Linking

todo

  • add markmind automatically

why

  • modular
  • ..

ELF Object File Format

  • ELF header
  • Segment header table
  • .text: should be .code, named for historical reason
  • .rodata: read only data, separating the HEAP into two parts.
  • .data: initialized global data
  • .bss: block starting symbol, un-initialized global(static) data,
  • .symtab: symbol table
  • .rel.text, .rel.data: hints for the linker?
  • .debug: debug symbols, line numbers, etc.
  • Section header table

Linker Symbols

  • Global: non-static func & var
  • External: refer symbols defined in other modules
  • Local: static var & func, C’s private, information hiding, abstraction

Linking

Step 1: Symbol Resolution

find where is the symbol we’re referencing

  • Local Symbols
    • Local non-static C var vs local static var
      • non-static: stored on the stack
      • ~ : stored in either .bss or .data

Step 2: Relocation

  • Merge
  • Combine .text together into a single .text section, do the same on other sections too.

Static lib

.a files archived into .o file, using ar

Linker’s algorithm: cmd order matters

- scan .o and .a files in the command line order
- keep a list of current unresolved reference
- as each obj file is encountered, try to resolve each unresolved ref in the list
- if any entries in the list at the end of scan, then **undefined reference**

Shared lib

  • if lib updated, no need to rebuild all the binary referencing it, saving space too
  • link at load time
  • runtime link dlopen
  • at run time, they are loaded in the middle the HEAP space, separating the HEAP into two parts.

Interpositioning

  • Compile time: by preprocessing, hijack the include header
  • Link time: gcc -Wl,--wrap,malloc -Wl,--wrap,free …
  • Load/Run-time: (LD_PRELOAD=...)