HOWTO: Use ON ERROR To Debug Applications

• Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b, 5.0, 5.0a, 6.0
• Microsoft Visual FoxPro for Macintosh, version 3.0b
• Microsoft FoxPro for Windows, versions 2.5, 2.5a, 2.5b, 2.6, 2.6a
• Microsoft FoxPro for Macintosh, versions 2.5b, 2.5c, 2.6a
• Microsoft FoxPro for MS-DOS, versions 2.5, 2.5a, 2.5b, 2.6, 2.6a
• Microsoft FoxPro for UNIX, version 2.6

SUMMARY

FoxPro provides the ON ERROR command to allow programmers to trap specific errors that occur within applications. This article describes how to use an error trapping routine that stores relevant debugging information in a FoxPro table.

The code included in this article traps errors that occur within a FoxPro application and stores the following information to a table:

• The computer name where the error occurred.



• The error number.



• The error message.



• The line of code where the error occurred if the source code is available.



• The program file where the error occurred.



• The line number where the error occurred.



• The name of the currently selected table when the error occurred.



• The date and time of the error.

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.

REFERENCES

FoxPro Help, search on: "ON ERROR"; "ERROR()"; "MESSAGE()"; "SYS()"; "PROGRAM()"; "DBF()"; "LINENO()"

Additional query words:

Keywords : kbcode kbnokeyword kbMAC kbVFp300 kbVFp500 kbVFp600
Version :
Platform :
Issue type : kbhowto