How To Use ON ERROR To Debug Applications

MORE INFORMATION

Faulty syntax and/or logic in an application can result in a wide array of trappable error conditions. Some of the errors that occur in an application may not be known until the application has been tested by end users. While the default FoxPro error message display contains useful information for a programmer, its appearance may be confusing to an end user. Likewise, end users usually have little time to carefully note and report errors that occur while an application is executing. This code may also prove useful in determining the cause of intermittent errors that occur when no one is normally present to witness the error.

1. Create a program called Errutil.prg with the following code.

         ON ERROR DO errhand IN errutil ;
         WITH SYS(0), ERROR(), MESSAGE(), MESSAGE(1), ;
         PROGRAM(), LINENO(1), DBF(), DATE(), TIME()
         * End of error trap setup.
   
         PROCEDURE errhand
         PARAMETER m.machine, m.messgnum, m.messg, m.linecode, ;
            m.callprog, m.inline, m.OPENTABL, m.errdate, ;
            m.errtime
            m.errspace=SELECT()      && Store current work area.
            m.errorder=ORDER()       && Store current order.
         IF LEN(ALLTRIM(m.callprog))=0
            m.callprog="Command Line"
           STORE SPACE(0) TO m.linecode
         ENDIF
         outmsgline="Error ; "+m.messg+CHR(13)+"Line "+STR(m.inline)+ ;
              CHR(13)+ ;
             "program name = "+m.callprog+CHR(13)+"Syntax is  :"+m.linecode
         * Visual FoxPro users use =MESSAGEBOX(outmsgline,32+0)
         * MAC uUse the FXALERT() Function in Foxtools.mlb
         * FoxPro For Windows users use the MsgBox() Function in Foxtools.fll
         WAIT WINDOW outmsgline TIMEOUT 5 && All versions can use this syntax.
           IF !USED("ERRORLOG")
         IF FILE("ERRORLOG.DBF")
            SELECT 0
            USE errorlog
         ELSE
            SELECT 0
            thisversion=VERSION()
            IF LEFT(ALLTRIM(thisversion),6)="Visual"
               * Create Free table for Visual FoxPro Versions
               CREATE TABLE errorlog FREE (machine c(20), messgnum N(4,0), ;
                  messg c(70), linecode c(70), callprog c(40), ;
                  inline N(6,0), OPENTABL c(25), errdate d, errtime c(8))
            ELSE
                  CREATE TABLE errorlog (machine c(20), messgnum N(4,0), ;
                       messg c(70), linecode c(70), callprog c(40), ;
                    inline N(6,0), OPENTABL c(25), errdate d, errtime c(8))
                  ENDIF
               ENDIF
           ENDIF
           INSERT INTO errorlog FROM MEMVAR
           SELECT errorlog                  && Select errorlog table.
           USE                              && Close errorlog table.
           SELECT (m.errspace)              && Return to stored work area.
           IF !EMPTY(ALIAS())
         SET ORDER TO (m.errorder)
          ENDIF
         RELEASE ALL LIKE m.messgnum, m.messg, m.linecode, m.callprog, ;
             m.inline
         RETURN
   							

2. Create a program called Ztest.prg that contains the following lines of code:

         DO errutil          && Activates ON ERROR routine in Errutil.prg.
         USE c:\noexist.dbf  && These files should not exist in order to
         DO C:\noexist.prg   && produce errors for testing purposes.
         ON ERROR            && Turns off active ON ERROR routine.
   							

3. Type the following in the Visual FoxPro Command window:

         Do ZTest.prg.
   							

   Two Wait windows appear identifying the offending lines of code. This information is placed into the Errorlog.dbf file. The windows disappear after five seconds.
4. Activate the Command window, then open and browse the Errorlog table.