Is it always a good practice to set pointers to NULL after free()-ing them? [duplicate]

Question

Possible Duplicate:
Setting variable to NULL after free …

I am learning about good C programming practices and my friend told me to always set the pointers to NULL after free()ing them (or calling a specific freeing function).

For example:

char* ptr = malloc(100);
...
free(ptr);
ptr = NULL;

or

struct graph* graph = create_graph();
...
destroy_graph(graph);
graph = NULL;

Why is this a good practice?

Update: After reading the answers, it seems an awful practice to me! I am hiding possible double-free() errors. How can this possibly be a good practice? I am shocked.

Thanks, Boda Cydo.

Answer 1

While it can't hurt, it doesn't always help. The issue to consider is that it's easy for there to be multiple copies of the pointer and most likely you are only going to set a single one to NULL. The classic example of where it doesn't help at all is:

void free_graph(graph *g)
{
    ...
    free(g);
    g = NULL;  // not useful in this context
}

The problem here is that you are only setting the pointer that is local to free_graph to NULL and the pointer is held by the caller of free_graph will still have it's original value.