Parse and Understand Object File Formats (ELF)

ELF and Object Files

Compilers emit object files (.o), not executables. ELF on Linux, Mach-O on macOS, COFF on Windows: all contain machine code, symbol tables, and relocation entries. readelf and LLVM's llvm-objdump are daily tools for backend engineers.

ELF Sections

.text holds code. .data and .bss hold initialized and zero-initialized data. .symtab lists symbols with binding (local/global/weak) and section index. .rela.text records fixup sites that the linker must patch.

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• Relocations patch addresses the compiler could not fix
• Symbols may be undefined until link time
• Debug sections (.debug_info) map to source lines
• Object files are the linker's input atoms

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will parse and understand object file structure for ELF. This exercise asks you to read section headers, symbol tables, and relocation entries from a simple compiled object.