Torvalds on Specs

28 Dec 2007 /

A “spec” is close to useless. I have _never_ seen a spec that was both big
enough to be useful _and_ accurate.

And I have seen _lots_ of total crap work that was based on specs. It’s
_the_ single worst way to write software, because it by definition means
that the software was written to match theory, not reality.

So there’s two MAJOR reasons to avoid specs:

– they’re dangerously wrong. Reality is different, and anybody who thinks
specs matter over reality should get out of kernel programming NOW.
When reality and specs clash, the spec has zero meaning. Zilch. Nada.
None.

It’s like real science: if you have a theory that doesn’t match
experiments, it doesn’t matter _how_ much you like that theory. It’s
wrong. You can use it as an approximation, but you MUST keep in mind
that it’s an approximation.

– specs have an inevitably tendency to try to introduce abstractions
levels and wording and documentation policies that make sense for a
written spec. Trying to implement actual code off the spec leads to the
code looking and working like CRAP.

The classic example of this is the OSI network model protocols. Classic
spec-design, which had absolutely _zero_ relevance for the real world.
We still talk about the seven layers model, because it’s a convenient
model for _discussion_, but that has absolutely zero to do with any
real-life software engineering. In other words, it’s a way to _talk_
about things, not to implement them.

And that’s important. Specs are a basis for _talking_about_ things. But
they are _not_ a basis for implementing software.

So please don’t bother talking about specs. Real standards grow up
_despite_ specs, not thanks to them.

Linus


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