Double pointers and linked lists.

simo_r · 05-05-2008, 09:59 AM

I'm creating an a* algorithm implementation using two double linked lists.

p_node is the node being copied. ppDest is the tail node and ppDestlist_h is the head node.

I'm not entirely sure about my double pointers and I might just be making a schoolboy error here. I only learnt about their use yesterday, and have converted my code to use it instead of returning pointers to the nodes.

Code:

int addtolist(p_node *node, p_node **ppDest, p_node *ppDestlist_h)
{ 

  if(( node == NULL ))
  {
       printf("Node pointer is null!\n");
       return 1;
  }

  /*edit* /
  ppDest = &(*ppDestlist_h);
  
  /*double pointers here are used to keep going forward until null is hit*/
  /* if null is hit immediately then copy the node right away*/
  while(*ppDest != NULL)
   { ppDest = &(*ppDest)->next;}
   
   memcpy(&(*ppDest), &node, sizeof(p_node));
   
   printf("node inside func: %d Current:  \n", (*ppDest)->x); 
  
  /*now I set the next value of the node to NULL*/
  
  (*ppDest)->next = NULL;
  
  return;
  
}

This seems like it should work, but doesnt want to.

has anyone got any pointers? (ouch that was bad)

thanks,

-Simo

vart · 05-05-2008, 10:23 AM

It took me some time to read this horrible code...

Code:

memcpy(&(*ppDest), &node, sizeof(p_node));

&(*ppDest) is just ppDest - pointer to pointer
&node - is pointer to pointer as well
so
*ppDest = node; should be enough...

instead of it you are trying to copy other the pointer the whole p_node structure of bytes (from location that does not contains enogh info) overwriting all memory after ppDest ...

It does not seems as it should work...

simo_r · 05-06-2008, 04:25 AM

Quote:

Originally Posted by vart

It took me some time to read this horrible code...

Code:

memcpy(&(*ppDest), &node, sizeof(p_node));

&(*ppDest) is just ppDest - pointer to pointer
&node - is pointer to pointer as well
so
*ppDest = node; should be enough...

instead of it you are trying to copy other the pointer the whole p_node structure of bytes (from location that does not contains enogh info) overwriting all memory after ppDest ...

It does not seems as it should work...

heh yeah you were right. I really had no idea what I was doing using memcpy. I rewrote it today and just copied the data over and It worked on the first compile. heh.

Thanks

05-05-2008, 09:59 AM	#1
simo_r Registered User Join Date: May 2008 Posts: 2	Double pointers and linked lists. I'm creating an a* algorithm implementation using two double linked lists. p_node is the node being copied. ppDest is the tail node and ppDestlist_h is the head node. I'm not entirely sure about my double pointers and I might just be making a schoolboy error here. I only learnt about their use yesterday, and have converted my code to use it instead of returning pointers to the nodes. Code: int addtolist(p_node node, p_node ppDest, p_node ppDestlist_h) { if(( node == NULL )) { printf("Node pointer is null!\n"); return 1; } /edit / ppDest = &(ppDestlist_h); /double pointers here are used to keep going forward until null is hit/ / if null is hit immediately then copy the node right away/ while(ppDest != NULL) { ppDest = &(ppDest)->next;} memcpy(&(ppDest), &node, sizeof(p_node)); printf("node inside func: %d Current: \n", (ppDest)->x); /now I set the next value of the node to NULL/ (ppDest)->next = NULL; return; } This seems like it should work, but doesnt want to. has anyone got any pointers? (ouch that was bad) thanks, -Simo Last edited by simo_r; 05-05-2008 at 10:26 AM. Reason: clarification
simo_r is offline

05-05-2008, 10:23 AM	#2
vart CSharpener Join Date: Oct 2006 Posts: 5,242	It took me some time to read this horrible code... Code: memcpy(&(ppDest), &node, sizeof(p_node)); &(ppDest) is just ppDest - pointer to pointer &node - is pointer to pointer as well so *ppDest = node; should be enough... instead of it you are trying to copy other the pointer the whole p_node structure of bytes (from location that does not contains enogh info) overwriting all memory after ppDest ... It does not seems as it should work... __________________ If I have eight hours for cutting wood, I spend six sharpening my axe.
vart is offline

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