I tried to use recursion here but it does not seem to free all the nodes.
The current node (a
) is free
d only if its a->higher
and a->lower
are NULL
. Since they are not set to NULL
anywhere, a
will not be free
d.
Instead a
should always be free
d as long as it is itself not NULL
.
void free_nodes(node*a) {
if (a == NULL) {
return; // nothing to do
}
free_nodes(a->higher); // no need to check for `NULL` here, it will be done in the recursive call
free_nodes(a->lower); // no need to check for `NULL` here, it will be done in the recursive call
free(a); // we know that a is not NULL since it was checked in the beggining
}
Note that none of the pointers are actually set to NULL
after free
ing them. For the sub nodes it does not really matter. But the root node should be set to NULL
by the caller after calling free_nodes
with it.
Alternatively you can change free_nodes
to accept a node**a
(pointer to pointer) and set it to NULL
inside it. This will require using *a
all over, which is a bit more cumbersome, but from a design point of view it has the advantage that it guarantees a pointer will not remain non-NULL after it was free
d, so there's no risk of double-free.
A final note:
The body of free_nodes
can also be written equivalently as:
if (a) { // the same as `if (a != NULL)`
free_nodes(a->higher); // no need to check for `NULL` here, it will be done in the recursive call
free_nodes(a->lower); // no need to check for `NULL` here, it will be done in the recursive call
free(a); // we know that a is not NULL since it was checked in the beggining
}
Which style is better is a matter of personal preference (I personally prefer the first version, when the edge case is handled first and the rest is unconditional).