How C Code Runs: Compiling and Linking

Turning hello.c into ./hello is actually several steps, usually hidden behind one gcc command:

Preprocessing -> Compiling -> Assembling -> Linking

The preprocessor handles lines starting with # (like #include) before real compilation even begins — textually pasting in header file contents and expanding macros. The compiler then translates the result into machine code, and the linker combines that with any library code (like printf's actual implementation) into one final executable.

gcc -E hello.c   # stop after preprocessing, show the result
gcc -S hello.c   # stop after compiling, show generated assembly

Common mistakes

• Treating a linker error ("undefined reference to foo") like a compiler error — it means the compiler understood your code fine, but couldn't find the actual implementation of something you called, often because a library wasn't linked in.