Wednesday, December 12, 2007

Top ten

The top ten ways to get screwed by the C language has 18 entries. I was a bit surprised that entries weren't numbered starting at zero. Entry 15 (which you might note is more than ten) talks about using an array past it's end.

Today, let's start with number one.

#1 Non-terminated comment, "accidentally" terminated by some subsequent comment, with the code in between swallowed.

a=b; /* this is a bug
c=d; /* c=d will never happen */

In the eighties, i wrote a small filter (in C, of course) that can do two things. First, it can show you all the comments in a C program. Second, it can remove all the comments in a C program. This basically solves the debugging of this problem. By looking at all the comments, it becomes obvious, as in the previous example, that some code has been commented out. Since about that time, i've used '#ifdef' to effectively comment out code. In the early days of C, however, before the evil C preprocessor was invented, one would use another idiom:

if (0) {

The optimizer would notice that the code couldn't be reached and left it out. But then, my comment filter wouldn't show it, right?

Where is this filter now? Well, i've planned releasing it, but it just hasn't happened. I might have published it here, but my home machine is down right now. I might finish repairs tonight.

This thing with comments is, of course, by design. The most memorable time that i was bitten by it was back in the 80x24 terminal days. Someone who worked for me submitted non-working code, but had run out of time to debug it. It turned out that this person used a comment block, where each line of the block had an open and close comment bit. However, the last line of the block's closing comment had a space between the '*' and the '/', so didn't close the comment. Why wasn't it obvious? The terminal didn't wrap the line (which was 81 characters long), it wrote all characters starting with the 80th in the last column. So, the space went there, and a millisecond later (9600 BAUD) the slash went there. At that point, it looked like '*/'. That may have been when i wrote the filter. But i may already have had it. Single stepping in the debugger showed that some code was missing.

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