Fix: Server.c:379:3: warning: format '%d' expects argument of type 'int', but argument 5 has type 'long int' [-Wformat=]
Fix: Server.c:1040:3: warning: passing argument 3 of 'pthread_create' from incompatible pointer type [enabled by default]
=> expected 'void * (*)(void *)

Please review this change.

From man:
       * It calls pthread_exit(3), specifying an exit status value that is available to another thread in the same process that calls pthread_join(3).
       * It returns from start_routine().  This is equivalent to calling pthread_exit(3) with the value supplied in the return statement.

So the return NULL, seems enough.
As the serv option is not used, having void * seems ok.