Q41390: Handling Events and ERRORs When BASIC Calls Non-BASIC Routines

Article: Q41390
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | docerr B_BasicCom SR# S890202-69 | mspl13_basic
Last Modified: 22-DEC-1989

This information applies to QuickBASIC Versions 4.00, 4.00b, and 4.50,
Microsoft BASIC Compiler Versions 6.00 and 6.00b for MS-DOS and OS/2,
and Microsoft BASIC PDS Version 7.00 for MS-DOS and MS OS/2.

The following corrections for documentation errors apply to Page 296
of the learning and using Microsoft QuickBASIC manual for QuickBASIC
4.00 and 4.00b and BASIC compiler 6.00 and 6.00b, and to Page 442-443
of the "Microsoft BASIC Version 7.0: Programmer's Guide" for BASIC PDS
7.00:

1. The section "Events and Errors" incorrectly implies that BASIC
   can call a subroutine written in a different language, have that
   language sense an EVENT or ERROR, and then jump to the error
   handling routine in BASIC. BASIC is not designed to do this. What
   happens is that the first event (but not subsequent events) that
   occurs during the other language procedure is stored on the stack
   until control returns to BASIC. If the other language procedure
   calls a BASIC procedure in which the ERROR statement forces an
   error (or a natural error occurs), then the error handler (if any)
   in the BASIC program handles the error and RESUMEs as described
   below.

2. The last sentence is incorrect ("The BASIC statement containing...
   is the statement that RESUME would reexecute"). This sentence should
   be changed to say the following:

   "The BASIC statement containing the error (ERROR x%) is the
   statement that RESUME would reexecute. RESUME NEXT would reexecute
   at the following statement."

The program shown below is an example of a BASIC program calling a C
function that calls a BASIC procedure. The program, as written, will
loop indefinitely between the error-handling routine and the BASIC
subprogram that generates the error. If the RESUME statement is
changed to RESUME NEXT, the program executes the next statement in the
subprogram: PRINT "Return from ERROR".

(The documentation incorrectly states that "The BASIC statement
containing the call to the non-BASIC code is the statement that RESUME
would reexecute." According to this statement, the CALL CSUB statement
would be executed; however, execution actually returns to the BASIC
subprogram where the error occurred, as described above.)

The following is a code example:

REM ** Here is the BASIC program
DECLARE SUB jump ()
DECLARE SUB csub CDECL ()
CLS
ON ERROR GOTO errhand
FOR i = 1 TO 5
  PRINT " Before call to c ", FRE(-2)
  CALL csub
  PRINT " out if c:", FRE(-2)
NEXT
PRINT "At end ", FRE(-2)
END
errhand:
   PRINT " error handle => ", FRE(-2)
   RESUME
SUB jump
  print " Generate and Error"
  ERROR 9
  print "Return from ERROR "
END SUB

extern void fortran jump(void);
void csub(void)
 {
   jump();
 }

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