Summary

OS provides foundation for libraries

• Dynamic libraries vs static libraries
  • How to manipulate dynamic loader
• Example of issues from ABI changes without API changes

Applications

• OS is a kernel + a bunch of different libraries

  

  Applications may pass through multiple layers of libraries

• To define an OS depends on the application consist of a kernel and libraries required for your application

  • Android and Debian use the Linux kernel, but the applications are different
  • They are the same OS if you only care about terminal applications

• Linux distributions may be considered GNU/Linux

Normal Compilation in C

Object files .o are just ELF files with code for functions!

Static libraries are included at link time

Dynamic Libraries

Loaded into memory at runtime, reducing size of executable, rather than being copied into the executable during the linking phase

Multiple applications can use the same library

• C standard library is a dynamic library (.so) like any other on the system
• Collection of .o files containing function definitions
• The OS only loads libc.so in memory once, and shares it
• ldd <executable> shows which dynamic libraries an executable uses
• Whenever we run a program using a dynamic library, and it changes, we always get the freshest copy of it

Static vs Dynamic Libraries