sorting a singly linked list

Hello,
I am trying to sort a singly linked list for the following typedef
typedef struct message {
int messageId;
char * messageText;
struct message * next;
} message;
I have successfully implemented add, delete, print, and search functions for the linked list but I am not sure why I am stuck with this one.


I posted my code below. I get a BUS error in the second iteration of the while loop. I am quite new to linked list so I will appreciate any explanation.

Thank you

Code:

	if(first != NULL && first->next != NULL) /* less than two nodes */
	{		
		message *a = first;
		message *b = first->next;
		message *beforeA =  NULL;

		while(b != NULL)
		{
			if(a->messageId > b->messageId)
			{
				a->next = b->next;
				b->next = a;
				if(a == first && beforeA == NULL)
				{
					first = b;
					beforeA = first;
				}
				else if (beforeA != NULL)
				{
					beforeA->next = b;
				}
				/* Move to the next couple of nodes */
				b = a->next;
				beforeA = beforeA->next;
			}else if (a->messageId < b->messageId)
			{

				a = a->next;
				b = b->next;
			}
		}
	}

You didn't advance beforeA when a->messageId < b->messageId, so when you fo to the second iteration beforeA is NULL and doesn't get updated because a is not first anymore. So at the line where beforeA = beforeA->next, you access NULL->next and get a bus error.

Proposed fix:

Code:

			}else if (a->messageId < b->messageId)
			{
                                beforeA = a;
				a = a->next;
				b = b->next;
			}

Thank you for your reply. That was indeed one of the problems but I still have a Bus error.
I am sure that the error is in the conditional statement. (After some debugging I guess this is the problem.)

Code:

			if(a->messageId > b->messageId)
			{
				a->next = b->next;
				b->next = a;
				if(a == first && beforeA == NULL)
				{
					first = b;
					beforeA = first;
				}
				else if (beforeA != NULL)
				{
					beforeA->next = b;
				}
				/* Move to the next couple of nodes */
				b = a->next;
				beforeA = beforeA->next;
			}