void star and & in C

Question

I have problems with pointer in C, this is an thread example in C. this code was written in "Advanced Linux Programming" book:

void* print_xs (void* unused)
{
    while (1)
    fputc (‘x’, stderr);
    return NULL;
}

and:

int main()
{
    pthread_t thread_id;
    pthread_create (&thread_id, NULL, &print_xs, NULL);
    while (1)
        fputc (‘o’, stderr);

   return 0;
}

1. why print_xs is void*?

   I found an answer, but it wasn't clear enough to me. the answer: This declares a pointer, but without specifying which data type it is pointing to
   

2. Is it related to return value?

3. also why void* data type is used for "unused" (void* unused)?

4. I'm not sure about why "&" is used before print_xs in pthread_create? Is it correct to say: pthread_create is in another library and we want to tell it to run pthread_create function but it doesn't know where is this function, so we tell the the address of this function to it.

Answer 1

A void pointer(void* ) is a pointer that has no associated data type with it. A void pointer can hold address of any type and can be typcasted to any type.

For example:

int a = 10;
char b = 'x';

void *unused = &a;  // void pointer holds address of int 'a'
unused = &b; // void pointer holds address of char 'b'

Yes, void* print_xs() has the return type of void pointer.

In pthread_create (&thread_id, NULL, &print_xs, NULL); pthread_create function passes the address of thread_id and print_xs