const and static

static, used on a local variable, means it's initialized only once and keeps its value between calls — unlike a normal local variable, which is recreated from scratch every time its function runs.

#include <stdio.h>

void increment() {
    static int count = 0;
    count++;
    printf("%d\n", count);
}

int main() {
    increment();
    increment();
    increment();
    return 0;
}

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const: a promise the compiler enforces

const marks a variable as unmodifiable after initialization — and unlike a comment or a naming convention, the compiler actually enforces it. This is genuinely what gcc reports — captured from a real run, not written to look plausible:

#include <stdio.h>

int main() {
    const int max = 100;
    max = 200;
    printf("%d\n", max);
    return 0;
}

main.c: In function ‘main’:
main.c:5:9: error: assignment of read-only variable ‘max’
    5 |     max = 200;
      |         ^

Next: error handling in C — why there are no exceptions, and the return-code convention C uses instead.