Why below code gives segmentation fault problem? [duplicate]

This question already has answers here: Closed 11 years ago.

Possible Duplicate:

Why does simple C code receive segmentation fault?

I have code below which will remove trailing spaces from a string but i don't what going in this code so as it gives segmentation fault problem??

 void main(void);

char* rtrim(char*);

void main(void)
 { 
 char* trail_str = "This string has trailing spaces in it.               ";


 printf("Before calling rtrim(), trail_str is '%s'\n", trail_str);

 printf("and has a length of %d.\n", strlen(trail_str));



 rtrim(trail_str);



 printf开发者_JS百科("After calling rtrim(), trail_str is '%s'\n", trail_str);

 printf("and has a length of %d.\n", strlen(trail_str));

 }


  char* rtrim(char* str)
  {
   int n = strlen(str) - 1;    

    while (n>0)            
   {
       if (*(str+n) != ' ')    
      {

           *(str+n+1) = '\0'; 



           break;             
      }

      else

      n--;
 }

 return str;      

 }

---

trail_str points to a constant area in the memory and thus cannot be changed in *(str+n+1) = '\0'

when initializing

char* trail_str = "This string has trailing spaces in it.               ";

you actualy generate a constant string: "This string has trailing spaces in it. " and tell trail_str to point to it.

---

char* trail_str = "This string has trailing spaces in it.               ";

The string pointed to by tail_str can be stored in read-only memory. You can't modify it.

If you want to modify it, you'll need to allocate storage for it and copy that string constant .

(Also, main should return an int, not void.)