Problems with pointers and malloc()

This is a discussion on Problems with pointers and malloc() within the C Programming forums, part of the General Programming Boards category; Hi there, I'm having problems with dynamic memory management and pointers. I guess the problem has got something to do ...

  1. #1
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    Unhappy Problems with pointers and malloc()

    Hi there,

    I'm having problems with dynamic memory management and pointers. I guess the problem has got something to do with the fact, that I don't want to malloc() (and check the result with if...) many times in my program. So I decided to create a function that gets the pointer, to where it is supposed to malloc() memory and write data.
    Btw, the program is supposed to do a depth first search on an 8-puzzle problem...

    Here the most important parts of the code, with the two problematic parts marked. Sorry for posting so much code, but I couldn't shorten it even more without losing the context.

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    
    int depth_max;
    
    typedef struct node {
    	int value[3][3];
    	struct node *next;
    	struct node *prev;
    } node;
    
    void printNode(node *k) { ... }
    
    int equal(int value1[3][3], int value2[3][3]) { ... }
    
    /**
    * Appends node2 after node1 (doubly-linked list)
    */
    void appendToList(node *k1, node *k2) {
    	printf("appendToList() entering ...\n");
    	k1->next = k2;
    	k2->prev = k1;
    	return;
    }
    
    /**
    * Creates a node which is similar to node k. Allocates memory for node "new"
    */
    void newSuccessor(node *k, node *new, int x1, int y1, int x2, int y2) {
    	node *tempNode;
    	if ((tempNode = (node *)malloc(sizeof(node))) == 0) {
    		printf("No more memory left!\n");
    		return;
    	}
    	*new = *tempNode; // !!! Error 1
    	memcpy(new->value, k->value, sizeof(k->value));
    	...
    	new->prev = NULL;
    	new->next = NULL;
    	return;
    }
    
    /**
    * Creates a new doubly-linked list with the successors of node k.
    * Allocates memory for node "first" (the first node of the list)
    */
    void successors(node *k, node *first) {
    	node *tmp1, *tmp2;
    	...
    	newSuccessor(k, start, 0,0, 0,1);
    	newSuccessor(k, tmp1, 0,0, 1,0);
    	appendToList(start, tmp1);
    	return;
    }
    
    int depthFirstSearch(node *k, int target[3][3], int depth) {
    	...
    	node *firstNode, *tempNode;
    
    	successors(k, firstNode);
    	
    	while (firstNode != NULL) {
    		
    		int res = depthFirstSearch(firstNode, target, depth+1);
    		if (res == 1) {
    			// Target was found
    			return erg;
    		}
    		*tempNode = *firstNode->next;
    		free(firstNode); // !!! Error 2
    		*firstNode = *tempNode;
    	}
    }
    
    int main(int argc, char* argv[]) {	
    	
    	depth_max = 3;
    	
    	int target[3][3];
    	...
    	node *k = (node *) malloc(sizeof(node));
    	...
    	depthFirstSearch(k, target, 0);
    }
    Error 2: free(firstNode);

    Here, the computer gives me this error:

    9223: symbol=free; lookup in file=./depthfirstsearch [0]
    9223: symbol=free; lookup in file=/lib/tls/i686/cmov/libc.so.6 [0]
    *** glibc detected *** ./depthfirstsearch: free(): invalid pointer: 0xb7fb7ff4 ***
    Segmentation fault (core dumped)
    Well, so I marked this line as a comment

    Error 1: *new = *tempNode;

    This produces after some tries (not at the first try) this message:

    Segmentation fault (core dumped)
    I guess there must be something wrong with the pointers. FYI I ran the program in Ubuntu linux 7.04.
    Any help would be greatly appreciated.

  2. #2
    Dr Dipshi++ mike_g's Avatar
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    I think you want to be doing:
    Code:
    new = tempNode;

  3. #3
    cas
    cas is offline
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    It appears that you have a small amount of confusion about passing pointers into functions. If you pass a pointer to a function, you cannot change where that pointer points. That is, even if you make it point somewhere new in your function, it won't be pointing there after the function returns:
    Code:
    #include <stdlib.h>
    #include <stdio.h>
    void f(void *x)
    {
      static int y;
      x = &y;
    }
    int main(void)
    {
      void *x = NULL;
      f(x);
      printf("&#37;p\n", x);
    }
    This will print out "null" (or whatever your printf() does with a null pointer). It doesn't matter that inside of f() you pointed it somewhere else; inside of f(), all you have is a copy of the pointer. You can manipulate the data the original pointer points to, but not where it points.

    There are a couple options: Pass in a pointer to the pointer:
    Code:
    void f(void **x)
    {
      static int y;
      *x = &y;
    }
    ..
    f(&x);
    Or you can return a pointer:
    Code:
    void *f(void)
    {
      static int x;
      return &x;
    }
    ..
    x = f();
    On an unrelated note, there's no need to cast malloc() in C. It's not wrong, per se, but it's superfluous. void* is special; it can be converted automatically to any object pointer type (and vice versa).
    Last edited by cas; 10-27-2007 at 06:56 PM. Reason: Fixed a slightly ambiguous wording.

  4. #4
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    Lightbulb

    Hi cas,

    thanks a lot for your help! That was indeed the mistake.

    Oh, and you're right, malloc() without a cast doesn't even produce a warning

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