char* repeat_char(char ch, size_t num_times) {
    char* s = malloc(sizeof(*s) * num_times);  // ▶ Whoops... didn't make room for '\0' ◀
    for(size_t i = 0; i < num_times; i++) {
        s[i] = ch;
    }
    s[num_times] = '\0';  // ▶▶▶ !!! INVALID WRITE … 0 bytes after !!! ◀◀◀
    return s;             //     AKA "buffer overflow"
}

void print_string(char* s) {
    for(int i = 0; s[i] != '\0'; i++) { // ▶▶▶ !!! INVALID READ … 0 bytes after !!! ◀◀◀
        fputc(s[i], stdout);            //     (when s==2)  AKA "buffer overread"
    }
}

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