Compiling with gcc

gcc (the GNU C compiler) turns C source code into a program the CPU can run. clang is another compiler that accepts the same commands.

gcc hello.c -o hello     # compile hello.c into the program hello
./hello                  # run it

Without -o, the program is called a.out (a.exe on Windows). For C++, use g++ in the same way.

The flags you should always use

A good everyday command:

gcc -Wall -Wextra -std=c11 -g main.c -o main

Why warnings matter

#include <stdio.h>

int main(void) {
    int stock = 3;
    if (stock = 0) {
        printf("sold out\n");
    } else {
        printf("in stock: %d\n", stock);
    }
    return 0;
}

stock = 0 is an assignment, not a comparison: it sets stock to 0, and the if sees 0, which is false. It is legal C, so plain gcc compiles it without a word, and the program prints "in stock: 0". With -Wall, gcc warns you:

warning.c:5:9: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
    5 |     if (stock = 0) {
      |         ^~~~~

Press Visualize: StepVioz is stricter and refuses to run it at all. Do the same with gcc: treat every warning as a bug until you have understood it. -Werror turns warnings into errors, so the program will not build until they are fixed.

The four steps inside gcc

1. Preprocess (gcc -E main.c): paste in the #include files, replace #define names, drop comments.
2. Compile (gcc -S main.c makes main.s): translate C into assembly for your CPU.
3. Assemble (gcc -c main.c makes main.o): turn assembly into machine code, an object file.
4. Link: join the object files and the C library (where printf lives) into one program.

Errors from steps 1 to 3 point at a line of your code. Errors from step 4 (the linker) say undefined reference: you used a function that is declared but never defined anywhere.

Mark lesson complete