Rotate a potentially large string about a pivot point

gistfile1.c

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

void reverse(char *data, int start, int end)
{
  int i; char tmp;

  for (i=0; i<(end-start)/2; i++)
    {
      tmp = data[i+start];
      data[i+start] = data[end-i-1];
      data[end-i-1] = tmp;
    }
}

void rotate(char *data, int pivot)
{
  int i, j, l, f;
  char tmp;

  l = strlen(data);
  printf("length = %d\n", l);
  for (i=0; i<pivot; i++)
    {
      tmp = data[i];
      f = i;
      j = i + pivot;
      while (j<=l-pivot+i)
        {
          if (i == 0 ) printf("f= %d, j = %d\n", f, j);
          data[f] = data[j];
          f = j;
          j += pivot;
        }
      data[f] = tmp;
      printf("%s.\n", data);
    }
}

int main(int argc, char **argv)
{
  char data[] = "The quick brown fox jumps over the lazy dog.";

  /* test solution #1 */
  reverse(data, 0, 4);
  reverse(data, 4, strlen(data));
  reverse(data, 0, strlen(data));
  printf("test 1 = '%s'\n", data);  /* should be 'quick brown fox jumps over the lazy dog.The ' */

  /* test solution #2 */
  rotate(data, 6);
  printf("test 2 = '%s'\n", data);  /* should be 'quick brown fox jumps over the lazy dog.The quick ' */

  return 0;
}