Thread: casting struct pointer to void**

Hello,

This might be silly but I just came across a strange code. I don't have any problem understanding most of the code except the part where the author casts a structs pointer to void** before derefencing it.

I have written some small test to see what effect this has but it doesn't look me like it is having any effect.

Can someone shed some light on what exactly this the casting to void** is trying to achieve?

Code:

static void free_dc_attr( DC_ATTR *dc_attr )
{
    struct dc_attr_bucket *bucket;


    pthread_mutex_lock( &dc_attr_lock );


    LIST_FOR_EACH_ENTRY( bucket, &dc_attr_buckets, struct dc_attr_bucket, entry )
    {
        if (bucket->entries > dc_attr || dc_attr >= bucket->next_unused) continue;
        *(void **)dc_attr = bucket->next_free;
        bucket->next_free = dc_attr;
        break;
    }


    pthread_mutex_unlock( &dc_attr_lock );
}

edit:

DC_ATTR is just a struct and dc_attr is a valid pointer.

Originally Posted by ghoul

Can someone shed some light on what exactly this the casting to void** is trying to achieve?

Okay - just disregard this. It seems to like I might not be providing enough information to figure out what is happen.

But, yes, this is Wine code, if someone wants to investigate this further.

Originally Posted by john.c

It is reinterpreting a pointer to DC_ATTR into a pointer to void* and then storing an address at that location to make it part of a linked list.

Code:

#include <stdio.h>
#include <stdlib.h>
 
#define prn(x) printf(#x ": %p\n", (void*)x)
 
struct Stuff {
    int a, b, c, d;
};
 
int main() {
    void *list = NULL;
 
    struct Stuff *a = malloc(sizeof *a);
    *(void**)a = list;
    list = a;
 
    struct Stuff *b = malloc(sizeof *b);
    *(void**)b = list;
    list = b;
 
    prn(a);
    prn(b);
    prn(list);
 
    struct Stuff *x = list;
    list = *(void**)list;
 
    x->a = 1;
    x->b = 2;
 
    prn(list);
 
    return 0;
}

Thanks, John C, as always.

I guess that's about all I can say.