Why we need separate library for static and dynamic linking?

Question

There are related post here and here.

According to my understanding, static linking directly insert code(what code?machine code?) from library into executables. However, dynamic linking only insert reference(pointer?) point to somewhere in the library.

Then I am wondering why we need two separate version of library of same functionality? For example, for intel MKL, we have libmkl_sequential.a and libmkl_sequential.so. And static linking must link static library, dynamic linking must link dynamic library. Why dynamic linking can not just simply point to static library?

What is the real difference between content of .so and .a of same functionaly?

Answer 1

Code which you want to execute needs to be loaded in memory. A function linked statically becomes a part of your program and so they are both loaded together when the program starts.

Why dynamic linking can not just simply point to static library? Static library is a disk file, how would you want to point inside this? There must be a mechanism (loader & binder) which investigates the starting executable program, asks which functions it wants to use, and loads the corresponding libraries into memory. Yes, the netto code (instructions) in both versions "libmkl_sequential.a" and "libmkl_sequential.so" may be identical, but static and dynamic types of libraries require different auxilliary metainformation dictated by the library format creator.