ON ERROR GOTO 0 in Basic PDS Won't Give Error Line's Address (61343)

SUMMARY

Because of new flexibility added to the ON ERROR GOTO handling in Microsoft Basic Professional Development System (PDS) versions 7.00 and 7.10, the address given when an .EXE program aborts from within an error handler may not be the actual address of the line that produced the error.

If error trapping is turned on, an error occurs, and an ON ERROR GOTO 0 or ERROR ERR statement is used in the error handler to abort the program, then the address of the ON ERROR GOTO 0 or ERROR ERR line displays when the program stops. This behavior differs from .EXE programs compiled in previous versions of Basic (including QuickBasic 4.x and Basic compiler 6.00 and 6.00b), which return the address of the line that originally caused the error.

This information applies to .EXE programs compiled in Microsoft Basic PDS versions 7.00 and 7.10 for MS-DOS and MS OS/2.

MORE INFORMATION

Because Basic PDS versions 7.00 and 7.10 allow errors generated in a module's error handler to be trapped in another module's error handler, the compiler must generate the address of the ON ERROR GOTO 0 (or ERROR ERR) line. Otherwise, if the error were transferred to another error handler that did a RESUME NEXT, the program would not resume to the error handler, but to the user code (because that is the address the compiler would have passed to it). This leaves the error handler still on the stack. If this occurred multiple times, the program would eventually run out of stack space.

Note that the QB.EXE interpreter environment found in QuickBasic 4.x and Basic compiler 6.00/6.00b treats an ON ERROR GOTO 0 in an error handler as if it were an ERROR ERR, causing QB.EXE to stop the program and flag the error on the ON ERROR GOTO 0 line. However, the BC.EXE compiler in these products goes to the extra work of making .EXE programs that report the error back where the error originally occurred.

The following program, ERR.BAS, when compiled using BC.EXE from Basic PDS version 7.00 or 7.10, will abort with an error and display the address of the ON ERROR GOTO 0 line. Compile and LINK as follows:

   BC /x ERR.BAS,,ERR.LST;
   LINK ERR ;

The following is ERR.BAS:

   ON ERROR GOTO handler
   y = 0
   x = 1/y
   END

   handler:
      ON ERROR GOTO 0
      RESUME

In Basic PDS version 7.00 or 7.10, when the program aborts, the following error message is given (note: the address given will be different on different machines):

Division by zero in line 0 of module ERR at address 23E5:0066

Looking at the following listing file (ERR.LST) produced by the compiler shows that the address given when the program stops in error is that of the ON ERROR GOTO 0 line:

   0030   0006
   0030   0006            ON ERROR GOTO handler
   003A   0006            y = 0
   0046   000A            x = 1/y
   0057   000E            end
   005C   000E            handler:
   005C   000E               ON ERROR GOTO 0
   0066   000E               RESUME
   006B   000E
   008E   000E

The address given (0066) shows as the RESUME statement. This is because the error occurs at the end of the ON ERROR GOTO 0 statement, which is the starting address of the RESUME statement line. In .EXE programs compiled in versions of Basic earlier than 7.00, the address given was of the actual line that the error occurred on (the line at address 0046: x = 1/y).

The address given in a program without ON ERROR trapping active is still the address of the actual error line, x = 1/y, in Basic 7.00 and 7.10 (and earlier versions of BC.EXE).