Dynamic libraries

Dynamic libraries are libraries that are linked at load time. Dynamic libraries are more like executables without an entry point. They contain code needed by your program, and the load-time linker is responsible for providing the location of each required function to your program while your program is being loaded.

It is also possible for your program to link itself while it executes during runtime as an optimization, only linking functions that are needed (a process known as lazy loading). 

Most of the libraries provided by the operating system are dynamic libraries. To see which dynamic libraries are needed by your program, you can use the LDD tool, as follows:

> ldd a.out
  linux-vdso.so.1 (0x00007ffdc5bfd000)
  libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f92878a0000)
  /lib64/ld-linux-x86-64.so.2 (0x00007f9287e93000)

In this example, we used the LDD tool to list the dynamic libraries needed by our simple Hello World\n example. As shown, the following libraries are needed:

• vdso: a library provided by the operating system to speed up the process of making system calls
• libc: the standard C library
• ld-linux-x86-64: the dynamic linker itself, responsible for lazy-loading