Question

I’m trying to create a struct that can hold generic values. The code below works but I’m getting a compiler warning about the cast from pointer to integer. This is on a 64-bit system.

struct node
{
    void *key;
    void *value;
};

void insert(struct node *ht, void *key, void *value)
{
    ht->key = key;
    ht->value = value;
    return;
}

int main()
{
    struct node *t = (struct node *)malloc(sizeof(struct node));
    insert(t, (void *)3, (void *)5);
    printf("[%d]->[%d]\n", (int)t->key,(int)t->value);
    free(t);
    return 0;
}

I’m not even sure if this is the correct way to go about it. I sort of hacked it up. Please let me know if there is a proper way to do this.

Answer 1

The compiler tries to warn you that you lose bits when casting from void * to int. It doesn’t know that the void * is actually an int cast, so the lost bits are meaningless.

A double cast would solve this (int)(uintptr_t)t->key.
It first casts void * to uintptr_t (same size, no warning), then uintptr_t to int (number to number, no warning).
You need to include <stdint.h> to have the uintptr_t type (an integral type with the same size as a pointer).