A string ends at the null terminator

A C string is a char array with a '\0' (the null terminator) after the last letter. "cat" takes four boxes.

my_strlen moves the pointer p one box at a time. Watch its arrow slide along word until it lands on '\0', then the length is how far it moved.

#include <stdio.h>

int my_strlen(const char *s) {
    const char *p = s;
    while (*p != '\0') {
        p++;
    }
    return (int)(p - s);
}

int main(void) {
    char word[] = "cat";
    int len = my_strlen(word);
    printf("%s has %d letters\n", word, len);
    return 0;
}

Output:

cat has 3 letters

From the lesson: Strings