ELF: Executable & Linkable Format

ELF (Executable and Linkable Format) is the file format for executables, shared libraries, and object files on Linux/BSD/Solaris/most UNIX-likes.

Three roles:

1. Object file (.o): output of assembler. Has unresolved symbols + relocations.
2. Executable: ready to run. Resolved.
3. Shared library (.so): position-independent code, loaded into other processes.

Same format, different use.

Why ELF?

• Standardized (System V ABI).
• Flexible: arbitrary number of sections.
• Extensible: vendor-specific data.
• Compact: minimal overhead.

Compare:

• PE (Portable Executable): Windows.
• Mach-O: macOS / iOS.
• a.out: ancient UNIX.

Tools:

• readelf -a foo.elf: dump everything.
• objdump -d foo.elf: disassemble.
• nm foo.o: list symbols.
• file foo: identify type.

ELF is what the kernel's execve() reads. The loader walks the ELF, maps segments into memory, applies relocations, and jumps to entry point.

We'll build an ELF parser + loader: read an ELF file, set up memory, jump to start. Same logic as ld-linux.so (the dynamic linker) and the kernel's binfmt_elf.

Reference: System V ABI / Linux Generic ABI specs.