c++cdllloader

How loader maps DLL into Process Address Space

I am curious to know how the Loader maps DLL into Process Address Space. How does the loader do that magic?

Solution

  • What level of detail are you looking for? On the basic level, all dynamic linkers work pretty much the same way:

    1. Dynamic libraries are compiled to relocatable code (using relative jumps instead of absolute, for example).
    2. The linker finds an appropriately-sized empty space in the memory map of the application, and reads the DLL's code and any static data into that space.
    3. The dynamic library contains a table of offsets to the start of each exported function, and calls to the DLL's functions in the client program are patched at load-time with a new destination address, based on where the library was loaded.
    4. Most dynamic linker systems have some system for setting a preferred base address for a particular library. If a library is loaded at its preferred address, then the relocation in steps 2 and 3 can be skipped.