What does #include actually do?

Question

In C (or a language based on C), one can happily use this statement:

#include "hello.h";

And voila, every function and variable in hello.h is automagically usable.

But what does it actually do? I looked through compiler docs and tutorials and spent some time searching online, but the only impression I could form about the magical #include command is that it "copy pastes" the contents of hello.h instead of that line. There's gotta be more than that.

Answer 1

Logically, that copy/paste is exactly what happens. I'm afraid there isn't any more to it. You don't need the ;, though.

Your specific example is covered by the spec, section 6.10.2 Source file inclusion, paragraph 3:

A preprocessing directive of the form

# include "q-char-sequence" new-line

causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters.

Answer 2

That (copy/paste) is exactly what #include "header.h" does.

Note that it will be different for #include <header.h> or when the compiler can't find the file "header.h" and it tries to #include <header.h> instead.