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Q62820: In SUB, Variable-Length Near String Array Element Losing Value

Article: Q62820
Product(s): See article
Version(s): 7.00   | 7.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | SR# S900603-3 buglist7.00 fixlist7.10 | mspl13_basic
Last Modified: 2-NOV-1990

A program compiled with the Near Strings option (without BC /Fs) may
lose the value of a variable-length string array element under the
specific conditions described in this article.

The same program will run correctly when run from the QBX.EXE
environment or when compiled with the Far Strings option (with BC
/Fs).

Microsoft has confirmed this to be a problem with the BC.EXE 7.00
compiler provided with Microsoft BASIC Professional Development System
(PDS) version 7.00 for MS-DOS and MS OS/2. This problem was corrected
in BASIC PDS 7.10.

This problem does not occur in earlier versions of Microsoft BASIC
Compiler (BC.EXE).

Here is a description of the conditions under which the problem occurs
(as shown in Examples 1 and 2):

Two variable-length string arrays are dimensioned. Array "A" may be
dimensioned either dynamically or statically. Array "B" must be
dimensioned dynamically. Array "B" will lose a value even though the
program never changes it. A string value is assigned to an element in
array "B" and this array element is passed as a parameter to a SUB.
Array "A" is passed to the same SUB either through a parameter list or
by using SHARED in the DIM statement. Inside the SUB, an element in
array "A" is directly assigned to a string value returned by the
RTRIM$, LTRIM$, LEFT$, MID$, or RIGHT$ function. Array "B" is not
assigned any values in the SUB. But when control is returned to the
main-level code (or calling procedure), the string value that was
assigned to an element in array "B" is lost.

The following two examples exhibit the problem. Both examples pass an
element in B$() as a parameter variable. In the first example, array
A$() is declared globally in a DIM SHARED statement. In the second
example, A$() is passed as a parameter to the SUB.

Example 1
---------

DECLARE SUB TheSub (temp$)
REM $DYNAMIC
DIM SHARED A$(1)
DIM B$(1)
B$(1) = "bas"
PRINT B$(1)
CALL TheSub(B$(1))
PRINT B$(1)         'There is no longer anything in B$(1).
END

SUB TheSub (temp$)
     A$(1) = RTRIM$("   bob    ")
END SUB

Example 2
---------

DECLARE SUB TheSub (temp1$(), temp2$)
REM $DYNAMIC
DIM A$(1), B$(1)
B$(1) = "bas"
PRINT B$(1)
CALL TheSub(A$(), B$(1))
PRINT B$(1)        'There is no longer anything in B$(1).
END

SUB TheSub (temp1$(), temp2$)
     temp1$(1) = RTRIM$("   bob    ")
END SUB

To work around the problem, alter the program with any one of the
following choices:

1. Compile with the Far Strings option (BC /Fs).

2. Statically dimension the array that loses its value.

3. Remove the RTRIM$ statement.

4. Assign a temporary string variable to the string returned by
   RTRIM$, then assign this temporary variable to the desired element
   in array "B".

The next program is an example of using workaround 4 described above:

DECLARE SUB TheSub (temp$)
REM $DYNAMIC
DIM SHARED A$(1)
DIM B$(1)
B$(1) = "bas"
PRINT B$(1)
CALL TheSub(B$(1))
PRINT B$(1)         'B$(1) now contains the correct value.
END

SUB TheSub (temp$)
     dummy$ = RTRIM$("   bob    ")  'Use a temporary string variable
     A$(1) = dummy$                 'to work around the problem.
END SUB

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