char* repeat_char(char ch, size_t num_times) {
    char* s = malloc(sizeof(*s) * (num_times + 1));
    for(size_t i = 0; i < num_times; i++) {
        s[i] = ch;
    }
    s[num_times] = '\0';
    return s;
}

void print_string(char* s) {
    for(int i = 0; s[i] != '\0'; i++) {
        fputc(s[i], stdout);
    }
}

int main(int argc, char* argv[]) {
    char* s = repeat_char('@', 3); // ⇒ "@@@"
    print_string(s); // same as printf("%s", s)
    free(s);

    s[1] = '_'; // change to "@_@" // ▶▶▶ !!! INVALID WRITE … free'd !!! ◀◀◀

    return EXIT_SUCCESS;
}