How Does the C Linker Relocate Addresses and Resolve Symbols?

How Does C Linking Work in Practice?

In the world of C programming, linking plays a crucial role in combining compiled object files into a single, executable program. Unlike the question of "how to link," which involves using specific commands, this article delves into the intricate workings of linking at a fundamental level.

1. Introduction:

   • The key function of linking is address relocation.
   • The compiler cannot determine the final addresses of variables and functions because it only sees individual input files at a time.
   • Linking resolves undefined symbols, avoids collisions between multiple sections, and assigns addresses consistently across all object files.

2. Understanding .text of .o Files:

   • Decompiling the .text section of an object file reveals placeholder values that represent the addresses of data or functions.
   • The compiler inserts these placeholders because it lacks the information about the final memory layout.

3. Introducing .rela.text:

   • The .rela.text section contains relocation entries.
   • Each entry specifies an offset within the .text section and provides information on the type of relocation required.
   • In this case, R_X86_64_64 denotes a 64-bit relocation with a simple addition of the value at the offset.

4. Relocation in the Executable:

   • During linking, the linker processes the .rela.text entries.
   • It calculates the final addresses and inserts them into the .text section of the executable.
   • The result is a fully resolved and relocated executable program that can be executed on the target machine.

5. Program Headers and Address Mapping:

   • The linker also generates program headers that describe the layout of sections in the executable.
   • The program headers include the starting address for each section, allowing the operating system to properly load and access the code and data.

6. Conclusion:

   • The linking process is a complex but essential step in C program execution.
   • It ensures that all object files are correctly combined, addresses are relocated, and sections are loaded at the appropriate memory locations.
   • Understanding this intricate process is crucial for optimizing the performance and correctness of C programs.

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