valgrind showing memory leaks. how do i stop the leak?

Question

I'm new to valgrind and I ran it with some code I wrote for a quadtree.

I've written a function that recursively frees the Nodes from the quadtree:

void destroyTree (Node* p)
{
    if (!p) return;

    for (size_t i = 0; i < 4; ++i)
        destroyTree(p->child[i]);

    free(p);
}

I call that function in the main function:

int main( int argc, char **argv ) {

  Node *head;

  // make the head node
  head = makeNode( 0.0,0.0, 0 );

  // make a tree
  makeChildren( head );
  makeChildren( head->child[0] );
  makeChildren( head->child[1] );
  makeChildren( head->child[2] );
  makeChildren( head->child[3] );

  // print the tree for Gnuplot
    writeTree( head );
  return 0;

  //destroy tree
  destroyTree (head);
  return 0;
}

When I run valgrind its showing that I lose some memory.

The structure is:

struct qnode {
  int level;
  double xy[2];
  struct qnode *child[4];
};
typedef struct qnode Node;

I call malloc in buildTree:

Node *makeNode( double x, double y, int level ) {

  int i;

  Node *node = (Node *)malloc(sizeof(Node));

  node->level = level;

  node->xy[0] = x;
  node->xy[1] = y;

  for( i=0;i<4;++i )
    node->child[i] = NULL;

  return node;
}

How do I stop the leak? Is there something wrong with my freeing function or is it somewhere else?

It says a `malloc` call in `makenode` is unfreed. You should share that function. — StoryTeller - Unslander Monica, Feb 23 '19 at 20:46
you're using `return 0` in the middle of your code so destroy function isn't called!! please enable warnings — Jean-François Fabre, Feb 23 '19 at 20:52
In `makenode`, you have a member variable called `xy` that appears to be either an array or pointer. How are you allocating space for that, and likewise, how are you freeing it? Before you free a node, you also have to free any pointers inside of it, first. — Alex Reynolds, Feb 23 '19 at 20:54
@Jean-FrançoisFabre I tried removing the `return 0` but that didn't make a difference — anon2000, Feb 23 '19 at 20:55
there are 2 return statements. Then please remove the one in the middle, that should fix it. Else [edit] your question — Jean-François Fabre, Feb 23 '19 at 20:55

Jean-François Fabre · Accepted Answer · 2019-02-23T21:34:07.050

1

okay, since noone spotted this, the end of main reads:

  // print the tree for Gnuplot
    writeTree( head );
  return 0;    // <----------- wooooot!

  //destroy tree
  destroyTree (head);
  return 0;
}

We notice that the 2 last lines are unreacheable because there's a return 0 statement just above this. So the destroyTree method is never called, which explains the memory leak.

Always enable compiler warnings and read them. The control flow is simple enough, every compiler detects dead code here.

That said, even -Wall -Wextra doesn't work here, the -Wunreachable-code flag must be added explicitly (Why does GCC not warn for unreachable code?)

edited Feb 23 '19 at 21:34

answered Feb 23 '19 at 20:54

Jean-François Fabre

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How do I enable compiler warnings? – anon2000 Feb 23 '19 at 20:59
1

`-Wall -Wextra -pedantic`. In that case, surprisingly it doesn't work!! – Jean-François Fabre Feb 23 '19 at 21:02
Is it okay to keep the `return 0` underneath `destroyTree (head);` ? – anon2000 Feb 23 '19 at 21:03
yes. C99 now considers that implied, but you can leave it. – Jean-François Fabre Feb 23 '19 at 21:04

valgrind showing memory leaks. how do i stop the leak?

1 Answers1