posted May 19, 2003 08:35 AM            

The problem with memory corruption issues is that they can often be very difficult to diagnose, and very time consuming to track down... especially since they originates (most often) in completely unrelated sections of a program, and giving no hint as to the source of the problem at the point where the symptom becomes apparent.

The most common source of such a problem can exist when a program writes to an array element that is beyond the boundaries of the array. Often this will cause a GPF, but if the memory address happens to be owned by the program, the effect will be one of writing data from (or reading data from) an area of memory that is actually being used by some other part of the program.

The first step in tracking such problems is to use #DEBUG ERROR ON. This tells PowerBASIC to perform array bounds testing in the compiled code, and to flag an ERR=9 code if an array element access would breach an array boundary.

However, unless the program actively tests for ERR or ERRCLEAR values, the run-time error report can be overlooked at the point the error is detected.

What we need is an easy way to log run-time errors, so we can track down the cause quickly... and with PB/Win 7.0x and PB/CC 3.0x, the TRACE statement fills this need very nicely!

First, just below the #COMPILE EXE metastatement (or at the very start of the source code if there is no #COMPILE metastatement present):

#COMPILE EXE
#DEBUG ERROR ON
#TOOLS ON
...

FUNCTION PBMAIN
  TRACE NEW "trace.txt"
  TRACE ON
  ...

FUNCTION PBMAIN
...
  TRACE OFF
  TRACE CLOSE
END FUNCTION

Finally, open the new TRACE.TXT file in any file viewer than can cope with large text files, and search for "error"... every run-time error that occurred while the program was running will be logged in that file, along with Labels, Sub/Function names (and their parameters), etc, providing a useful context "map" that will show the trail of program execution up to the point where the error(s) are occurring.

Naturally you will want to investigate any errors that are logged, but do pay special attention to any ERROR 9 reports, since these will definitely indicate an array bounds (or null-pointer) problem is occurring in the program.

PS: Dont forget to switch off the trace file operation with #TOOLS OFF after you finish debugging the program code!