Knuth 7.2.1.2: Algorithm L (Lexicographic Permutation Generation) in C.

permutation.c

static inline void Exchange(int* data, int a, int b) {
  int temp = data[a];
  data[a]=data[b];
  data[b]=temp;
}

//generates all permutations of initially sorted array `a` with `n` elements
//returns 0 when no more permutations exist
int genPermutation(int a[], int n) 
{
  int l,j;
  for (j = --n; j > 0 && a[j-1] >= a[j]; --j) { ; }
  if (j == 0) return 0; 
  for (l = n; a[j-1] >= a[l]; --l) { ; }
  Exchange(a, j-1, l);
  while (j < n) { Exchange(a, j++ ,n--); }
  return 1;
}