Linus directly contradicts you in that linked thread, for this specific situation: And as mentioned, there _are_ exceptions. Some types just get _sooo_ com

Linus directly contradicts you in that linked thread, for this specific situation: And as mentioned, there _are_ exceptions. Some types just get _sooo_ complex that it's inconvenient to type them out, even if they are perfectly regular types, and don't depend on any config option. The "filldir_t" typedef in fs.h is such an example - it's not really opaque, _nor_ is it a config option, but it sure as hell would be inconvenient for all low-level filesystems to do int my_readdir(struct file *filp, void *dirent, int (*filldir)(void *, const char *, int, loff_t, u64, unsigned)) { ... } because let's face it, having to write out that "filldir" type just made me use two lines (and potential for totally unnecessary tupos) because the thing was so complex. So at that point, using a typedef is just common sense, and we can do int my_readdir(struct file *filp, void *dirent, filldir_t filldir) { ... } instead. But it's really quite hard to make that kind of complex type in C. It's almost always a function pointer that takes complex arguments.