use plain recursion
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
void swap(char* src, char* dst)
{
        char ch = *dst;
        *dst = *src;
        *src = ch;
}
/* permute [set[begin], set[end]) */
int permute(char* set, int begin, int end)
{
        int i;
        int range = end - begin;
        if (range == 1) {
                printf("set: %s\n", set);
        } else {
                for(i=0; i<range; i++) {
                        swap(&set[begin], &set[begin+i]);
                        permute(set, begin+1, end);
                        swap(&set[begin], &set[begin+i]);       
/* set back */
                }
        }
        return 0;
}
int main()
{
        char str[] = "abcd";
        permute(str, 0, strlen(str));
        return 0;
}