GWBASIC User's Manual

GW-BASIC User's Guide
LOCK Statement
Purpose:
To restrict the access to all or part of a file that has been opened by another process. This is used
in a multi-device environment, often referred to as a network or network environment.
Syntax:
LOCK [#]n [,[record number] [TO record number]]
Comments:
n is the number that was assigned to the file as it was originally numbered in the program.
record number is the number of the individual record that is to be locked. Or, if a range of
records are to be locked, record number designates the beginning and ending record of the
specified range.
The range of legal record numbers is 1 to 2 32 -1. The limit on record size is 32767 bytes.
The record range specified must be from lower to (the same or) higher record numbers.
If a starting record number is not specified, the record number 1 is assumed.
If an ending record number is not specified, then only the specified record is locked.
The following are examples of legal LOCK statements:
LOCK #n locks the entire file n
LOCK #n, X locks record X only
LOCK #n TO Y locks records 1 through Y
LOCK #n, X TO Ylocks records X through Y
With a random-access file, the entire opened file, or a range of records within an opened file, may
be locked, thus denying access to those records to any other process which has also opened the
file.
With a sequential access file that has been opened for input or output, the entire file is locked,
regardless of any record range specified. This is not considered an error. The specification of a
range in the LOCK statement regarding the sequential file will simply be disregarded.
The LOCK statement should be executed on a file or record range within a file before attempting
to read or write to that file.
The locked file or record range should be unlocked before the file is closed. Failure to execute the
UNLOCK statement can jeopardize future access to that file in a network environment.
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