/* ex: set ts=2 et: */
/*
 * Question:
 *
 * What is the hard limit of total simultaneous TCP connections on a
 * Windows XP machine? What is the limiting factor?
 *
 * The system which brought this question to mind uses the select() function
 * call, and so this test will be utilizing select().
 *
 * In this test program we are interested in determining the maximum number
 * of sockets we can manage with the select() function.
 * select() depends on fd_set, a set of file-descriptors.
 *  what is the maximum size of an fd_set?
 * for windows' winsock2's select(), fd_sets are composed of only SOCKETs.
 *  what is the maximum number of SOCKETs creatable via winsock2?
 * 
 * Answer:
 *
 * Interestingly the first hard limit we reach is neither of the above...
 * it is TCP's 16-bit port number. Since each side or a TCP connection
 * requires a unique port number allocated from within an operating systems'
 * TCP stack's port address space, we can not possibly ever have more than
 * 2^16 (65536) simultaneous TCP connections on a single operating system
 * instance.
 *
 * This is further limited by ports reserved or in use by the TCP standard
 * and the host operating system. Several ports and port ranges (i.e. port 0)
 * are reserved by the IANA. Unix-style systems will generally not return a
 * port in the range of 0-1023 from accept(); I do not know of Windows'
 * behavior in this regard. Furthermore, any socket-utilizing processes in
 * existence further limit the address space for local port numbers available
 * to our application; on my current Windows XP workstation 'netstat -na'
 * returns several dozen TCP sockets in various states.
 */

#define FD_SETSIZE 64 * 1024 /* defined *before* winsock2.h is included, obviously.
                              * this can basically be an arbitrary integer;
                              * however if it is very large ensure you don't
                              * overflow your stack! */
#include <winsock2.h>
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
  static fd_set fd; /* large FD_SETSIZEs will overflow the stack,
                       protect your stack from overflow with static
                       allocation */
  long cnt = 0;
  SOCKET sock;
  WSADATA data;
  if (SOCKET_ERROR == WSAStartup(MAKEWORD(2,0), &data)) {
    perror("WSAStartup");
    exit(EXIT_FAILURE);
  }
  FD_ZERO(&fd);
  /* allocate sockets until we produce an error or run out of (16-bit) port address space */
  while (SOCKET_ERROR != (sock = socket(AF_INET, SOCK_STREAM, 0)) && cnt <= 0xFFFF) {
    cnt++;
    if (0 == (cnt & (cnt-1)))
      printf("socket[%ld]=%d\n", cnt, (int)sock);
    FD_SET(sock, &fd);
  }
  printf("allocated %ld sockets before limit reached\n", cnt);
  return 0;
}