quartz/content/notes/linker.md

title	aliases	tags
linker	Linker, linking	cosc202

• Linkers facilitate use of code in multiple languages, e.g.
  • use C for programming speed of developer;
  • use Assembly language for running speed of code
  • But these days: often hard to beat optimising compilers!
• Linkers effect interaction with libraries in two ways:
  • Static linking: build libraries’ code into your application
  • Dynamic linking: library connected in at load or run-time
• Linkers facilitate interaction with OS

Static linking

• Statically linked libraries are built into executable
  • We’ve previously discussed the dependency problems:
    • Every executable carries the weight of the library code
    • Also, bug fixing requires rebuilding the whole program
• However, static linking is straightforward
  • e.g., nothing much OS-specific about static linking
    • Library calls are just function calls within userspace
• Alternative is for linking not to finish at compile-time
  • Linker could add code to link at runtime
  • More commonly, linker tells OS more linking needed later

Dynamic linking

• Dynamic linking is common on modern OSs, e.g.,
  • DLLs on Windows: Dynamic Link Libraries
  • SOs on Linux, Solaris: Shared Objects
  • DyLibs on macOS: Dynamically loaded modules
• One copy of dynamic library shared by all executables
  • efficiently uses memory: load into RAM once; share to all
• Fixing bugs / security flaws is streamlined:
  • just upgrade the single copy of the library
  • all processes will adopt fixed library (when they next start)

Managing dynamic libraries

• Dynamic libraries may have different ownership
  • Some dynamic libraries will be part of the OS distribution
  • Others installed along with apps that need those libraries
• Can track installable library use with reference counters
  • Software installer checks whether library is already present:
    • if not, install library, set reference count to one
    • if so, increase reference count by one
  • Uninstaller decreases reference count: deletes if at zero
  • But your reference counting needs to be reliable
    • ... unlike early Windows OSs: so called ‘DLL hell’