uecker/Quirks of C.md

I thought that I’d throw together the quirks of the C language that I learned about over time for all to see. Some are plain weird, some are actually kinda neat. Most are plain weird.

1. Combined type and variable/field declaration, inside a struct scope: https://godbolt.org/g/Rh94Go
2. Compound literals are lvalues: https://godbolt.org/g/Zup5ZB
3. Switch cases anywhere: https://godbolt.org/g/fSeL18 (also see: Duff's Device)
4. Flexible array members: https://godbolt.org/g/HCjfzX
5. {0} as a universal initializer: https://godbolt.org/g/MPKkXv
6. Function typedefs: https://godbolt.org/g/5ctrLv
7. Array pointers: https://godbolt.org/g/N85dvv
8. Modifiers to array sizes in parameter definitions: https://godbolt.org/z/SKS38s
9. Flat initializer lists: https://godbolt.org/g/RmwnoG
10. What’s an lvalue, anyway: https://godbolt.org/g/5echfM

Special mentions:

1. The power of UB: https://godbolt.org/g/H6mBFT. This happens because:
2. LLVM sees that side_effects has only two possible values: NULL (the initial value) or this_is_not_directly_called_by_main (if bar is called)
3. LLVM sees that side_effects is called, and it is UB to call a null pointer
4. UB is impossible, so LLVM assumes that bar will have executed by the time main runs rather than face the consequences
5. Under this assumption, side_effects is always this_is_not_directly_called_by_main.
6. A macro that tells you if an expression is an integer constant: https://godbolt.org/g/a41gmx (from Martin Uecker, on the Linux kernel ML)
7. You can make some pretty weird stuff in C, but for a real disaster, you need C++. Labels inside expression statements in really weird places: https://godbolt.org/g/k9wDRf.

(I have a bunch of weird things in C++ too, but so does literally everyone who’s used the language for more than an hour, so it’s not as interesting.)