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Compiling a C program

C source is text. A compiler turns it into a program the machine runs, and if it cannot, nothing runs at all. Learn the two kinds of error.

C8 min

Source, compiler, program

The text you type is source code, saved in a file like main.c. The machine cannot run text. A compiler reads the whole file first and translates it into machine instructions: the program. Only then does anything run.

That order has a big consequence. In Python, a typo on line 20 only stops you when line 20 runs. In C, a typo anywhere means there is no program yet, so not even line 1 runs. No output, no half-finished run, just the compiler's message.

Two kinds of error

Compile errors come from the compiler, before running. The program is not valid C: a missing ;, a name it has never seen, the wrong number of arguments. The message names the line:

Output
main.c(5): `coisn' undeclared

Runtime errors happen while the program runs. It compiled fine, then did something impossible, like dividing by zero or touching memory it does not own:

Output
RuntimeError: divide by zero

A compile error is about what you wrote. A runtime error is about what happened with the values. Fix compile errors top to bottom, starting with the first one.

Run it: compiled, then run

Run it, then change one thing and run it again.

main.c to run
#include <stdio.h>

int main(void) {
    int kills = 12;
    int deaths = 4;
    printf("K/D ratio: %d\n", kills / deaths);

    // Try 1: delete the ; above. Nothing prints: a compile error.
    // Try 2: set deaths to 0. It compiles, then stops while running.
    printf("done\n");
    return 0;
}

Nothing prints at all

This should print two lines, but it prints nothing, not even the first line. Run it, read the message, and fix it.

exercise_1.c to run
#include <stdio.h>

int main(void) {
    printf("Loading level 1\n");
    int coins = 30;
    printf("Coins: %d\n", coins)
    return 0;
}