Miscellaneous APIs

Miscellaneous APIs

In this section, we'll cover a few Winsock API functions that you might find useful when you put together your own network applications.

getpeername

This function is used to obtain the peer's socket address information on a connected socket. The function is defined as

int getpeername(     SOCKET s,     struct sockaddr FAR* name,      int FAR* namelen );

The first parameter is the socket for the connection; the last two parameters are a pointer to a SOCKADDR structure of the underlying protocol type and its length. For datagram sockets, this function returns the address passed to a connect call; however, it will not return the address passed to a sendto or WSASendTo call.

getsockname

This function is the opposite of getpeername. It returns the address information for the local interface of a given socket. The function is defined as follows:

int getsockname(     SOCKET s,      struct sockaddr FAR* name,      int FAR* namelen );

The parameters are the same as the getpeername parameters except that the address information returned for socket s is the local address information. In the case of TCP, the address is the same as the server socket listening on a specific port and IP interface.

WSADuplicateSocket

The WSADuplicateSocket function is used to create a WSAPROTOCOL_INFO structure that can be passed to another process, thus enabling the other process to open a handle to the same underlying socket so that it too can perform operations on that resource. Note that this is necessary only between processes; threads in the same process can freely pass the socket descriptors. This function is defined as

int WSADuplicateSocket(     SOCKET s,     DWORD dwProcessId,      LPWSAPROTOCOL_INFO lpProtocolInfo );

The first parameter is the socket handle to duplicate. The second parameter, dwProcessId, is the process ID of the process that intends to use the duplicated socket. Third, the lpProtocolInfo parameter is a pointer to a WSAPROTOCOL_INFO structure that will contain the necessary information for the target process to open a duplicate handle. Some form of interprocess communication must occur so that the current process can pass the WSAPROTOCOL_INFO structure to the target process, which then uses this structure to create a handle to the socket (using the WSASocket function).

Both socket descriptors can be used independently for I/O. Winsock provides no access control, however, so it is up to the programmer to enforce some kind of synchronization. All of the state information associated with a socket is held in common across all the descriptors because the socket descriptors are duplicated, not the actual socket. For example, any socket option set by the setsockopt function on one of the descriptors is subsequently visible using the getsockopt function from any or all descriptors. If a process calls closesocket on a duplicated socket, it causes the descriptor in that process to become deallocated. The underlying socket, however, will remain open until closesocket is called on the last remaining descriptor.

In addition, be aware of some issues with notification on shared sockets when using WSAAsyncSelect and WSAEventSelect. These two functions are discussed in Chapter 5. Issuing either of these calls using any of the shared descriptors cancels any previous event registration for the socket regardless of which descriptor was used to make that registration. Thus, for example, a shared socket cannot deliver FD_READ events to process A and FD_WRITE events to process B. If you require event notifications on both descriptors, you should rethink your application design so that it uses threads instead of processes.