How can I get structure from void* array - C?

Question

I am trying to retrieve value from void* array passed to function as argument.

This is what is passed to my function as void * data argument:

void *sredp[3];
sredp[0] = (void*)buf;
sredp[1] = (void*)(&len);
sredp[2] = (void*)(&ri);

source: https://github.com/kamailio/kamailio/blob/master/udp_server.c#L481

For the dereferencing of first two fields I am using this code:

int my_fnc(void *data)
{
        str *obuf;
        obuf = (str*)data;
}

str is internal type of Kamailio server (http://www.asipto.com/pub/kamailio-devel-guide/#c05str).

I am wondering how can I dereference ri field from void *data. I am trying to do this like this:

int my_fnc(void *data)
{
        struct receive_info * ri;
        ri = (struct receive_info*)(data + 2*sizeof(void*));
}

And when I am trying to print receive_info structure (https://github.com/sipwise/kamailio/blob/4ee1c6a799713b999aac23de74b26badbe06d0aa/ip_addr.h#L132) it seems to giving me some data but I have no idea if theese are just some random bytes from memory or it is correct data that I should retrieve.

So my question is if I am doing it right and if there is another/better way to reach my goal?

Answer 1

data in my_fnc has to be treated as void** to be able to extract the real data.

int my_fnc(void *data)
{
   void** sredp = (void**)data;
   str* obuf = (str*)srdep[0];

   // The pointer that contains len can be obtained from srdep[1]
   // The pointer that contains ri can be obtained from srdep[2]
}

Here's a simple program that illustrates that:

#include <stdio.h>

void foo(void* data)
{
   void** realData = (void**)data;
   void* p1 = realData[0];
   void* p2 = realData[1];
   void* p3 = realData[2];

   printf("sredp[0]: %p\n", p1);
   printf("sredp[1]: %p\n", p2);
   printf("sredp[2]: %p\n", p3);

   int a1 = *(int*)p1;
   float a2 = *(float*)p2;
   double a3 = *(double*)p3;

   printf("a1: %d\n", a1);
   printf("a2: %f\n", a2);
   printf("a3: %lf\n", a3);
}

int main()
{
   int a1 = 10;
   float a2 = 20.0f;
   double a3 = 40.0;
   void *sredp[3];
   sredp[0] = &a1;
   sredp[1] = &a2;
   sredp[2] = &a3;

   printf("sredp[0]: %p\n", sredp[0]);
   printf("sredp[1]: %p\n", sredp[1]);
   printf("sredp[2]: %p\n", sredp[2]);

   foo(sredp);
}

Sample output from running the program:

sredp[0]: 0x28ac2c
sredp[1]: 0x28ac28
sredp[2]: 0x28ac20
sredp[0]: 0x28ac2c
sredp[1]: 0x28ac28
sredp[2]: 0x28ac20
a1: 10
a2: 20.000000
a3: 40.000000

Answer 2

You can't do pointer arithmetic with void* in C. It would require it to use sizeof(void), which doesn't really exists.

The problem is in your function signature :

int my_fnc(void *data)

Your pointer is not a void*, it's a void**. So cast it before using it :

void** d = (void**) data;
ri = (struct receive_info*)(d + 2);

This will use a sizeof(void*), which is a standard pointer size.