perm.c

#include <stdio.h>
#define SWAP(a,b) do{ typeof(a) tmp = a; a = b; b = tmp; }while(0)

void permutate(int* v, int n, int i) {
  int j;
  if (i == n) {
    for(j = 0; j < n; j++) printf("%d ", v[j]);
    printf("\n");
  }else
    for(int j = i; j < n; j++) {
      SWAP(v[i], v[j]);
      permutate(v, n, i+1);
      SWAP(v[i], v[j]);
    }
}

void combinate(int v[], int start, int n, int k, int maxk) {
  int i;

  if(k > maxk) {
    permutate(v+i, maxk, 0);
    return;
  }

  for(i=start; i<=n; i++) {
    v[k] = i;
    combinate(v, i+1, n, k+1, maxk);
  }
}

int main(){
  int v[] = {1,2,3,4,5,6,7,8,9};
  combinate(v, 1, 9, 1, 4);
  return 0;
}