I suffer from constiaption. Always have, since I was a little kid. I know it's common for a constipated person to pass poop that has 'surface cracks' in it. Because it's so hard. But from what you are describing, it sounds like your poop looks like swiss cheese.
If you are concerned about colon cancer, I would suggest you go to your doctor ASAP. But it could be other things as well. All of which your doctor may be able to help with. How is your diet? Exercise Routine? Do you take anything for your constipation? like laxatives or stool softeners?
I know that my constipation symptoms were pretty much solved with changing my diet to more fruits & veggies, salad bars, whole wheat, and very little red meat. I drink strictly water and iced tea during the day. I also exercise more too. Mainly just light cardio, like walking a mile everyday. I also take a colon cleanser called Oxypowder , which helps regulate my constipation. I would recommend a good colon cleansing so that you can flush out some of the impacted fecal matter in there, as well as any parasites that may be in there. This will help with the constipation.
But I'm no expert on any of this. I can only tell you what worked best for me. Go to your doctor and have him run some tests, just to make sure you don't have something more serious then chronic constipation...