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Q61343: ON ERROR GOTO 0 in BASIC PDS Won’t Give Error Line’s Address

Article: Q61343
Product(s): See article
Version(s): 7.00 7.10 | 7.00 7.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | SR# S900415-2 | mspl13_basic
Last Modified: 15-JAN-1991

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.

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).

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