Q28166: QB Uses Up Default String Space When Concatenating Far String

Article: Q28166
Product(s): See article
Version(s): 6.00 6.00b 7.00 | 6.00 6.00b 7.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | B_QuickBas | mspl13_basic
Last Modified: 2-FEB-1990

Problem:

When running the program below inside the QB.EXE environment, after
about 72 iterations of concatenating a variable-length string onto a
fixed-length string, an "Out of String Space" error is generated.
However, the FRE("") and FRE(-1) functions both return the same value
all the way through the loop. Note that the array of fixed-length
strings is dynamically allocated, putting it into the far heap.

The program runs properly when compiled and run as an EXE file.

Response:

The problem is that QuickBASIC assumes that a far fixed-length string
will always stay far.

However, the BASIC run-time routines must have the string in DGROUP in
order 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 in DGROUP. This will also happen 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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