C Compilation Model

Four Stages From Source to ELF

A C file on disk is not yet a running process. Clang (via the gcc driver) walks four stages that Make, CMake, and Bazel all assume you understand.

1. Preprocess (.c → .i): expand #include, #define, strip comments (gcc -E)
2. Compile (.i → .s): parse C, emit assembly (gcc -S)
3. Assemble (.s → .o): machine code in a relocatable object (gcc -c)
4. Link (.o → executable): resolve symbols across translation units (gcc file.o -o prog)

Each .c file is its own translation unit. add.o can compile while main.o is untouched, which is why incremental builds work.

What You See at Each Stage

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"Undefined reference to foo" is a linker error: something declared, never defined in any .o. "Unknown type name" is a compiler error in stage 2.

For this exercise, you will run a small program with macros through all four stages and report what changed. This task asks you to connect error messages to stages, because debugging build failures without that map wastes hours in every systems shop.