C/C++ How Does Dynamic Linking Work On Different Platforms?

Question

How does dynamic linking work generally?

On Windows (LoadLibrary), you need a .dll to call at runtime, but at link time, you need to provide a corresponding .lib fi

Answer 1

In shared library, such as .dll .dylib and .so, there is some information about symbol's name and address, like this:

------------------------------------
| symbol's name | symbol's address |
|----------------------------------|
| Foo           | 0x12341234       |
| Bar           | 0xabcdabcd       |
------------------------------------

And the load function, such as LoadLibrary and dlopen, loads shared library and make it available to use.

GetProcAddress and dlsym find you symbol's address. For example:

HMODULE shared_lib = LoadLibrary("asdf.dll");
void *symbol = GetProcAddress("Foo");
// symbol is 0x12341234

---

In windows, there is .lib file to use .dll. When you link to this .lib file, you don't need to call LoadLibrary and GetProcAddress, and just use shared library's function as if they're "normal" functions. How can it work?

In fact, the .lib contains an import information. It's like that:

void *Foo; // please put the address of Foo there
void *Bar; // please put the address of Bar there

When the operating system loads your program (strictly speaking, your module), operating system performs LoadLibrary and GetProcAddress automatically.

And if you write code such as Foo();, compiler convert it into (*Foo)(); automatically. So you can use them as if they're "normal" functions.