4.9 - What are the "64 sockets" limitations?

There are two limitations in Winsock where you're limited to 64 sockets.

The Win32 event mechanism (e.g. WaitForMultipleObjects()) can only wait on 64 event objects at a time. Winsock 2 provides the WSAEventSelect() function which lets you use Win32's event mechanism to wait for events on sockets. Because it uses Win32's event mechanism, you can only wait for events on 64 sockets at a time. If you want to wait on more than 64 Winsock event objects at a time, you need to use multiple threads, each waiting on no more than 64 of the sockets.

The select() function is also limited in certain situations to waiting on 64 sockets at a time. The FD_SETSIZE constant defined in winsock.h determines the size of the fd_set structures you pass to select(). It's defined by default to 64. You can define this constant to a higher value before you #include winsock.h, and this will override the default value. Unfortunately, at least one non-Microsoft Winsock stack and some Layered Service Providers assume the default of 64; they will ignore sockets beyond the 64th in larger fd_sets.

You can write a test program to try this on the systems you plan on supporting, to see if they are not limited. If they are, you can get around this with threads, just as you would with event objects.