C/C++ uses the famous make-stuff-up-as-we-go-along naming convention. In general, the only consistent things you can say about the C/C++ standard library naming convention is that it doesn’t use camel case (except for C++ STL template typenames), it uses lower-case class and function names, and (sometimes) uses underscores to separate words. But the usage of underscores is more of a C++ thing; C in particular tends to abbreviate words, e.g. strlen, memcpy. This is probably a result of the tendency to use very terse names for things in the early days of UNIX.