Q27489: "Out of String Space," Dynamic Array of Fixed-Length Strings

Article: Q27489
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 7-FEB-1989

When running the program below inside the QuickBASIC Version 4.00,
4.00b, or 4.50 environment, an "Out of String Space" error is
generated after fewer than 70 iterations of concatenating a variable
length string onto a fixed-length string array element. However, the
FRE("") and FRE(-1) functions both return the same value all the way
through the loop. Note that the fixed-length string array element is
part of a dynamic array, which puts it in the far heap; therefore, it
normally does not use up any of the normal string space in the default
data segment.

The program below demonstrates an internal limitation of the QB.EXE
interpreter that is rarely encountered. Microsoft currently does not
plan to change this limitation.

Note that fixed-length strings are not supported by QuickBASIC
versions earlier than Version 4.00.

As a workaround, please note that the problem only occurs inside the
QuickBASIC Version 4.00 editor. When compiled to an .EXE file, the
sample program works properly.

The problem is that QuickBASIC assumes that a far fixed-length string
will always stay far. The BASIC run-time routines must have the string
in DGROUP to manipulate it with string functions like LTRIM$ and
RTRIM$.

LTRIM$ and RTRIM$ require two copies of the string at one point, and
the program eventually runs out of string space in the default data
segment, DGROUP. This behavior also occurs in a version of the
program compiled with BC /O if the strings are made longer. This
limitation of BASIC is inherent in its construction, and Microsoft
currently has no plans to change this construction.

The following is a code example:

 REM $DYNAMIC
 DIM srv(0) AS STRING * 28000
 abc$ = STRING$(330,"0")
 srv(0) = abc$
 FOR t = 1 TO 79
   srv(0) = RTRIM$(srv(0)) + abc$
   PRINT t, LEN(RTRIM$(srv(0))), FRE(""), FRE(-1)
 NEXT T

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