Article ID: 42595
Article Last Modified on 11/21/2006
This article was previously published under Q42595
SYMPTOMS
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.
STATUS
Microsoft has confirmed this to be a bug 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).
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:
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.
Additional query words: QuickBas BasicCom B_BasicCom buglist4.50
Keywords: KB42595
