is interrupted with any signal.  The
           alarm(2) command may be used to provide a timeout facility
           in applications which require this facility.

      ERRORS
           The lockf utility will fail if one or more of the following
           are true:


                Fildes is not a valid open descriptor.  [EBADF]

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

                Cmd is F_LOCK or F_TLOCK and a deadlock would occur.
                [EDEADLK]

                There are not enough entries in the system lock table
                to honor the request.  [ENOLCK]



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      LOCKF(3C)                   (UNIX 5.0)                  LOCKF(3C)



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

      SEE ALSO
           chmod(2), close(2), creat(2), fcntl(2), intro(2), open(2),
           read(2), write(2).















































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