3 Linking

1. Static Linking

Static linkers such as the Linux ld: a collection of relocatable object files → a fully linked executable object file that can be loaded and run

Symbol Resolution:

• Symbols: a function, a global variable, or a static variable
• associate each symbol reference with exactly one symbol definition

Relocation:

• Compilers and assemblers generate code and data sections that start at address 0.
• The linker relocates these sections by associating a memory location with each symbol definition,
• and then modifying all of the references to those symbols so that they point to this memory location
• relocation entries

2. Object Files

• Relocatable object file: compilers and assemblers (ELF files)
• Executable object file: linkers
• Shared object file: compilers and assemblers

3. Symbols and Symbol Tables

• Global symbols #1
  • defined by module m
  • can be referenced by other modules
  • nonstatic C functions and global variables
• Global symbols #2
  • referenced by module m
  • defined by some other module
  • externals
  • nonstatic C functions and global variables defined in other modules
• Local symbols
  • defined and referenced exclusively by module m
  • static C functions and global variables that are defined with the static attribute
  • visible anywhere within module m, cannot be referenced by other modules

4. Symbol Resolution

4.1 Resolve Duplicate Symbol Names

• Strong symbols: functions and initialized global variables
• Weak symbols: uninitialized global variables

Rule 1. Multiple strong symbols with the same name are not allowed.

Rule 2. Given a strong symbol and multiple weak symbols with the same name, choose the strong symbol.

Rule 3. Given multiple weak symbols with the same name, choose any of the weak symbols.

4.2 Linking with Static Libraries

On Linux systems, static libraries are stored on disk in a particular file format known as an archive, which is a collection of concatenated relocatable object files.

To create a static library of these functions, we would use the ar tool as follows:

gcc -c addvec.c multvec.c
ar rcs libvector.a addvec.o multvec.o
# use libvector.a to build the executable
gcc -static -o prog2c main2.o ./libvector.a

During the symbol resolution phase, the linker scans the relocatable object files and archives left to right in the same sequential order that they appear on the compiler driver’s command line.

The general rule for libraries is to place them at the end of the command line.

5. Relocation

• Relocating sections and symbol definitions
• Relocating symbol references within sections

• R_X86_64_PC32: Relocate a reference that uses a 32-bit PC-relative address.
• R_X86_64_32: Relocate a reference that uses a 32-bit absolute address.

6. Loading

7. Shared Libraries

7.1 At Linking Time

A shared library is an object module that, at either run time or load time, can be loaded at an arbitrary memory address and linked with a program in memory.

.so on Linux and .dll on Windows.

• in any given file system, there is exactly one .so file for a particular library
• a single copy of the .text section of a shared library in memory can be shared by different running processes

Build a shared library:

# pic: position-independent code
gcc -shared -fpic -o libvector.so addvec.c multvec.c
# link into program
gcc -o prog2l main2.c ./libvector.so

7.2 At Run Time

8. PIC

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Last update: August 18, 2021

Authors: Co1lin