17.6 Interactive Debugging Tips and Tricks

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Interactive debuggers work well for most programs, but sometimes they need a little help. Consider Example 17-4. We try to debug it and find it fails when point_number is 735. Why it fails on call 735 to lookup we don't know, but the first 734 calls work and the next one doesn't. All we know is that we call float_point_color 800 times, it calls lookup 800 times, and something goes wrong at 735.

We want to put a breakpoint before the calculation is made. When the debugger inserts a breakpoint into a program, the program will execute normally until it hits the breakpoint, then control will return to the debugger. This allows the user to examine and change variables , as well as perform other debugging commands. When a cont command is typed, the program will continue execution as though nothing happened . The problem is that there are 734 points before the one we want, and we don't want to stop for each of them.

Example 17-4. debug/cstop.cpp

 float point_color(int point_number) {     float correction;           // color correction factor      extern float red,green,blue;// current colors      // Lookup color correction     extern lookup(int point_number);     correction = lookup(point_number);     return (red*correction * 100.0 +              blue*correction * 10.0 +             green*correction); } 

How do we force the debugger to stop only when point_number == 735 ? We can do this by adding the following temporary code:

 48:    if (point_number == 735)  /* ### Temp code ### */  49:        point_number = point_number;   /* ### Line to stop on ### */ 

Line 49 does nothing useful except serve as a line that the debugger can stop on. We can put a breakpoint on that line with the command break 49 . The program will process the first 734 points, then execute line 49, hitting the breakpoint. (Some debuggers have a conditional breakpoint. The advanced GDB command break 49 if point_number == 735 would also work; however, your debugger may not have such advanced features.)

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