Section 13,65: UNLOCK

 Frees the current record (that is, the last record read) in an
 indexed (VMS only), relative, or sequential file.  By default, a
 record is locked when it is read.  The lock is normally held until
 your program performs another I/O operation on the unit (for
 example, rewriting the record, reading another record, or closing
 the file).

 Statement format:

    UNLOCK ([UNIT=]u[,ERR=s][,IOSTAT=ios])
    UNLOCK u

    u    An integer variable or constant specifying the 
         logical unit number of the file, optionally 
         prefaced by UNIT=.  UNIT= is required if unit is
         not the first I/O specifier.

    s    The label of a statement to which control is 
         transferred if an error condition occurs.

    ios  An integer scalar memory reference that is 
         defined as a positive integer if an error occurs 
         and zero if no error occurs.