Q43008: Debug Does Not Display Changes Made by _dos_setvect()

Article: Q43008
Product(s): See article
Version(s): 2.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | buglist2.00 | mspl13_c
Last Modified: 2-MAY-1989

The QuickC Version 2.00 debug window will not update the display of an
entry in the interrupt vector table after _dos_setvect is used to
change that entry. The program below will reproduce the problem.

To demonstrate the problem, do the following:

1. Start QuickC.

2. Load the program below.

3. Open the debug window and set watches on the following:

    hptr
    saveint
    ptr
    *ptr (use hex format)
    lp (use hex format)

Use the F8 key to step into the program. Saveint contains the address
of the old interrupt handler. Hptr contains the address of the new
interrupt handler. Ptr points to the interrupt vector, thus *ptr shows
the address of the interrupt handler and should be different after the
call to _dos_setvect. However, it is not. Notice that lp does contain
the correct address, demonstrating that internally all addresses are
being held correctly. The watch window is simply not displaying the
altered interrupt vector properly.

The following sample program will demonstrate the problem:

#include <stdio.h>
#include <dos.h>
#define THE_INT 0x12

void interrupt far handler() ;

void (interrupt far *hptr)() ;
void (interrupt far *saveint)() ;
long far* ptr ;
unsigned long lp ;

void main ( void )
{
hptr = handler ;
saveint = _dos_getvect ( THE_INT ) ;

FP_SEG ( ptr ) = 0 ;
FP_OFF ( ptr ) = 0x48 ;    /* make ptr point to where the interrupt
                            * THE_INT is in interrupt vector table
                            */

/* following will do the same thing by searching for the address
 * rather than setting ptr directly:
 */
FP_SEG ( ptr ) = 0 ;
FP_OFF ( ptr ) = 0 ;
while (*ptr++ != (long) saveint) ;
ptr -- ;
*/

_dos_setvect ( THE_INT, hptr ) ;   /* set to new handler */
lp = *ptr ;

_dos_setvect ( THE_INT, saveint ) ; /* reset handler */
}

void interrupt far handler()
{
}

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