How Does the C Linker Handle Address Relocation During the Linking Process?

How does C linking work in practice?

Linking in C involves combining multiple object files (.o files) to create an executable or shared library. The process involves various steps, including address relocation and symbol resolution.

Address Relocation

One crucial function of linking is _address relocation_. When the compiler compiles each source file into an object file, it generates code that contains references to symbols (functions, variables, etc.) defined in other source files. However, it doesn't know the final addresses of these symbols until all object files are linked together.

During linking, the linker reads the relocation sections in the object files. These sections contain information about which addresses need to be modified in the .text and .data sections to point to the correct locations for symbols. The linker then adjusts these addresses based on the final locations of the symbols.

Example

Consider the following example to illustrate address relocation:

; object file hello_world.o
.data
hello_world db "Hello world!", 10
.text
.global _start
_start:
    mov rax, 1 ; sys_write
    mov rdi, 1 ; stdout
    mov rsi, hello_world ; placeholder for string address
    mov rdx, 13 ; number of characters to write
    syscall

When this .o file is compiled, the compiler inserts a placeholder value (e.g., 0) for the address of the "Hello world!" string. The linker reads the .rela.text section in this file:

Offset          Info           Type           Sym. Value    Sym. Name + Addend
00000000000c  000200000001 R_X86_64_64       0000000000000000 .data + 0

This entry tells the linker that at offset 0x0c (inside the mov rsi, hello_world instruction), it needs to modify a 64-bit value to point to the .data section.

When the linker combines all object files, it adjusts the value at offset 0x0c to point to the correct address of the string in the final executable.

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