Thread: Malloc problem

Hi, I'm getting a crash on the routine on a device and i'm not sure why:

I have the following structures:

Code:

struct NodeSlotFrame
{
  word addr;
  byte slot;
  byte frame;
  struct NodeSlotFrame* next;
};

typedef struct
{
  struct NodeSlotFrame own;
  byte nrOfNeighbours;
  struct NodeSlotFrame neighbours;
  byte nrOfHiddenNodes;
  struct NodeSlotFrame hiddenNodes;
}INF;

And in a function i try to have dynamic memory allocation:

Code:

word extractINF(byte* src, INF* dest)
{
...
  pNode = &dest->neighbours
  //Get the remote neighbourhood's information
   for(i = 0;i < dest->nrOfNeighbours;i++)
   {
	//Allocate memory for neighbours table
	pNode->next = (struct NodeSlotFrame *) malloc(sizeof(struct NodeSlotFrame));
        ...		
	pNode = pNode->next;
   }
...
}

I can't debug the system so it's hard for me to know what's exactly going on. Any ideas ?

Thanks a million

Inderjit

This is not the right way to implement a linked list. The neighbours field of the INF struct should be a pointer, not a real structure. It points to the head of the list.

Furthermore, I do not see the code that makes sure the last node of the list has a NULL next pointer. So the crash probably comes when you try to free this thing. It could be crashing at the head (which isn't dynamically allocated) or the tail (which is uninitialized). Or you could be walking off the end of the list somewhere, again, because you didn't terminate the list with a NULL next pointer.

Hi again,

I changed the structure to this:

Code:

typedef struct
{
  word addr;
  byte slot;
  byte frame;
}NodeSlotFrame;

struct NSFList
{
  NodeSlotFrame nsf;
  struct NSFList *next;
};


typedef struct
{
  NodeSlotFrame own;
  byte nrOfNeighbours;
  struct NSFList *neighbours;
  byte nrOfHiddenNodes;
  struct NSFList *hiddenNodes;
}INF;

And then I allocate new node like this:

Code:

void myfunction()
{
        struct NSFList* neighbour = dat.inf.neighbours;

        if(dat.inf.nrOfNeighbours == 0)
	{
		dat.inf.neighbours = (struct NSFList*) malloc(sizeof(struct NSFList));
		dat.inf.neighbours->next = NULL; //first node

		//Keep the information of new node
		memcpy(&dat.inf.neighbours->nsf, &rDat.inf.own, sizeof(NodeSlotFrame));

		//Increase the count
		dat.inf.nrOfNeighbours++;

		return;
	}

        //SEARCH THE LIST
        while (neighbour->next != NULL)
        {
              ...
        }


        if(newNeighbour == TRUE)
	{
		//Memeory for new neighbour
		neighbour->next = (struct NSFList*) malloc(sizeof(struct NSFList));
		neighbour->next->next = NULL; //This is the last node

		//Add to the list
		memcpy(&neighbour->next->nsf, &rDat.inf.own, sizeof(NodeSlotFrame));
		
		//Increase the count
		dat.inf.nrOfNeighbours++;
	}
}

I am assuming that this should be correct. if not, please let me know. I have done link lists many times but it's always confusing when it's done in c or c++. I also problem debugging the algorithm in KDevelop with g++ (Which is not the compiler for the actual device) and am not sure where the problem lies. I wish do debug it directly for the device but that is just not possible at the moment...

I can't pass parameters I'm afraid so please accept the there are no functions for adding/removing items. I'm working in NesC, a flavor of C for Wireless Sensor Networks and it doesn't 'allow' passing parameters directly.

Thanks again

Inderjit

Thanks for the code. Looks a lot better. Regarding NesC. You're right, it is possible but only if you create an component. The reason goes in basic implementation of a nesc/tinyos design. Creating a real-time app on tinyos with nesc, it is recommended to create a job split into many tasks for avoidance in blocking the system.

Inderjit