Linking (Computing)

• Static Linking: at compile time
• Dynamic Linking: at runtime, operating system looks for .so (linux) .dll (win) .dylib (mac) extensions

Dynamic Linking §

Where Are They? §

OS looks for dynamic libraries in:

• Mac
  1. Paths baked into the binary (via @rpath, @executable_path, @loader_path)
  2. DYLD_LIBRARY_PATH (if not restricted by SIP).
  3. Default system locations:
     • /usr/lib/
     • /usr/local/lib/
• Linux
  1. RPATH, RUNPATH in the binary (ldd shows them).
  2. LD_LIBRARY_PATH environment variable.
  3. Configured dirs in /etc/ld.so.conf and /etc/ld.so.conf.d/.
  4. Standard locations:
     • /lib
     • /usr/lib
     • /usr/local/lib
• Windows
  1. The directory containing the executable.
  2. System directories (C:\Windows\System32, C:\Windows\SysWOW64).
  3. Current working directory.
  4. Directories in the PATH environment variable.

At Runtime §

Dynamic Linker (ld.so (linux), dyld on mac, Windows loader on win)matches all undefined symbols in the symbol table in the program with exported symbols by the dynamic libraries