Q42595: VARSEG Incorrect for COMMON String Array Passed Through CHAIN

Article: Q42595
Product(s): See article
Version(s): 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom buglist4.50 | mspl13_basic
Last Modified: 7-FEB-1990

The programs shown below demonstrate that VARSEG does not return the
correct segment address of a string array passed in a COMMON block
through a CHAIN.

Microsoft has confirmed this to be a problem in QuickBASIC Version
4.50 for MS-DOS and in Microsoft BASIC Compiler Version 6.00b
(buglist6.00b) for MS-DOS and MS OS/2. This problem was corrected in
Microsoft BASIC Professional Development System (PDS) Version 7.00 for
MS-DOS and MS OS/2 (fixlist7.00).

The following program is TEST.BAS, which dimensions the string array
and passes it in COMMON:

   COMMON a$()
   DIM a$(4)
   CHAIN "test2"
   END

The following separately compiled program is TEST2.BAS, which
dimensions another string array and passes both to the C routine. The
second string array is used for comparison.

   DECLARE SUB crot CDECL (BYVAL plo AS INTEGER, BYVAL pls AS INTEGER)
   COMMON a$()
   DIM b$(1)
   a$(0) = "a0" + chr$(0)
   b$(0) = "b0" + chr$(0)
   print varseg(a$(0))
   print varseg(b$(0))
   CALL crot(VARPTR(a$(0)), VARSEG(a$(0)))
   CALL crot(VARPTR(b$(0)), VARSEG(b$(0)))

The following is the C routine CR.C, which should print out the first
string in each array:

   #include <stdio.h>
   struct struct_string {
       int length;
       char *address;
                        };
    void crot(struct struct_string far *string)
    {
         printf("%s\n", string->address);
    }

To demonstrate the problem from .EXE programs, compile and link as
follows:

     BC TEST.BAS ;
     LINK TEST.OBJ ;

     BC TEST2.BAS ;

     CL /AM /c CR.C ;

     LINK /NOE TEST2.OBJ CR.OBJ ;

When TEST.EXE is run, the first line prints garbled, while the second
prints correctly, as shown below:

   $#lkds
   b0

Note: If the C routine is changed so that near addressing is used, the
routine works correctly, as follows:

   #include <stdio.h>
   struct struct_string {
       int length;
       char *address;
                        };
   void crot(struct struct_string *string) /* changed to near pointer */
    {
         printf("%s\n", string->address);
    }

Making the above change correctly displays the following:

   a0
   b0

This change works because with near addressing, the C routine ignores
the VARSEG part of the address. This only works if the array a$ lies
within the default data segment.

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