Compiling with gcc

Installing a compiler

• Linux (Ubuntu/Debian): sudo apt install build-essential (gives gcc, make and libraries).
• Windows: install MSYS2 and its MinGW-w64 gcc package, or use WSL (Windows Subsystem for Linux) and install gcc inside Ubuntu. WSL is recommended because tools like valgrind (module 11) work there.
• macOS: run xcode-select --install; the "gcc" command there is actually clang, which accepts the same common options.

Check the install with gcc --version. Any editor works; VS Code with the C/C++ extension is a popular free choice.

Compile and run

gcc hello.c -o hello                          # compile hello.c into an executable named hello
./hello                                       # run it (Linux, macOS, WSL)
hello.exe                                     # run it (Windows cmd, MinGW)

gcc -std=c17 -Wall -Wextra -g hello.c -o hello   # the recommended everyday command

The four stages

gcc -E hello.c -o hello.i      # 1. preprocess: expand #include and #define (text)
gcc -S hello.i -o hello.s      # 2. compile: C → assembly language
gcc -c hello.s -o hello.o      # 3. assemble: assembly → machine code object file
gcc hello.o -o hello           # 4. link: join object files + libraries → executable

The linker matters because your program calls functions you did not write, like printf. Their machine code lives in the C standard library; the linker connects your calls to it. "undefined reference to …" errors come from this last stage.

Errors and warnings

hello.c: In function 'main':
hello.c:5:5: error: expected ';' before 'return'
hello.c:4:12: warning: format '%d' expects argument of type 'int', but argument 2 has type 'double' [-Wformat=]

Exit status

When main returns, the number is the program's exit status: 0 means success, anything else means failure. Scripts and other programs check it: on Linux run echo $? right after your program; on Windows cmd, echo %ERRORLEVEL%.