realloc and segfaulting problem

SciYro · 05-30-2005, 04:42 AM

ok,

Code:

void tree_add_child (parent, child)
	tree *parent;
	tree *child;
{

		int i;

		parent->number_of_children++;

		// segfault in the line below
		parent->children = (tree **) realloc (parent->children, sizeof (tree * [parent->number_of_children]);
		if (parent->children == NULL) exit (-1);

		parent->children[parent->number_of_children] = child;

}

ok, the function is simple, each struct "tree" has a list of children, its a pointer to a array of pointers to other struct "tree"s, this should increase the number of children, then use realloc to make a extra space, then insert the pointer at the end.

Code:

 typedef struct _tree {
   int number_of_children;
   _tree **children;
} tree;

the main function inits multiple tree struct, with the children set to NULL, and number to 0. Then this function is called to add children to 1 tree struct, it works fine for the first 3, but on the forth it segfaults at the realloc line, i been trying for days but cant seem to find out why, or make it go away. any ideas on why its doing this?, thanks.

itsme86 · 05-30-2005, 06:38 AM

You're incrementing parent->number_of_children and then (skip a few lines) you're using parent->number_of_children as the index to set the child. So in essence you're completely skipping parent->children[0]. The first child is set to parent->children[1]. The problem here is that you've only reserved room for 1 child in your realloc() line so parent->children[1] on the first added child is out of bounds.

To fix this, don't increment parent->number_of_children until after using the original value as the index for added child. And in the realloc() line, allocate sizeof(tree *) * (parent->number_of_children+1) bytes.

SciYro · 05-30-2005, 08:46 AM

i see,
thanks itsme86, i changed the needed parts and all is well.