February 3, 2010
Stupid mistakes.
I just saw a post on /r/programming asking people to describe their dumbest coding mistakes. Instead of just posting one of mine there, I’ll do it here.
Many, many years ago, back in 2005 when I was a green grad student and an all around n00b, I got the opportunity to work on the auto-discovery daemon for OpenMosix (may it rest in peace). OpenMosix was a self-assembling cluster system. You would just install it on several machines in the same subnet, and bam you’d have a cluster that automatically distributed jobs. On with the story.
I don’t remember what part of the daemon I was working on. It was already doing some auto-discovery and self assembly. I was pretty pleased. The basic idea was that it would fork off a listener and have another process that would chatter on broadcast announcing its presence. At some point, about 5 minutes into the run, the whole thing would segfault. More precisely, at 4:38 in, it would segfault. Wait, what? The same exact amount of time to crash? That’s just weird! Well, no kidding.
This killed me for several hours. I had no idea what was up. That’s the day I got familiar with using GDB to attach to running processes. And what did I discover? I out clevered myself. I had code like this:
fprintf(stdout, "This is some stupid debugging \
message that I'll eventually remove\n");
That looks all well and good. Why is a printf breaking? Well, it’s breaking because of this line:
fclose(stdout);
See above where I said that I out-clevered myself. Basically, I wanted to turn off all that annoying debugging chatter. So, what the hell, why not just close stdout? It worked! No more chatter. Well, that’s not all. See, whenever using printf, I was writing to a closed file handler… but it was buffered. So, my program would continue to run, dumping silly messages to this buffer until the buffer ran out of space and it smashed some import part of memory causing the segfault. Oops.
So, what did I learn? Debugging messages are important, but use some kind of clever logging system, even if it’s just a series of macros. Sometimes, if your idea seems really clever, take a step back because it might lead you to more trouble than it’s worth.
