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From: joe@athena.mit.edu (Joseph C Wang)
Newsgroups: comp.windows.x
Subject: Problem using processes with XtAddInput
Message-ID: <1991Jun13.064942.11737@athena.mit.edu>
Date: 13 Jun 91 06:49:42 GMT
Sender: news@athena.mit.edu (News system)
Organization: Massachusetts Institute of Technology
Lines: 70

I'd like to use XtAddInput to communicate to and from a separate process.
The code which I use to do this is (in c++) is below.

This works unless the child process starts up a process of it's own.  
Then the whole thing blocks until I kill the grandchild process.

So what am I doing wrong.



------- Code follows

{
 // sock1[1] is to write to child, sock2[1] is to read from child

  int sock1[2], sock2[2];

  // open socket pairs
  if (socketpair(AF_UNIX, SOCK_STREAM, 0, sock1) < 0 ||
      socketpair(AF_UNIX, SOCK_STREAM, 0, sock2) < 0) {
    Error("Unable to open sockets.");
    return;
  }
  // fork

  if ((child_pid = vfork()) == -1) {
    Error("Unable to start child process.");
    return;
  }
  // this is the parent process (i.e. CDS)
  else if (child_pid > 0) {
    // close child sockets
    close(sock1[0]); close(sock2[0]);

    stream_write = fdopen(sock1[1], "w");
    stream_read = fdopen(sock2[1], "r");

    input_id =
      XtAddInput(sock2[1], XtInputReadMask, CmModelReceiveInput, this);

  }
  // this is the child process
  // this code follows the popen() code, in the Unix programmers
  // manual, except it establishes two-way communication.
  else {
    // close parent sockets
    // close(sock1[1]); close(sock2[1]);


    // duplicate stdin and stdout into child sockets
    // dup2 closes the old sockets automatically
    dup2(sock1[0], fileno(stdin));
    dup2(sock2[0], fileno(stdout));

    int n = getdtablesize();
    for (int i=3; i<n; i++)
      (void) close(i);

    execl("/bin/sh", "sh", "-c", command.Chars(), 0);  // outta here...
    _exit(1);
  }
}



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