PE - IAT resolve mechanism

Question

I'm trying to understand how Windows is resolving functions with the IAT.

I have noticed that when a call is made to a Win API function, the structure of that call is not always the same (it's still consistent inside a binary, but not between two differents binary).

Sometime, if i follow the target address of that call, i find a jump to the resolved Win API function. And sometime, it's directly a call to the resolved function.

For instance:

• the binary A is using call like : call ds:GetSystemDirectoryW

• the binary B is calling like that: call GetSystemDirectoryW -> jmp ds:__imp_GetSystemDirectoryW

Can someone explain me the this difference in the calling procedure ?

Answer 1

The direct call can be generated by the compiler when it knows that the function comes from a DLL at compile time, or whole program optimization is used. If the target function is not marked as dllimport, the compiler generates a simple call to an external symbol and at link time this external symbol is resolved to a stub which actually jumps to the DLL import. For more info:

Importing function calls using __declspec(dllimport)

What is DLL import binding?