bits and bytes

What’s a breakpoint?

A breakpoint is defined as the location where a debugger breaks execution to allow the user to have a look or to modify the execution context.

What’s new with breakpoints?

With visual studio 2005 and 2008 behavior of breakpoint has changed. Some features that were added are as follows…

1. Know hit count of a break point, no more need to keep a temp debugger variable for counting hits. For e.g. you can set a conditional breakpoint and then enable hit count. You’ll see how many times the condition was satisfied, you can also disable breaking of execution so that the program keeps running.
2. Trace local variables/function name and more to the visual studio immediate/output window.
3. Run a macro when a breakpoint is hit.
4. We can disable breaking of execution which means there will only be tracing going on and no breaking of execution.

So to access these features, after adding a breakpoint, open the breakpoint window and right click on this breakpoint. Lower half of the dropdown contains these features, here is a screenshot of the context menu…

Breakpoint Context Menu

There are different types of break points available in visual studio. There are four in my knowledge…

1. File breakpoint – Breaks at a location in a file
2. Address breakpoint – Breaks at an address
3. Function breakpoint – Breaks at a function
4. Data breakpoint – Breaks at specific byte locations

Hit count

Let me show you how to count the number of even numbers from 1 to 100, I know it’s quite easy, it’s fifty, but via debugger? Here is a small function with a loop from 1 to 100.

void CountEven( const int From, const int To )
{
   for( int Index = From; Index < To; ++Index )
   {
      // Set a conditional breakpoint to get the count of odd numbers,
      // Dummy code to allow set a breakpoint
      ::SendMessage( AfxGetMainWnd()->GetSafeHwnd(), WM_NULL, 0, 0 );
   }
}

Now set a break point inside the for loop (Press F9) and right click and select “Breakpoint->Condition” item. You’ll get the following dialog…

Setting a Conditional breakpoint

So here I’ve set the condition that whenever Index%2 is zero then we have an even number. So when this happens I’m asking the debugger to break execution. So this works as expected but our aim is to count the number of even numbers using debugger, so for that again right click on the breakpoint line and select “Breakpoint->Hit Count” item, following dialog pops up…

Enable hit count in the debugger

You can see that I’ve selected an option called “break when the hit count is a multiple of”. I’ve given 10 as the option, so that frequency of breaking of execution is less. When I run the code this is what I get in the breakpoints window…

Hit count result

See the “Hit Count” item?  Execution broke 5 times this means for every 10 hits, hence count is 50. Isn’t this fun?

Tracing

Remember the times when we had to trace statements just for debugging purpose, for e.g. we wanted to trace a certain variable’s value when it satisfies a condition. Let’s take the above example, we’ve already enabled a conditional breakpoint such that it’s breaks execution whenever “Index” is an even number. Right click on the breakpoint line and select “Breakpoint” and then “When Hit” option. Following dialog pops up…

When Hit Dialog

Check the first option Print a message”, this is the option that prints a message either to your “immediate window” or to your “Output window” based on the options that you’ve set. Now run the program to have some fun, this is the output in my computer…

Output of "when hit"

All even index values are traced to the output window, isn’t this cool?

Filter

This one is even more cool. It allows us to set breakpoint filters. This is kind of a conditional but with a larger scope. For e.g. if you want to break execution only for a certain thread or only for a certain process. So for this right click on the breakpoint line and select “Breakpionts->Filter”. Following dialog pops up…

Breakpoint filter

Pretty easy to use. So this means for a function that is called from multiple threads you can explicitly tell the debugger for which threads to break execution.

Tid bits

1. Did you know that you can set a breakpoint in the call stack window by press F9?
2. Did you know that you can delete all breakpoints by press Ctrl + Shift + F9?
3. Did you know that you can run to cursor by pressing Ctrl + F10?
4. Did you know that you display breakpoints window by pressing Alt + F9?
5. Did you know that you can use “Set Next Statement” by pressing Ctrl + Shift + F10?

Yeap that’s it from me for now, all the best.