I'm learning about static and dynamic libraries in C and how to make them.
One thing that keeps bothering me is this:
Suppose a file is using the library mylibrary by doing #include <mylibrary.h>.
Does this mean that C libraries are distributed along with matching textual header files? Or is mylibrary.h somehow magically exported from the binary library file?
Does this vary between different approaches, or whether the library is static or dynamic?
Yes, and depending on the platform, you get even more files to distribute with it. It's a quite messy story. At least, it doesn't matter whether the library is static or dynamic (aside from linker parameters).
The header file is necessary because the compiled binary does not contain enough information to be usable by the compiler. With some platform-based variance, a C binary typically only has enough metadata to identify functions and global variables by their name. That metadata does not include the types (or count) of parameters, return types, structure or union definitions, the type or size of global variables, etc. All of this information typically is encoded in the headers that are distributed with the library. (Conveniently, it also means that anything that does not exist in the header is hidden from the developer; this is what allows you to create non-public functions in a library, that users shouldn't call directly.)
On some platforms, binaries don't even contain function names. Instead, functions are referenced by their position in an "ordinal table". On those platforms, the library has to ship a header, the executable binary, and an additional file that translates from the name of the function in the header to the index of the function in the ordinal table, such that "void hello(void)" might be "function at index 3 in ordinal table" to the linker.
Conversely, including a header does not (usually) link against the library that it accompanies. This is possible on some platform, like Windows, on which there are special compiler directives that you can put in a header and that tells the linker to link against some library, but it is not standard behavior and you can't expect it to be a reality on any other platform.
Up and coming are modules, which provide a better user experience to link against binaries. A module is yet another file that you can package with your binary and that says "here are all my headers and here are all my libraries". Using modules, it's possible to write something like "import MyLibrary;" and it'll get you all the headers and all the linker arguments that you need. I believe that there are no C-standard modules yet; C++ is getting there with C++20.