Q45055: Bad Random GET Record Order with TYPE’s Element as Next Record

Article: Q45055
Product(s): See article
Version(s): 4.00 4.00B 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S890515-70 B_BasicCom | mspl13_basic
Last Modified: 1-JUN-1989

The following issue can arise when you use user-defined TYPEs, and
each record in your random-access file contains a field that points
to the next record number.

It has been reported that when running a compiled BASIC program, it is
possible to get inconsistent results when the record-number argument
of a random-access GET is based upon an element of a user-defined-TYPE
record that is input in the same GET statement.

The following is an example:

   GET #1, ARec.NextRec, ARec

ARec.NextRec is an element of the user-defined record ARec, and is
also the pointer to the next record number in the file (which is known
as a linked-list file structure).

The behavior of such a GET statement has been reported as
inconsistent from a compiled .EXE program, and records may be read in
the wrong order. The same program works correctly running from within
the QB.EXE environment.

Microsoft does not advise using an element that you are reading data
into as the pointer to the record being read in the same GET
statement. Instead, assign the element containing the next record to
be read to a temporary variable, and use that temporary variable in
the next GET statement.

This information applies to QuickBASIC Versions 4.00, 4.00b, and 4.50,
and to the BASIC Compiler Versions 6.00 and 6.00b.

The following code fragment helps explain the problem (but is not
sufficiently complete to reproduce the error):

   TYPE atype
      NextRec as integer
      CurrentData as single
   END TYPE
   DIM ARec as atype
   ' Please avoid doing GETs such as the following, which use an
   ' element of the input record as the next record number to input
   ' in the same GET statement:
   GET #1, ARec.NextRec, ARec

As a workaround, use a temporary variable to specify the next record
to GET:

   TYPE atype
      NextRec as integer
      CurrentData as single
   END TYPE
   DIM ARec as atype
   temp%=ARec.NextRec
   GET #1, temp%, ARec

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