Socket inheritance on different Windows platforms (150523)

MORE INFORMATION

Under Windows NT and Windows 2000, socket handles are inheritable by default. This feature is often used by a process that wants to spawn a child process and have the child process interact with the remote application on the other end of the connection.

It is also common practice on Windows NT to set the standard handles (standard input, output, or error) of the child process to the socket handle. In such cases, the child process usually does not know that its standard handles are actually sockets.

Windows 9x differs from Windows NT/Windows 2000 in the following manner:

• Socket handles are not inheritable when created. To ensure that a child process can obtain and use a socket handle created in the parent, the handle must be explicitly duplicated using the Win32 API DuplicateHandle. Set the bInheritHandle parameter of the API to TRUE.
• Socket handles cannot be set to the standard handles of the child process. A programmer may use other mechanisms to pass the socket handle to the client, such as passing the handle values as command line arguments so that the child process can simply look at its argument vector.

Sample code

The following sample code illustrates how to write applications that will inherit sockets in child processes on both Windows 9x and Windows NT/Windows 2000.

Note This is 32-bit code only; 16-bit applications cannot inherit socket handles.

   // This is a Winsock server that is listening on a port.

   // When a client connects, the server spawns a child process and
   // passes the socket handle to the child.
   // The child can use this socket handle to interact with the
   // client and the parent is free to go back to waiting for
   // other clients to connect.

   OrigSock=accept(listen_socket,(struct sockaddr *)&from,&fromlen);

   if (OrigSock == INVALID_SOCKET)  {
      fprintf(stderr,"accept failed %d\n",GetLastError());
      return -1;
   }
   {
      STARTUPINFO si;
      PROCESS_INFORMATION pi;
      char argbuf[256];

      memset(&si,0,sizeof(si));

      // 
      // Duplicate the socket OrigSock to create an inheritable copy.
      // 
      if (!DuplicateHandle(GetCurrentProcess(),
            (HANDLE)OrigSock,
            GetCurrentProcess(),
            (HANDLE*)&DuplicateSock,
            0,
            TRUE, // Inheritable
            DUPLICATE_SAME_ACCESS)) {

         fprintf(stderr,"dup error %d\n",GetLastError());
         return -1;
      }
      // 
      // Spawn the child process.
      // The first command line argument (argv[1]) is the socket handle.
      // 

      wsprintf(argbuf,"child.exe %d",DuplicateSock);
      if (!CreateProcess(NULL,argbuf,NULL,NULL,
               TRUE, // inherit handles
               0,NULL,NULL,&si,&pi) ){
         fprintf(stderr,"createprocess failed %d\n",GetLastError());
         return -1;
      }

      // 
      // On Windows 95, the parent needs to wait until the child
      // is done with the duplicated handle before closing it.
      // 
      WaitForSingleObject(pi.hProcess, INFINITE);
   }
   // 

   // The duplicated socket handle must be closed by the owner
   // process--the parent. Otherwise, socket handle leakage
   // occurs. On the other hand, closing the handle prematurely
   // would make the duplicated handle invalid in the child. In this
   // sample, we use WaitForSingleObject(pi.hProcess, INFINITE) to
   // wait for the child.
   // 
   closesocket(OrigSock);
   closesocket(DuplicateSock);
				

The following sample code illustrates how the newly created process extracts the socket handle from its command line:

   main(int argc, char *argv[]){
      SOCKET Sock;

      /* WSAStartup etc. */ 
      if (2 == argc){
         Sock = atoi(argv[1]);   // use Sock
      }
   }