Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!umich!samsung!usc!wuarchive!udel!rochester!ken
From: ken@cs.rochester.edu (Ken Yap)
Newsgroups: comp.lang.c
Subject: Re: Computing the absolute value of an integer
Message-ID: <1990May9.164513.20739@cs.rochester.edu>
Date: 9 May 90 16:45:13 GMT
References: <11679@june.cs.washington.edu> <1990May4.121950.22726@agate.berkeley.edu> <1990May5.011945.9284@sq.sq.com>
Reply-To: ken@cs.rochester.edu
Organization: University of Rochester Computer Science Department
Lines: 25
Address: Rochester, NY 14627, (716) 275-1448

|?  I want a function that will compute the absolute value of any integer
|?  that can be stored in a variable of type int.  What makes this difficult
|?  is that on some machines the absolute value of the most negative integer
|?  will not fit in an int.
|
|Which in turn means that the operation "-a" on a variable a of any
|signed integer type is capable of causing overflow, which is allowed
|to cause the program to abort.  You are right to be worried.

I came across a bug that was caused by exactly this problem. In a
boundary case, a fp division by zero happened giving negative infinity
silently (another story). This was assigned to an integer. The problem
was that the author decided to do his own conversion to a string
instead of using printf or sprintf. The code said, in effect:

	if (x < 0)
		print leading -
		x = -x
	proceed as in positive case

Problem is, -maxint is -maxint, so the program printed a minus sign
and nothing else. Since this was a converter to Postscript, this bare
minus sign caused the printer to barf, aborting the job.

Moral of story? Make up your own.