VARSEG Incorrect for COMMON String Array Passed Through CHAIN (42595)

MORE INFORMATION

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: 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.