Reverse a linked list in group of k

This is one of a candidate interview question. I don't see any practical application of this-

Given a linked list, reverse the list in groups of size k. 

I am sure it is not clear to you, so here is a example-
Input -    1=>2=>3=>4=>5=>6=>7=>8
Output -  3=>2=>1=>6=>5=>4=>8=>7

Try solving it yourself before going through the code.

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

typedef struct node 
{
 int v;
 struct node *next;
} node;

node * revk(node *head, int k)
{
 int i = 1;
 node *p, *b, *s1, *s2, *t;
 p = head;
 b = NULL;
 s1 = NULL;
 s2 = head;

 while (p)
 {
  t = p;
  p = p->next;
  if (t)
   t->next = b;
  b = t;

  if (i == k)
  {
   if (s1)
    s1->next = b;
   else
    head = b;
   s1 = s2;
   s2 = p;
   i = 0;
  }
  i++;
 }
 s1->next = b;
 s2->next = NULL;
 return head;
}

void print(node *head)
{
 node *p = head;
 while (p)
 {
  printf("%d -> ", p->v);
  p = p->next;
 }
 printf("$\n");
}

void insert(node **t, int v)
{
 node *p = malloc(sizeof(node));
 p->v = v;
 p->next = NULL;
 (*t)->next = p;
 *t = p;
}

int main(int argc, char *argv[])
{
 node *head, *tail = NULL;
 head = malloc(sizeof(node));
 head->next = NULL;
 head->v = 1;
 tail = head;
 insert(&tail, 2); 
 insert(&tail, 3); 
 insert(&tail, 4); 
 insert(&tail, 5); 
 insert(&tail, 6); 
 insert(&tail, 7); 
 insert(&tail, 8);

 print(head);

 head = revk(head, 3);

 print(head);

 tail = head;

 while (tail)
 {
  head = tail;
  tail = tail->next;
  free(head);
 }

 printf("\n");
 return 0;
}