In which it becomes clear, the C Preprocessor was designed by a Kafka fan
So you have heard rumors whispered between peers, that a rare few people somehow manage to make compile-time recursion work in C? And you want to have some insight into how that might be possible??
I should warn you, you’re risking your sanity… but I’ll indulge you.
Wait, did I really just say that? I must be a glutton for punishment, because the macro system is, by far, the thing I like least about C.
C has many advantages that have led to its longevity (60 years as perhaps the most important language). It has many quirks due to its age, most of which are easy to look past. But despite 30 years writing C, I still bristle at C’s macro system.
That’s not just because there are many languages (including C++) with more modern takes on compile-time execution. Macros appear simple, but have subtleties that make them poorly suited for anything other than light wrappers.
Still, being C’s only compile-time execution capability (currently), it is still both critical and important. Critical, in that many venerable critical systems heavily depend on them, and wouldn’t compile without them. Important, in that it’s often the only way to abstract out complexity that would lead to safety or security issues if exposed, such as automatically adding sentinels or static type checks.
C macros being hard to use does discourage their overuse. It’s easy for too much abstraction to make it too difficult for other people to maintain the code, so in some ways, as painful as they are, I can find some things to appreciate, and do occasionally find reason to use them for something non-trivial.
But it doesn’t take much for a macro to be non-trivial, because, while C macros can look like functions, they cannot be called recursively (at least, not easily, as we will see).
I have never been able to find out why recursion is limited in C macros. It could have started off intentional, but compile time execution wasn’t really on people’s minds then; the challenge was abstracting over many platform differences as cheaply as possible.
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