Single-Line DEF FN Can Cause Program to Rerun ON ERROR (63272)

SYMPTOMS

A program will incorrectly return from an error-handling routine if an error occurs in a single-line DEF FN function. If the error occurs during the execution of a single-line DEF FN function, when the error handler executes a RESUME or RESUME NEXT, the program will resume on the line after the DEF FN definition instead of on the line or line after where the DEF FN function was invoked. This usually results in an infinite loop that eventually causes an "Out of stack space" error after the FN function invocations have been nested too many times.

STATUS

Microsoft has confirmed this to be a bug in QuickBasic versions 4.00, 4.00b, and 4.50; in Microsoft Basic Compiler versions 6.00 and 6.00b (buglist6.00, buglist6.00b); and in Microsoft Basic Professional Development System (PDS) versions 7.00 and 7.10 for MS-DOS (buglist7.00, buglist7.10). This problem occurs in both the QB.EXE/QBX.EXE editor environment and the BC.EXE compiler. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

The following code example demonstrates this problem:

   10 ON ERROR GOTO handler
   20 REM For the following program:
   30 CLS
   40 DEF fnx (x%) = 1 / x%
   45
   50 PRINT "line after the DEF FN": PRINT
   60 PRINT fnx(0)
   70 END
   80 handler:
   90   RESUME NEXT

In the example above, an error will occur on the PRINT fnx(0). In the error handler, the RESUME NEXT should cause the program to RESUME on line 70 after the PRINT fnx(0), but instead the program will resume on the line after the DEF FN definition (line 45). This will cause the PRINT fnx(0) on line 60 to be executed again, causing another error, which effectively is an infinite loop. The program will eventually run out of stack space and quit with a run-time error.

To work around this problem, simply expand the single-line DEF FN to a multiple-line DEF FN ... END DEF. To correct the example above, you would make the following changes:

   10 ON ERROR GOTO handler
   20 REM For the following program:
   30 CLS
   40 DEF fnx (x%)
        fnx = 1 / x%
      END DEF
   45
   50 PRINT "line after the DEF FN": PRINT
   60 PRINT fnx(0)
   70 END
   80 handler:
   90   RESUME NEXT

This program will execute correctly. The FN function will return a 0 and the program will resume on the line after the PRINT fnx(0) statement.