Question

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Why is this C code causing a segmentation fault?

I am writing a simple string reverse script.

I added print statements to debug. I keep getting a run time exception before Error1. But I can’t seem to figure out the reason.

Here’s my code:

#include <iostream>
#include <cstdlib>

using namespace std;

int strlen(char* s){ 
  int i = 0;
  while(*s != '\0'){
    i++;
    s++;
  }
  return i;
}

void reverse(char* src){

  char* dest = src+strlen(src)-1;
  char temp;

  while(src < dest){
    temp = *src;
    cout << "Error0" << endl;
    *src = *dest;
    cout << "Error1" << endl;
    *dest = temp;
    cout << "Error2" << endl;
    src++;
    dest--;
  }

}

int main (void){

  char* s = "Hello world";
  cout << s << endl;
  int i = strlen(s);
  cout << i << endl;
  reverse(s);
  cout << s << endl;

  getchar();
  return 0;
}

Here’s my output:

            Hello world
            11
            Error0

Answer 1

this

char* s = "Hello world";

needs to be

char s[] = "Hello world";

Your orignal is trying to change constant memory that you aren’t allowed to change, so you need to allocate space and initialize it with the string