Q21863: Passing Variables to ON ERROR and ON Event Handlers

Article: Q21863
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 29-DEC-1989

This article discusses how a QuickBASIC program can I pass variables
to ON ERROR and ON "event" handler routines compiled with subprograms
that are separate from the main program.

(The different ON "event" statements are ON TIMER GOSUB, ON KEY GOSUB,
ON PLAY GOSUB, ON STRIG GOSUB, and ON PEN GOSUB.)

This information applies to Microsoft QuickBASIC 1.00 1.01 1.02 2.00
2.01 3.00 4.00 4.00B and 4.50, to Microsoft BASIC Compiler 6.00 and
6.00B, and to Microsoft BASIC PDS Version 7.00 for MS-DOS and MS OS/2.

Variables can be passed through the COMMON SHARED statement to ON
ERROR and ON "event" handler routines compiled separately from the
main program. Variables that are not in COMMON SHARED are not
accessible to the handler routines (unless the handler is in the same
source file as the main program).

The following is an example of a separately compiled subprogram that
shares a flag variable with its error handler routine:

   ' TEST.BAS
   COMMON SHARED FLAG
   errortrap:
      ' The error is handled here:
      PRINT "Error Number Trapped="ERR
      FLAG=5
      RESUME NEXT  ' Resumes to line after where error occurred.
   SUB test STATIC
   ON ERROR GOTO errortrap
   FLAG=0
   ERROR 5  ' Forces a test error number 5 to occur.
   PRINT "FLAG=",FLAG  ' Execution resumes here after the error.
   END SUB

This program is called by the following separate main program module:

   ' MAIN.BAS
   CALL TEST

First, compile as follows:

   BC MAIN/X;
   BC TEST/X;

You then can LINK MAIN+TEST; run the resulting program as an example.

You cannot use the linenumber or linelabel option of RETURN or RESUME
in conjunction with subprograms.

In QuickBASIC Versions 1.00, 1.01, 1.02, 2.00, 2.01, 3.00, and 4.00,
error and event handling routines are local to each separately
compiled module. A given error or event handling routine serves all
subprograms compiled in the same module. The ON ERROR GOTO or ON event
GOSUB statement should appear in each subprogram that is compiled
separately from the main program.

QuickBASIC Versions 4.00b and 4.50, Microsoft BASIC Compiler Versions
6.00 and 6.00b, and Microsoft BASIC PDS Version 7.00 offer global
error handling, which is described in a separate article. For more
information, query on the following words:

   global and error and handling and QuickBASIC

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