Question

Possible Duplicate:
How do you reverse a string in place in C or C++?
Why is this C code causing a segmentation fault?
Modifying value of char pointer in c produces segfault

Running a very simple code example

#include <stdlib.h>
#include <iostream>

char* last_char(char* s){
  char* last = s;
  while (*last) ++last;
  return last;
}

char* in_place_reverse(char* s) {
  char* left = s;
  char* right = last_char(s);
  char temp;

  while( left < right ) {
    temp = *left;
    *left = *right;
    *right = temp;

    left++;
    right--;
  }

  return s;
}

int main(){
  char * s = "letters\n";
  std::cout << in_place_reverse(s);
}

All the time I get

 Segmentation fault

But from my point of view I’m not doing anything illegal within the code.
Please help me to determine what’s wrong.

P.S. I compile with

g++ example.c

Answer 1

Two problems:

1. You are trying to modify a string literal. This may work, it may not, or it may crash. This is invoking undefined behavior. Use char s[] = "letters\n" to make a mutable copy.
2. last_char() in fact returns a pointer to the sentinel '\0' at the end of the string — it points beyond the last character. Change return last to return last - 1. Otherwise you are going to move the sentinel around too, and that’s almost certainly not what you want. (Note that this will return a pointer to garbage if the string is zero-length. You should fast-succeed in in_place_reverse() if *s == '\0' to avoid this complexity.)