access element of struct passed into a void* pointer

Question

I\'m working with a binary search tree data structure to sort a series of structs with the type definitions:

typedef struct {
    char c;
    int index;
} da         


        

Answer 1

Sure it's possible, you'd cast key as type *data_t. (As long as that's really what key points to!)

key                     /* argument of type void* */
(data_t*)key            /* cast as type data_t*   */
((data_t*)key)->index   /* dereferenced */

Here is a simple example:

#include 
#include 

typedef struct {
    char    c;
    int     index;
} data_t;

typedef struct node {
    void    *data;
    struct node *left;
    struct node *right;
} node_t;

static int cmp(void *lhs, void *rhs)
{
    return ((data_t *)lhs)->index - ((data_t *)rhs)->index;
}

int main(void)
{
    data_t d0;
    data_t d1;

    d0.c     = 'A';
    d0.index = 1;
    d1.c     = 'B';
    d1.index = 2;

    printf("d0 < d1? %s\n", (cmp((void *)&d0, (void *)&d1) < 0 ? "yes" : "no"));
    printf("d1 < d0? %s\n", (cmp((void *)&d1, (void *)&d0) < 0 ? "yes" : "no"));

    return EXIT_SUCCESS;
}