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Date: Fri, 3 May 2002 17:52:22 -0400 (EDT)
From: David E Euresti <davie AT MIT DOT EDU>
To: <cygwin AT cygwin DOT com>
Subject: Fork/Select discrepancy
Message-ID: <Pine.GSO.4.30L.0205031743270.7214-100000@cathedral-seven.mit.edu>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII

Hello,
	So I was trying to create a process, which does something
difficult that may crash and have this process spawn another process to
cleanup just in case.  The way it works is that they share a socketpair
and when the dangerous process dies the safe process gets an EOF from the
socket.  Unfortunately there seems to be a problem with cygwin in that
this behavior totally depends on whether the second process is a child or
a parent.

Dig this:
#include <unistd.h>
#include <sys/socket.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

void
recvdata (int fd)
{
  fd_set fds;
  char buf[1024];
  int n;

  FD_ZERO (&fds);
  for (;;) {
    FD_SET (fd, &fds);
    printf("Selecting");
    select (fd + 1, &fds, NULL, NULL, NULL);
    printf("Selected");
    if (FD_ISSET (fd, &fds)) {
      if ((n = read (fd, buf, sizeof (buf))) < 0) {
	printf ("%s, read (net)", strerror(errno));
	return;
      }
      if (!n) {
	FD_ZERO (&fds);
	shutdown (fd, 0);
	return;
      }
      write (1, buf, n);
    }
  }
}

int main() {
  int fds[2];
  int pid;
  int xfd;

  if (socketpair (AF_INET, SOCK_STREAM, 0, fds) < 0) {
    printf ("socketpair: %s\n", strerror(errno));
    return 0;
  }

  pid = fork ();
  if (pid == -1) {
    printf("Error fork");
    exit(0);
  }
  else if (pid == 0) {
    // child
    xfd = fds[0];
    close (fds[1]);
    printf("Child: Exiting\n");

  } else {
    // parent
    xfd = fds[1];
    close (fds[0]);
    recvdata(xfd);
    printf("Parent: Exiting");
  }
}

When the parent receives the data.  (As in the code above) this is the
output.


$ ./a.exe
Selecting
Child: Exiting
Selected
Connection reset by peer, read (net)Parent: Exiting

When the child receives the data.  (We move the recvdata(xfd) call to
right before Child:Exiting  we get the following

$ ./a.exe
Parent: ExitingSelecting

and ps shows that the child process is still running.
     1640       1     704       1640  con  500 17:50:45
/home/davie/unixsock/a

Let me know if there's some trick that I can do.  I tried it with AF_INET
also and it didn't work either.  Or if it's something that cygwin should
fix.

Thanks,
David Euresti





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