Sort a String in alphabetic order

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void sort_string(char*);
int main()
{
   char string[100];

   printf("Enter some text\n");
   gets(string);

   sort_string(string);
   printf("%s\n", string);

   return 0;
}
void sort_string(char *s)
{
   int c, d = 0;
   int length;
   char *pointer, *result, ch;

   length = strlen(s);
   result = (char*)malloc(length+1);
   pointer = s;

   for ( ch = 'a' ; ch <= 'z' ; ch++ )
   {
      for ( c = 0 ; c < length ; c++ )
      {
         if ( *pointer == ch )
         {
            *(result+d) = *pointer;
            d++;
         }
         pointer++;
      }
      pointer = s;
   }
   *(result+d) = '\0';
   strcpy(s, result);
   free(result);
}