Why a Program Might Hang with BLOAD/BSAVE

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This article was previously published under Q61344

SUMMARY

There are two reasons a program might hang when using the BLOAD statement in Microsoft QuickBasic. First, it is possible to hang your computer by BLOADing to the incorrect area in memory. Second, if the array you are BLOADing into is not large enough, the computer might hang or data and code might become corrupt.

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MORE INFORMATION

The most common programming error with BLOAD is to BLOAD into an array and only use the offset value [VARPTR()] of the array. If the array you BLOAD into is being stored as a far object and you only use the offset of the variable [VARPTR()], you will write over data in your DGROUP and possibly some of your code, and if you are using QB.EXE, you could write over the code for the editor. To avoid this problem, always use the VARSEG function to get the segment of the array and then invoke a DEF SEG beforehand to set the current segment to the one containing the array variable. This ensures that the data will go into the correct memory location, whether the array is being stored as a near or far object.

The second programming error is to BLOAD into a data-item that is smaller than the one that was used to BSAVE the data. This is similar to the error above in that this might cause you to write over both code and data in memory.

Both of these programming errors can cause a variety of problems, as follows:

You might receive a "String space corrupt" error.
You might receive a "Far heap corrupt" error.
Data values might be corrupt or incorrect.
The program could display totally erratic behavior because of corrupt code segments.
The computer might hang.

The following code examples demonstrate the correct and incorrect ways of executing a binary save.

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Code Examples

'***********************************************
'*  This program creates the file to BLOAD     *
'*                  MAKIT.BAS                  *
'***********************************************

REM $DYNAMIC
DEFINT A-Z
DIM buffer(16384)
DEF SEG = VARSEG(buffer(0))
BSAVE "test.bin", VARPTR(buffer(0)), 32768
DEF SEG

'***********************************************
'*  Correct method for BLOAD:  CORRECT.BAS     *
'***********************************************

REM $DYNAMIC
DEFINT A-Z
DIM buffer(16384)
' For far data objects, you must first set the segment
' to the segment of that object.
DEF SEG = VARSEG(buffer(0))
BLOAD "test.bin", VARPTR(buffer(0))
' The next line is necessary because you MUST
' reset the segment back to the Basic default segment.
DEF SEG
END


'***********************************************
'*  Incorrect method for BLOAD:  INCORRECT.BAS *
'***********************************************
' WARNING!!!!!: This probably will hang the computer.
' This example is just for comparison with the one above.

REM $DYNAMIC
DEFINT A-Z
DIM buffer(16384)
BLOAD "test.bin", VARPTR(buffer(0))
END

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APPLIES TO

Microsoft QuickBasic 3.0
Microsoft QuickBasic 4.0
Microsoft QuickBASIC 4.0b
Microsoft QuickBasic 4.5 for MS-DOS
Microsoft BASIC Compiler 6.0
Microsoft BASIC Compiler 6.0b
Microsoft BASIC Professional Development System 7.0

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KB61344

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Retired KB Content Disclaimer

This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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