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List:       freebsd-hackers
Subject:    Re: Properly reaping children from a fork()
From:       Terry Lambert <tlambert2 () mindspring ! com>
Date:       2003-02-24 21:51:40
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G-der wrote:
> This is a first attempt for me but I seem to have problems when it comes
> to ensuring that all the children exit like they should.  What happens is
> that each child process remains in a zombied state (as seen through ps).
> Also if you check sockstat you can see that each zombied process still has
> a connection open.

Unless you explicitly close the connection, it's open.  In the case
of a zombie, though, there is a zombie status structure, and the files
are in fact closed.  Probably what you are seeing is the TCP connection
is not being properly closed by the client, or the connection rate is
high enough that the 2MSL timer has not expired by the time you are
looking at it, so they appear open.


> I'm not sure if the problem is how I am trying to close the socket or if
> it is with my signal handling.  I'm currently reading through intro(2) to
> see if there is something simple I've missed.

Who is closing the socket, the client or the server?  If the client
is closing the socket, it must call "shutdown(2)" on the socket, and
then explicitly call close.   This is because certain user space TCP
implementations, such as those in Windows, do not have proper resource
tracking for sockets, which are implemented in user space, instead of
being implemented in kernel space.

There are also a number fo "test" applications, which are nothing
more than "SYN-guns", i.e. they do not implement the full handshake,
and their only purpose is to connection-load the server.  Many of
these send RST, and then drop all knowledge of the connection.  This
will, over time, result in a large number of outstanding open sockets
on the server, since RST packets do not time out and repeat, since
they do not require ACK'ing: because of that, if they get lost, then
they are lost forever.  This can happen even on a local wire, if you
get a collision or some other event (for example).


> /* The commented code was supposed to reap the children in but it didn't
>    the call to signal() is supposed to do the same thing, but it doesn't */
> 
> /*      sa.sa_handler = sigchld_handler;
>    sigemptyset(&sa.sa_mask);
>    sa.sa_flags = SA_RESTART;
>    if (sigaction(SIGCHLD, &sa, NULL) == -1) {
>       perror("main:sigaction");
>       exit(-1);
>       }                         */
> 
>         signal(SIGCHLD, sigchld_handler);

This is bogus.  You don't want SA_RESTART behaviour here, since it's
not like you have to worry about a system call.  Calling the old
"signal" code overrides pretty much everything you do with the
sigaction() in any case.

Your main() exits, instead of hanging in wait() or sleep(), and
looping forever, in order to reap connections.

Your fork() returns the wrong way to main() (i.e. child exits and
parent stays around forever), so you have some identity confusion.


NB: An alternate way of setting up automatic reaping, since your
handler seems to not care about exit status, is to set SIG_IGN
as the handler for SIGCLD.

Actually, you'd do well to get a copy of "UNIX Network Programming"
by Stevens, or, minimally, download the example source code from
the publisher's web site.  It would help you out by providing you
working "simple server" example source code.

-- Terry

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