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Lock, Unlock statements

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Controls access by other processes to all or part of a file opened by using the Open statement.

Syntax

Lock [ # ] filenumber, [ recordrange ] . . . Unlock [ # ] filenumber, [ recordrange ]

The Lock and Unlock statement syntax has these parts:

Part Description
filenumber Required. Any valid file number.
recordrange Optional. The range of records to lock or unlock.

Settings

The recordrange argument settings are:

recnumber | [ start ] To end

Setting Description
recnumber Record number (Random mode files) or byte number (Binary mode files) at which locking or unlocking begins.
start Number of the first record or byte to lock or unlock.
end Number of the last record or byte to lock or unlock.

Remarks

The Lock and Unlock statements are used in environments where several processes might need access to the same file.

Lock and Unlock statements are always used in pairs. The arguments to Lock and Unlock must match exactly.

The first record or byte in a file is at position 1, the second record or byte is at position 2, and so on. If you specify just one record, only that record is locked or unlocked. If you specify a range of records and omit a starting record (start), all records from the first record to the end of the range (end) are locked or unlocked. Using Lock without recnumber locks the entire file; using Unlock without recnumber unlocks the entire file.

If the file has been opened for sequential input or output, Lock and Unlock affect the entire file, regardless of the range specified by start and end.

Important

Be sure to remove all locks with an Unlock statement before closing a file or quitting your program. Failure to remove locks produces unpredictable results.

Example

This example illustrates the use of the Lock and Unlock statements. While a record is being modified, access by other processes to the record is denied. This example assumes that TESTFILE is a file containing five records of the user-defined type Record.

Type Record    ' Define user-defined type. 
    ID As Integer 
    Name As String * 20 
End Type 
 
Dim MyRecord As Record, RecordNumber    ' Declare variables. 
' Open sample file for random access. 
Open "TESTFILE" For Random Shared As #1 Len = Len(MyRecord) 
RecordNumber = 4    ' Define record number. 
Lock #1, RecordNumber    ' Lock record. 
Get #1, RecordNumber, MyRecord    ' Read record. 
MyRecord.ID = 234    ' Modify record. 
MyRecord.Name = "John Smith" 
Put #1, RecordNumber, MyRecord    ' Write modified record. 
Unlock #1, RecordNumber    ' Unlock current record. 
Close #1    ' Close file.

See also

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