Reverse strings stored in array of pointers

Question

Write a program to reverse the strings stored in the following array of pointers to strings:

char *s[ ] = {
  "To err is human...",
  "But to really mess things up...",
  "One needs to know C!!"
};

Hint: write a function xstrrev ( string ) which should reverse the contents of one string. Call this function for reversing each string stored in s.

Why am I not getting correct output with this code?

#include "stdafx.h"
#include "conio.h"
#include "stdio.h"
#include "string.h"

using namespace System;

void xstrrev (char str[])
{
    int i,l;
    char temp;
    l=strlen(str);

    for (i=0;i<(l/2);++i)
    {
        temp=*(str+i);
        *(str+i)=*(str+i+l-1);
        *(str+i+l-1)=temp;
    }
}

void main ()
{
char *s[] = {
        "To err is human...",
        "But to really mess things up...",
        "One needs to know C!!"
    };
    xstrrev(s[0]);
    xstrrev(s[1]);
    xstrrev(s[2]);
    puts(s[0]);
    puts(s[1]);
    puts(s[2]);
    getch();
}

Attempting to modify a string literal causes undefined behaviour. This question is a duplicate hundreds of times over. — Carl Norum, Mar 25 '13 at 18:29
Here's an example: [Why does char\* cause undefined behaviour while char\[\] doesn't?](http://stackoverflow.com/questions/8302290/why-does-char-cause-undefined-behaviour-while-char-doesnt). — Carl Norum, Mar 25 '13 at 18:31
Also, `main` should return `int`, and I think you have a couple of bugs in your reverse function, too. `*(str+i+l-1)` should probably be `*(str-i+l-1)`. — Carl Norum, Mar 25 '13 at 18:32
little surprised your xstrrev() function is very nice and main() is very bad ..How ? — Grijesh Chauhan, Mar 25 '13 at 18:53
well, for the bad main()... i saved the index variable :P... — vaibhav talwar, Mar 25 '13 at 19:18
@vaibhavtalwar The way you are writing your code look you need revers three string in order. But not reverse each string of `s[]` Am I correct ? — Grijesh Chauhan, Mar 25 '13 at 19:21
@vaibhavtalwar Yes if language says undefined means compiler dependent. So in TCC it worked well. remember Linux does perform strict checking then windows. tcc is windows based. — Grijesh Chauhan, Mar 25 '13 at 21:32
Undefined behaviour does not have to mean "compiler dependent". It means "undefined". That means a particular compiler can do whatever it likes in this situation, or choose to do different things at different times for any reason or no reason. — Carl Norum, Mar 26 '13 at 04:52

Grijesh Chauhan · Answer 1 · 2013-03-25T19:44:29.917

Are I can notice only thing you are doing wrong in declaration of s[]. You should declare you s[] in main() like this:

char str1[] = "To err is human...";
char str2[] = "But to really mess things up...";
char str3[] = "One needs to know C!!";
char *s[] = { str1, str2, str3};

Reason:

if you do like:

char* str = "yourname";
str[2] = 'c';    // you are trying to write on read only memory

you are trying to write on read only memory. that cause of segmentation fault (undefined behavior) Because str points to a constant string literal.

Where are if you do like:

char str[] = "yourname";
str[2] = 'c';    // ok no error

Then its ok because this time str[] is an array string and its value and size defined at point of declaration.

I think algorithm wise you are correct

EDIT and I am correct. see here you code is working after my suggestion Codepad and what @carlNorm said is correct. He also posted a good link.

Reverse strings stored in array of pointers

1 Answers1