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Asked: 2026-05-11T09:33:55+00:00

Here’s a simple example of a program that concatenates two strings. #include <stdio.h> void

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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++ != '\0');     s--;     while ((*s++ = *t++) != '\0'); }  int main() {     char *s = 'hello';     strcat(s, ' world');     while (*s != '\0') {         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?

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1 Answer

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

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