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Language Lawyers (and Nasal Demons)

Friday, July 30, 2004
As soon as a development language has an official specification behind it, the first language lawyer appears. If you’ve never seen one in action, they are the type who can rattle off section and paragraph numbers from a specification while explaining the bizarre behavior of some code snippet. You might see them write something along the lines of:

The values of a and b are automatically promoted back to int [], which is then the type of the result of the / operator []. Since -32768 is exactly divisible by -1, and the result of 32768 is representable in the type int, the division must yield this value.

Nearly every language with a specification will describe situations with “undefined behavior”. Undefined behavior means there is something so horribly screwed up in the code, or in the data, that the program is probably going to crash spectacularly. Except specifications never use the word ‘crash’. Many of them, including the C# spec, will use the more dignified phrase: “unpredictable results”.

Now language lawyers, not really being lawyers, have a sense of humor. Since the phrase “unpredictable results” leaves the outcome so open-ended, they will say undefined behavior could, in fact, lead to demons flying out of your nostrils.

The first apparent reference to demons and noses came in a 1992 post to the comp.std.c newsgroup. The post contains many paragraph and section number references to the C language specification, but ends with this little gem:

In short, you can't use sizeof() on a structure whose elements haven't been defined, and if you do, demons may fly out of your nose.

The phrase caught on.

In the mid 90’s I was maintaining a C code base intended for both an 8 bit Hitachi chip and the PC, so I was interested in writing portable ANSI C programs and followed the comp.std.c and comp.lang.c newsgroups. I don’t know how many times someone would come along and post code which started like this:

void main (void)

As soon as the above line appeared the language lawyers appeared in force. You could look at the code, and see it was only one semi-colon away from curing world hunger, but it didn’t matter, because the response was invariably something like:

The Standard says that declaring main as returning void invokes undefined behavior. Beware the nasal demons.

Although sometimes the lawyers would make other amusing comments:

if you write:

void main (void) 

it's *possible* (under the rules of C) that your computer will pogo round the room whistling the star spangled banner. It's very unlikely, but C isn't interested in "likely".

Whenever I was having a really bad day, I’d write a program with main returning void just to see what would happen. I tried it with the Microsoft C compiler, the Borland C compiler, the Intel C compiler, and the now defunct Boston Systems Office C compiler. I was hoping to create a horrible, slimy demon who I could train to stand at my office door and terrify people trying to enter with requirements documentation in hand. Not only did I never get a demon, the programs always worked without any errors. Shows you what the language lawyers know.

In any case, I’m hoping to find some good examples of undefined behavior in C# or VB so I can try to invoke nasal demons from .NET. I imagine it will be much harder to do with a managed runtime in place, but if you know of any opportunities, please let me know.