int to (void *)

The best way to do this is by using the intptr_t type. This type has the same size of a pointer (not int) in every architecture. It is defined as follows :

Integer type capable of holding a value converted from a void pointer and then be converted back to that type with a value that compares equal to the original pointer.

So you can do something like this:

#include <stdint.h>

void *threadfunc(void *param)
{
    int id = (intptr_t) param;
    ...
}

int i, r;
r = pthread_create(&thread, NULL, threadfunc, (void *) (intptr_t) i);

(This example code has been taken from : How to cast an integer to void pointer?)

However, there is not guarantee that the size of int is the same of the size of intptr_t, but it's really unlikely that some data is lost in the conversion process.