const *const pointers in function parameters

Question

I have a question about best practices involving pointers in function parameters and whether they should be specified as *const or const *const. I know there are varying opinions on use, or excessive use, of const, but at least some use is a good way to catch accidental mistakes.

Say you have the following function, which is part of the implementation of a queue based on a linked list:

void enqueue(struct queue *q, void *data)
{
    struct queue_node *item = malloc(sizeof(*item));
    item->data = data;

    [add node to linked list, specifics are not relevant here]
}

struct queue_node is defined as

struct queue_node {
        struct queue_node *next;
        void *data;
};

data is not meant to be modified in this function, so it seems like a good idea for data to be of type void *const so you can't accidentally do something like data = item->data instead of item->data = data.

The next level would be to define data as const void *const which means you also can't change what data points to. However, this leads to a warning about discarding the const qualifier from the pointer. And if I understand correctly, further use of such a pointer is considered undefined behavior. Using a cast (item->data = (void *)data) seems clunky.

What's the best approach here? Is const void *const unnecessarily over-protective? It feels like void *const is sufficient to catch most errors.

If item->data has type void *, I think it makes sense the the data parameter to be void * or void * const even though the function does not modify the object pointed to by data. I would only use const void * data or const void * const data if item->data had type const void *. — Ian Abbott
– Ian Abbott, Commented May 12, 2021 at 15:19

dbush · Accepted Answer · 2021-05-12 15:19:38Z

2

Since changes to function parameters aren't reflected in the calling function, there's little reason to make sure the parameters themselves are read-only. It is useful however to ensure that what a pointer parameter points to doesn't change. So if you don't want what data points to to be changed, you would change the function signature to:

void enqueue(struct queue *q, const void *data)

Note that this will work only if the data member of struct queue_node has type const void *. If it doesn't, then you don't want to use const on the data parameter.

answered May 12, 2021 at 15:19

dbush

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3 Comments

sj95126 Over a year ago

(original post edited to include struct definition) data is type void * so later retrieval can be cast-less and the object pointed to can be modified.

dbush Over a year ago

@sj95126 That means the function doesn't guarantee data won't be modified and therefore the parameter should not be const.

sj95126 Over a year ago

OK, thanks. It sounds like the solution is to just stick with void *. This won't protect against something like data = item->data instead of item->data = data but the effect of that should be obvious when there's an item in the queue with missing or corrupt data.

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