c - Is modifying a string pointed to by a pointer valid?

Question

Here's a simple example of a program that concatenates two strings.

#include <stdio.h>

void strcat(char *s, char *t);

void strcat(char *s, char *t) {
    while (*s++ != '');
    s--;
    while ((*s++ = *t++) != '');
}

int main() {
    char *s = "hello";
    strcat(s, " world");
    while (*s != '') {
        putchar(*s++);
    }
    return 0;
}

I'm wondering why it works. In main(), I have a pointer to the string "hello". According to the K&R book, modifying a string like that is undefined behavior. So why is the program able to modify it by appending " world"? Or is appending not considered as modifying?

Answer 1

Undefined behavior means a compiler can emit code that does anything. Working is a subset of undefined.