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ifdef Considered Harmful, or Portability Experience With C News

By Henry Spencer (Zoology Computer Systems, University of Toronto) and Geoff Collyer (Software Tool & Die)

"We believe that a C programmer's impulse to use #ifdef in an attempt at portability is usually a mistake. Portability is generally the result of advance planning rather than trench warfare involving #ifdef. In the course of developing C News on different systems, we evolved various tactics for dealing with differences among systems without producing a welter of #ifdefs at points of difference. We discuss the alternatives to, and occasional proper use of, #ifdef."

Introduction

"With UNIX running on many different computers, vaguely UNIX-like systems running on still more, and C running on practically everything, many people are suddenly finding it necessary to port C software from one machine to another. When differences among systems cause trouble, the usual first impulse is to write two different versions of the code—one per system—and use #ifdef to choose the appropriate one. This is usually a mistake."

"Simple use of #ifdef works acceptably well when differences are localized and only two versions are present. Unfortunately, as software using this approach is ported to more and more systems, the #ifdefs proliferate, nest, and interlock. After a while, the result is usually an unreadable, unmaintainable mess. Portability without tears requires better advance planning."

"When we wrote C News Coll87a