void x(void);
void perm(int n, char a[n]){
  // This routine permutes the n characters starting at a and for
  // each permutation, calls x.
  // The characters are finally left as they were found.
  char t = a[0]; int j=n; while(j--) {
    char u = a[j]; a[0] = u; a[j] = t;
    if(n<3) x(); else perm(n-1, a+1);
    a[j] = u;} a[0] = t;}

// Demo and test:
#include <stdio.h>
#include <string.h>
char q[8];
int main () {strcpy(q, "abcdefg");
perm(7, q); printf("\n --%s--\n", q);
return 0;}
void x() {static int c=0;
  if(!(c--)) {printf("\n"); c=6;}
  printf("%s ", q);}