Linking #

Memory #

Process memory layout:

Process creation #

Linkers #

• Unix: ld
• Windows: LINK.exe

What does it do?

1. Combine object files (compiled from source)
2. Computes memory layout
3. Fix memory addresses
4. Produces executable

Problems it has to deal with:

• Assembler doesn’t know where code or data will go in memory
  • Makes a guess: Assume sections will be located at 0, write everything as such
• Assembler doesn’t know addresses of external objects
  • Makes a guess: Assume 0 and leave a note for the linker

Object file #

Contains:

• Code and data sections
  • Size
  • Starting address
  • Initial contents
• Symbol table
  • Name and location for each of the methods and variables
• Unresolved references: info for linker
  • Location of the reference
  • What should the reference point to
• Additional info:
  • Debugging info

Linking operation #

1. Read in section sizes, computes memory layout
2. Reads symbols, build symbol table
3. Read section data, fix addresses, write executable

Dynamic linking #

• Shared libraries, don’t know locations at link time

At startup, dynamic loader scans jump table (for example, maps printf to libc.so)

1. Loads file into memory (mmap)
2. Looks up symbols
3. Fills address in jump table