Problem with the function creating updating a linked list

nkhambal · #1 28-Oct-2004, 17:08

Hi all,
I have this function,which builds dynamic linked list from the data received in connections. It complies fine. But when I build the first node in the list, the next pointer is somehow now getting assigned to NULL, which causes my while loops where in I check for NULL value, go on indefinitely. Following my function. I guess it must be something simple that i am missing but just cant figure out where. Pls help. I have sprinkled lots of printf statment in function for debugging purpose.

CPP / C++ / C Code:

boolean push_to_conn_queue(_conn_q_ptr **conn_q_ptr, _ppacket qpacket, _conn_q **queue)
{
				/*
                                                   typedef struct connq  _conn_q;
				struct connq
				{	
					short conn_id;
					_command command;
					long src_session_id;
					long dst_session_id;
					_conn_q *next;
				};

				struct connqptr
				{
					_conn_q *head;
					_conn_q *tail;
				};

								                                      typedef struct connqptr  _conn_q_ptr;
				*/
	
	static short conn_id=1;
	_conn_q *temp=NULL;

	printf("\nqueue: %d\n",conn_id);

	if ((*queue)==NULL)
	{	
		temp=(_conn_q *)malloc(sizeof (_conn_q *));
		if (temp==NULL)
		{
			fprintf(stderr,"%s(): Malloc Failed\n",__FUNCTION__);
			return FALSE;
		}
		(*conn_q_ptr)=(_conn_q_ptr *)malloc(sizeof (_conn_q_ptr *));
		if (*conn_q_ptr==NULL)
		{
			fprintf(stderr,"%s(): Malloc Failed\n",__FUNCTION__);
			return FALSE;
		}
		fprintf(stderr,"%s(): In If making temp node\n",__FUNCTION__);
		temp->conn_id=conn_id;
		temp->command=qpacket.packet.command;
		temp->src_session_id=qpacket.packet.src_session_id;
		temp->dst_session_id=qpacket.packet.dst_session_id;
		temp->next=NULL;
		fprintf(stderr,"%s(): Finished making temp node\n",__FUNCTION__);
		(*queue)=temp;
		(*conn_q_ptr)->head=temp;
		(*conn_q_ptr)->tail=temp;
		fprintf(stderr,"%s(): Created head and tail of queue\n",__FUNCTION__);
		printf("\nConnection %d queued\n",conn_id);
		conn_id++;
		return TRUE;
	} else {
		temp=(_conn_q *)malloc(sizeof (_conn_q *));
		if (temp==NULL)
		{
			printf("\n%s(): Malloc Failed\n",__FUNCTION__);
			return FALSE;
		}
		fprintf(stderr,"%s(): In else creating temp node\n",__FUNCTION__);
		temp->conn_id=conn_id;
		temp->command=qpacket.packet.command;
		temp->src_session_id=qpacket.packet.src_session_id;
		temp->dst_session_id=qpacket.packet.dst_session_id;
		temp->next=NULL;
		fprintf(stderr,"\n%s(): In else finished creating temp node\n",__FUNCTION__);
		int i=0;
		while ((*queue)->next!=NULL)  //looping starts here. 
		{
			(*queue)=(*queue)->next;
			printf("i= %d\n",++i);
			continue;
		}
		(*queue)->next=temp;
		(*conn_q_ptr)->tail=temp;
		fprintf(stderr,"%s(): In else changed tail\n",__FUNCTION__);
		printf("\nConnection %d queued\n",conn_id);
		conn_id++;
		return TRUE;
	}
	return TRUE;
}

this how I am defining queue and queue pointer variables in the parent function and calling this function.

CPP / C++ / C Code:

boolean create_open_command_channel(int *tx_qid,int *rx_qid,int *cmd_ch_proc_id,struct process_stack **proc_stack)
{
_conn_q *conn_q=NULL;
_conn_q_ptr *conn_q_ptr=NULL;

*cmd_ch_proc_id=fork();
		if (*cmd_ch_proc_id==0)
		{
		//printf("\nchild proc_id: %d\n",getpid());
		

		for (; ; )
		{
			if ((bytes_rcvd=msgrcv(*rx_qid,&rpacket,sizeof(_packet),1,IPC_NOWAIT))==-1)
			{
				msgctl((*rx_qid, IPC_RMID, NULL);
			} else {
				
					if (bytes_rcvd>0)
					{
							printf("\nbytes received: %d\n",bytes_rcvd);
							printf("\nConnection received.\n");
							if ((result=push_to_conn_queue (&conn_q_ptr,rpacket,&conn_q))==TRUE)
							{
								printf("\nConnection pushed onto queue\n");
							} else {
								printf("\nFailed push connection on to thequeue\n");
							} 

....rest of the code for function

Any suggestions ?

Thanks,

davekw7x · #2 28-Oct-2004, 18:30

Quote:

Originally Posted by nkhambal

Hi all,
I

Any suggestions ?

Thanks,

I haven't looked at everything, but I don't think you are malloc'ing what you need.

For example, you are getting storage for the struct, but you have

CPP / C++ / C Code:

 temp=(_conn_q *)malloc(sizeof (_conn_q *));

Shouldn't this be

CPP / C++ / C Code:

 temp=(_conn_q *)malloc(sizeof (_conn_q));

???

Look at your other malloc() stuff also.

Regards,

Dave

nkhambal · #3 28-Oct-2004, 20:26

Thanks dave,

That was the problem, I guess? I got little confused there with pointer to pointer convension that I didn't realize it. However I still wonder how it worked for first and second call to the function. I tried printing queue in the parent function and it kept printing the first connection data cause NULL was never hit. I guess, i was just lucky not to see a crash.

Thanks,

davekw7x · #4 28-Oct-2004, 20:45

Quote:

Originally Posted by nkhambal

Thanks dave,

That was the problem, I guess? I got little confused there with pointer to pointer convension that I didn't realize it. However I still wonder how it worked for first and second call to the function. I tried printing queue in the parent function and it kept printing the first connection data cause NULL was never hit. I guess, i was just lucky not to see a crash.

Thanks,

When a pointer points outside of its allocated area, it points somewhere. If it happens to point to some other allocated area, things may seem to be OK --- until the rightful owner of that area uses it (or tries to use it). Then all heck breaks loose --- sometimes.

Sometimes everything seems to be OK, but then at some later date it falls apart. Sometimes everything seems to work OK with a given set of test data, but then blows up when actual user data is encountered.

In your case you were lucky to have it misbehave with a fairly small test.

The bottom line: test, test, test, but remember that you can't prove a program is correct just by testing.

Good Luck, and

Best regards,

Dave

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