x2.c

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

void swap(int *p, int i, int j)
{
        int t = p[i];
        p[i] = p[j];
        p[j] = t;
}

void generate(int n, int *seed, char **p, unsigned long *i)
{
        int c;
        if(n == 0) {
                p[*i] = calloc(11, sizeof(char));
                if(p[*i]) {
                        for(c = 0; c < 10; ++c) {
                                p[*i][c] = seed[c] + '0';
                        }
                        (*i)++;
                }
                return;
        }

        for(c = 0; c <= n; ++c) {
                swap(seed, c, n);
                generate(n - 1, seed, p, i);
                swap(seed, c, n);
        }
}

int vstrcmp(const void *a, const void *b)
{
        return strcmp(* (char * const *)a, * (char * const *)b);
}

int main()
{
        int seed[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        char **output = NULL;
        unsigned long i = 0;

        output = calloc(3628800, sizeof(char*));
        if(output == NULL) {
                perror("uh oh");
                return 1;
        }


        generate(9, seed, output, &i);
        qsort(output, i, sizeof(char*), vstrcmp);

        do {
                if(output[i]) {
                        printf("%s\n", output[i]);
                        free(output[i]);
                }
        } while(i-- != 0);

        free(output);
        return 0;
}