Casting an int to void pointer in c++

I was a bit confused with the following piece of code which just creates a number of threads and assigns them an id.

#include "pthread.h"
#include "stdio.h"
#include "stdlib.h"

#define N 5

void *worker_thread(void *arg) {
    printf("This is worker_thread #%ld\n", (long) arg);
    pthread_exit(NULL);
}

int main() {
    pthread_t my_thread[N];

    long id;
    for (id = 1; id <= N; id++) {
        int ret = pthread_create(&my_thread[id], NULL, &worker_thread, (void*) id);
        if (ret != 0) {
            printf("Error: pthread_create() failed\n");
            exit(EXIT_FAILURE);
        }
    }

    pthread_exit(NULL);
}

The function prototype was;

int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                          void *(*start_routine) (void *), void *arg);

The arg was void*, Shouldn't we have;
int ret = pthread_create(&my_thread[id], NULL, &worker_thread, (void*) &id);

You can of course but this will mean that the referred value should be there when the function exists, which isn't a problem here because we are in main(). The more prominent problem is that the value of id might change when the thread routine actually uses it, so it's not a good idea to pass the argument by reference.
But how can we do that?
In fact casting a long to a void* is allowed, it just  means that the void* is actually not storing any pointer to a real memory location but a mere number represented by id. (void*) id returns some value and this is not a real memory address at all.

You can do the same with integers.

#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);