Valgrind "Invalid write of size n" when using malloc

Question

I'm creating a graph using this structures

struct node
{
  int id;
  struct node *ad;
};

struct graph
{
  int numVert;
  struct node **adjList;
  int *visited;
  int *back;
};

And using this to allocate the memory

static struct graph *graph_create(struct world *W, int v)
{
  struct graph *G = NULL;

  if ((G = graph_alloc(v)) != NULL)
  {
    /* some stuff */
  }

  return G;
}

static struct graph *graph_alloc(int v)
{
  struct graph *G = NULL;
  int i;
  struct node **newNode = NULL;

  if ((G = (struct graph *)malloc(sizeof(struct graph *))) != NULL)
  {
    G->numVert = v;
    G->adjList = (struct node **)malloc(v * sizeof(struct node *));
  }

  /* other stuff */

  return G;
}

But valgrind show me this when i compile using -s --leak-check=full --show-leak-kinds=all and i don't know why

==65390== Invalid write of size 8
==65390==    at 0x10985D: graph_alloc (main.c:299)
==65390==    by 0x109801: graph_create (main.c:283)
==65390==    by 0x10925F: main (main.c:75)
==65390==  Address 0x4a429c8 is 0 bytes after a block of size 8 alloc'd
==65390==    at 0x483F7B5: malloc (vg_replace_malloc.c:381)
==65390==    by 0x109830: graph_alloc (main.c:296)
==65390==    by 0x109801: graph_create (main.c:283)
==65390==    by 0x10925F: main (main.c:75)

I think the problem is that row becouse without it there is no error

G->adjList = (struct node **)malloc(v * sizeof(struct node *));

Answer 1

When you try to allocate memory for your graph structure here:

struct graph *G = NULL;

G = (struct graph *) malloc(sizeof(struct graph *));

you allocate memory only for a pointer to the structure, but you need to allocate memory for the thing the pointer points at:

struct graph *G = malloc(sizeof(*G));

You could also specify the size of the type:

struct graph *G = malloc(sizeof(struct graph));

but I like the idiom Type *p = malloc(sizeof(*p)) . The pointer you get from malloc is a handle to the memory of the given size you have allocated. It's not the size of the pointer p that matters, but the size of what it points to, namely *p .

In your case the graph struct is made up of three pointers and an integers, so the graph struct requires more memory than a pointer to the graph struct. That is what causes the invalid writes.

Two observations:

• In C, the cast to (struct graph *) is not necessary, because void * can be converted to a pointer to something else.

• I find doing both the assignment and the null check in the if clause clutters the code and makes it hard to see what's going on. Instead of

  struct graph *G = NULL; if ((G = malloc(sizeof(*G))) != NULL) ...

  I'd prefer:

   struct graph *G = malloc(sizeof(*G)); if (G != NULL) ...

  which separates the memory allocation from the test for success. It also has way fewer parentheses. :) (That's a matter of personal taste perhaps, but my impression is that beginners are very fond of these complicated constructions. Keep it simple.)