Find all possible combinations of alphabets of a string

Given a string, find the possible selections made from the alphabets of the string, such that they form unique combination.

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
int main(void) {
   char *v="abc";
   int s=pow(2,strlen(v))-1,i;
   int t;
   for(;s;s--)
   {
     t=s;
     for(i=0;i<strlen(v);i++)
     {
      if(t&1)
       printf("%c",*(v+i));
      t>>=1;
     }
     printf("\t");
   }
   return 0;
}