Debugging Techniques

Always look at the top error message in the spew that comes out from the compiler. This is usually where the problem is triggered. Junk after that is often spurious, especially if the first problem is a syntax error.

Use a revision control system like Git or Mercurial, and make a checkpoint of your code when ever you have made a significant addition. This way, you don’t get into the situation where you had it working, you broke it, and you can’t get it running again. Git has a little bit of a learning curve, but it rocks. If you are comfortable posting the code where other people can see, you can push your changes to github, and then if you have a catastrophic machine failure, you don’t lose your work.

The binary search is your friend. If you know that the error is somewhere between lines 10 and 20, Comment out lines 15-20 and see if you still have the problem. If so, comment out lines 13 and 14, and so on. A binary search means that you can search 1000 lines of code in 10 comparisons, and save yourself a metric shit-ton of time.  A metric shit-ton is an industry term.

Step through your code in a debugger, line by line, to make sure you know what it is actually doing, not what you think it is doing. Very illuminating. If you don’t have access to a debugger, make liberal use of trace statements. In gcc I often use:

#define TRACE() printf (“%s:%s:%d\n”,__FILE__,__FUNCTION__,__LINE__)

There is something comparable in most languages.

Always work from success. When I start a new C program, I start from

int main(){ printf (“Hello, World.”);}

And compile and run between each minor change.

Don’t be afraid to ask for help. A second set of eyes will often quickly see what you missed. Even more so, when you explain the code to someone else, you often solve the problem yourself. This is known as “verbal debugging.”

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