Question

Just for fun I have written a function to check if string given is palindrome. When I run the prog it throws segmentation fault. Could anyone please throw light on it.

int palindrome( const char *input )         
{    
  char * reverse;    
  int len     = 0 ;     
  int i       = 0;     
  bool result = false;    
  len         = strlen(input);    

  if( len <= 1)   
    return  -1;     
  reverse  = (char *)malloc( sizeof ( char)* len);     
  printf( " the len of character is %d", len);        
  while( input[i++] != '/0')     
  {     
     reverse[ --len] = input[i];         
  }
  reverse[len] = '/0';

 printf(" the reverse string is %s", reverse);
 if( !strcmp( input, reverse) )
   return 1;
 else
 return 0;
}

Thanks
Sam

---

Changing the code as per suggestions to:

int palindrome( const char *input )         
{    
  char * reverse;    
  int len     = 0 ;     
  int i       = 0;     
  bool result = false;    
  len         = strlen(input)+1;    

  if( len <= 2)   
    return  -1;     
  reverse  = (char *)malloc(len);     
  printf( " the len of character is %d", len); 
  reverse[len] = '\0' ;      
  while( input[i++] != '\0')     
  {     
     reverse[ --len] = input[i];         
  }

 printf(" the reverse string is %s", reverse);
 if( !strcmp( input, reverse) )
   return 1;
 else
 return 0;
}

I still have a problem. The segmentation fault has disappeared but the reversed string is empty.

Answer 1

A few points:

• the null terminator for strings is '\0', not '/0'.
• sizeof(char) is always 1, you don’t need to multiply by it.
• you should always allocate enough space for a string and its null terminator such as with strlen(s) + 1.
• you should not cast the return value from malloc in C, it hides certain errors that you would be better off knowing about.
• since you’re gradually reducing len to populate the reverse string, it will end up as 0, not usable for placing the null terminator at the end of that string. This is probably the immediate cause of your core dump since calling strcmp on a non-null-terminated string is a bad idea.
• it’s not really necessary to create a reversed string at all, you can just check the first and last characters, then the second and second-to-last and so on (until they meet in the middle, or cross over).

On that last point, what I mean is something like (pseudo-code):

def isPalindrome (str):
    left = 0
    right = strlen(str) - 1
    while left < right:
        if str[left] <> str[right]:
            return false
        left = left + 1
        right = right - 1
    return true

---

And, with the update, you have the right idea, setting the null terminator fist before reducing len.

But since, len is now the string length plus one, you’re populating indexes 1 thru LEN rather than 0 thru LEN-1.

Change:

reverse[len] = '\0';

to:

reverse[--len] = '\0';

Or, better yet, change it so that len is still the length of the string (for printf):

len = strlen (input) + 1;
:
reverse = malloc (len);

to:

len = strlen (input);
:
reverse = malloc (len + 1);