How to do a memory watch.... and Torque helps an indie album hit the Grammys?

Let me introduce myself – and Torque did what?

Hi all. Short introduction to start. My name’s Drew and I’ve been a Torque freelancer for 5 years. This has given me the financial support and freedom to pursue my first true love – music. Though it took almost four years and the hard work of many, my first indie pop/rock album is complete with a few accomplishments to boot (hopefully more to come)

I recently performed live on a TV talk show a song called “Sticky Rice” off my debut album, “On My Way Home.” The album also made it to the first round of the 2009 Grammy Awards (hard to do, say like 100-200 per category make it out of 50,000-60,000 submissions).

Video of me performing on TV a song off my debut indie pop/rock album

So here is me performing "Sticky Rice" from my debut album "On My Way Home”. It’s a song about growing up in Tokyo, Japan. Plus the song has two video game references, so you have to watch it. :)



Me playing “Sticky Rice” on TV on youTube

Moving right along

In this blog, I’d like to do two things:
1) Say “thanks” to GG and the GG Community for helping to make this dream of releasing an indie pop/rock album a reality, by helping me with Torque
2) Show you how to fire a breakpoint when a variable in memory changes

THANK YOU GG and GG community

Thank you GG for making fantastic high-quality products and releasing them at indie prices, so that I can make a living and have a blast doing it! Thank you GG community for answering a bazillion of my questions even when it seemed they would never end! And most of all, thank you Torque for being so awesome!!

Watching a memory location (Setting up a data breakpoint)

This is some good stuff to have in your debugging toolkit. Setting a data breakpoint allows you to breakpoint a memory location instead of a code line, and when that memory location changes, the IDE will break wherever that variable changed. Is that awesome or what?

Why is that handy? Let me give you an example. Have you ever had times where something is changing a certain variable, but you don’t know what it is? You could have a bug messing up a variable but can’t find the cause, or your investigating 3rd party code to try and understand how it works.

This technique also lets you know when "some other code is inadvertently stomping your memory location without ever directly accessing your variable (like a string buffer overrunning)." (Thanks Matt Fairfax)

The old solution I use to have for this was to search the code for function calls, do code traces, and when it gets worse (like there are multiple paths to the same variable) start putting breakpoints everywhere to see which path was changing the variable. And obviously that won't work for a buffer overrun anyway.

Now the first thing I try is setting a data breakpoint, and it has saved me boatloads of time. So let’s get started!

My setup

In this tutorial I’ve used Microsoft Visual Studio 2005 Express Edition (C++). But for any good IDE I’m betting they will have this feature.

Getting your variable

First, figure out what variable you want to watch. In this case I want to watch the mStretch variable of one of my classes. I put a breakpoint in a function that can SEE this variable before it changes in a way I care about (maybe a constructor, or some processing loop). Below is a cut and paste from the IDE watch window of the variable:

mStretch	0.00000000	float

Next, use the IDE watch window to discover its memory location this application run (when you restart the application, the memory location will probably change). Do that by adding a & directly in front of it.

&mStretch	0x0117cf44 	float

You'll have to figure out how to display the value correctly in the watch window with just the memory address. It will take some casting and playing around. Here is how to do a float:

*(float*)(0x0117af44)	0.00000000	float

In this case, the variable we want to watch is mStretch. It's memory location is 0x0117cf44. To watch that memory location in the watch window, paste this line into a watch window entry:

*(float*)(0x0117af44)

Setup a data break point

Now that you have your memory location, you need to add a data breakpoint. This will stop the program from executing whenever that data location is changed.

In the application menu at the top of the screen, do: Debug->New Breakpoint->New Data Breakpoint

In the "Address" field, put the memory location you set up previously: 0x0117af44
Adjust the byte count if you need to. Since we're watching a 32 bit (4 byte) float variable, 4 bytes is the correct size for this example. (It is also the max byte size allowed). However, if you were watching a char (U8) variable, the correct byte size would be 1.

Now Hit the OK button.

The data breakpoint will show up in the breakpoint window. It will say:

When '0x0117af44' changes (4 bytes)

Now the program will stop whenever that memory location changes. This may or may not be what you want. If the program is stopping all the time, you may have to filter out some changes by setting a condition.

Setting a stop condition

Sometimes you will only want the program to break when the memory location changes in certain ways, not when it changes in ANY way. So you have to set a condition. Right-click on the breakpoint and select "Condition."

A new popup appears:

Enter an expression. I wanted to detect when the value had changed by a certain amount, so this is what I did:

*(float*)(0x0117af44) > 0.3

Click the "Is true" radio button.

Let her rip!

Now run the program and turn on the breakpoint. The program may run noticeably slower. But when the watched variable mStretch gets above 0.3, the program will break! Then you can look at the callstack to find out what was causing the change.

Yeah!

I've found I can have a handful of data breakpoints and Torque will execute in debug build ok. But if I have over 5 or more data breakpoints, it really starts to chug.

Wrap up

That’s a wrap, and I hope that was helpful for somebody!

If you liked this write-up and want to get in my good books, you can watch my music video above to help raise the play count. :)

If you are really happy and want to get on Santa’s nice list, if you liked the video you can give it a good rating by signing into youTube. That will boost it on youTube’s page, giving it exposure to more viewers.

Thanks for reading, and thanks for being such an awesome community!

- Drew