KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q48059: "String Space Corrupt" If BSAVE Variable-Length-String Array

Article: Q48059
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S890724-71 B_BasicCom | mspl13_basic
Last Modified: 15-DEC-1989

If you want to use BSAVE and BLOAD with string arrays, you must use an
array of fixed-length strings. Fixed-length strings are available in
Microsoft QuickBASIC Versions 4.00, 4.00b, and 4.50 for MS-DOS, in
Microsoft BASIC Compiler Versions 6.00 and 6.00b for MS-DOS, and in
Microsoft BASIC PDS Version 7.00, but not in earlier versions.

Arrays of variable-length strings CANNOT be BSAVEd to a file, nor can
a file that was BSAVEd from a variable-length-string array be BLOADed
into another variable-length-string array. A "String space corrupt"
error message can display if you attempt to BLOAD a file into a
variable-length-string array, because the pointers in the BSAVEd
string descriptors will overlay and tangle existing pointers to string
space. This is the same mistake as POKEing a spurious value into a
string descriptor, which can corrupt the integrity of string space.

This information applies to Microsoft QuickBASIC Versions 1.00, 1.01,
1.02, 2.00, 2.01, 3.00, 4.00, 4.00b, and 4.50 for MS-DOS, to Microsoft
BASIC Compiler Versions 6.00 and 6.00b for MS-DOS and MS OS/2, and to
Microsoft BASIC PDS Version 7.00 for MS-DOS and MS OS/2.

Each element of variable-length-string array has a 4-byte string
descriptor composed of an offset (a 2-byte pointer) and length field
(2 bytes). The array of string descriptors is stored sequentially, but
the actual contents of the strings are stored separately in the
dynamic string space. Each 2-byte offset points to a location in the
string space. The string space memory is very dynamic, and strings are
given new offsets whenever new string values are reassigned. The
string contents of an array are not usually adjacent, especially if
they have been reassigned values. As a result, BSAVEing a certain
number of bytes does not mean that you've BSAVEd the contents of the
variable-length-string array.

The following code example attempts to BSAVE a variable-length-string
array, but generates the error "String Space Corrupt" when run within
the QuickBASIC QB.EXE Version 4.00, 4.00b, or 4.50 environment.

To work around this situation, create fixed-length-string arrays and
BSAVE that information. Fixed-length-string space is allocated
statically and sequentially in memory, and can be BSAVEd and BLOADed.

Code Example
------------

This example requires Microsoft QuickBASIC Version 4.00, 4.00b, or
4.50 for MS-DOS, Microsoft BASIC Compiler Version 6.00 or 6.00b for
MS-DOS, or Microsoft BASIC PDS Version 7.00 for MS-DOS.

To alter this program to work correctly, change the DIMension
statements to create a fixed-length-string array and BSAVE just that
many bytes.

OPTION BASE 1
DIM Arr1$(10)   ' Instead, use DIM Arr1(10) AS STRING*20
DIM Arr2$(10)   ' Instead, use DIM Arr2(10) AS STRING*20
StrDesc% = 4
ArrayLength% = 0
PRINT "This is the BSAVE array:"
PRINT
FOR I = 1 TO 10
   Arr1$(I) = "TEST" + STR$(I)
   ArrayLength% = ArrayLength% + LEN(Arr1$(I))
   PRINT Arr1$(I)
NEXT I

DEF SEG = VARSEG(Arr1$(1))
' In BC.EXE and QBX.EXE for BASIC 7.00 use SSEG for far variable
' length strings.

BSAVE "Test.Dat", VARPTR(Arr1$(1)), ArrayLength% + StrDesc%
DEF SEG
PRINT
PRINT "Hit a Key"
PRINT
SLEEP
DEF SEG = VARSEG(Arr2$(1))
' In BC.EXE and QBX.EXE for BASIC 7.00 use SSEG for far variable
' length strings.

BLOAD "Test.Dat", VARPTR(Arr2$(1))
DEF SEG
PRINT "This is the BLOADed array:"
PRINT
FOR I = 1 TO 10
   PRINT Arr2$(I)
NEXT I

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.