Re: TCP/IP Sockets

Patrick,

I think we are all saying the same thing only differently.

For blocking reads, the read() will return a value less than or equal
to 0 on a disconnect.

And we (at least Tim & I) would like to know how to get SO_KEEPALIVE to
work.  Perhaps somebody from IBM will see this thread and respond on
the SO_KEEPALIVE issue.

Bob

-----Original Message-----
From: Patrick Townsend <townsend@patownsend.com>
To: MIDRANGE-L@midrange.com <MIDRANGE-L@midrange.com>
Date: Saturday, June 12, 1999 10:02 PM
Subject: Re: TCP/IP Sockets


>Tim,
>
>We are not in disagreement. What you've said is exactly what I was
>trying to say. I may have added some confusion into the picture by
>talking about non-blocking reads, but the behaviour you describe is
>exactly what I've found, too, and what the IBM'er (Mike Mundy) was
>trying to teach me.
>
>Patrick
>
>Tim McCarthy wrote:
>>
>> I missed the original post but here goes. Patrick, I hate to
disagree
>> with an IBM'er :) but in my experience blocking is not the
determining
>> factor here. There are other ways to get a -1 return code on a read
even
>> with blocking on - in fact a disconnect before a read would NOT
block
>> therefore EWOULDBLOCK is not returned. Even more curiously, reads
>> following a disconnect can succeed with a zero result even with no
>> blocking! Even if EWOULDBLOCK was returned it really doesn't tell
you
>> anything about the state of the connection just that there is no
data
>> currently buffered from the remote. The remote could have just sent
>> data. Whether you get a 0 or -1 as a result of a remote disconnect
on a
>> READ depends on a variety of peculiar circumstances. I've found that
if
>> I send a steam of data prior to performing a series of reads I will
get
>> a -1 return code on a read when a disconnect is detected. If my
program
>> performs reads from a socket only, I get 0 returned. SO_KEEPALIVE
would
>> be the obvious answer to this problem but like Bob I've never been
able
>> to get it to work either.
>>
>> Tim
>> > -----Original Message-----
>> > From: Patrick Townsend [SMTP:townsend@patownsend.com]
>> > Sent: Saturday, June 12, 1999 12:09 PM
>> > To:   MIDRANGE-L@midrange.com
>> > Subject:      Re: TCP/IP Sockets
>> >
>> >
>> > I had some help from Michael Mundy at IBM on this issue a while
back.
>> > I
>> > hope I'm correctly reflecting what he said:
>> >
>> > If you are in non-blocking mode on the read (through the use of
>> > ioctl()
>> > or fcntl()) and there is no data to be read, you will get a -1
return
>> > code and an errno of EWOULDBLOCK.
>> >
>> > If you are in blocking mode on the read (the default) and you get
a 0
>> > return code for bytes read the socket has shut down or
disconnected.
>> >
>> > Patrick
>> >
>> > Bob Crothers wrote:
>> > >
>> > > Rich,
>> > >
>> > > I disagree.  A value of -1 indicates that the operation was
>> > > unsuccessful.  A value of 0 indicates the value was successful,
but
>> > 0
>> > > bytes of data where received.  Of course, I don't send 0 bytes
to
>> > often
>> > > (never), but it still indicates success and you can only check
errno
>> > > when it is -1.
>> > >
>> > > Per the "AS/400 Sockets Programming Manual (V3R1)"
(SC41-3422-00)
>> > (And
>> > > I very seriously doubt that this has changed since V3R1):
>> > >
>> > > Section 3.1.16:
>> > >
>> > > >>The read() function is used to receive data from a file or a
>> > socket.
>> > >
>> > > >>Parameters
>> > >
>> > > >>descriptor
>> > > >>    (Input)  The descriptor that is to be read from.  The
>> > descriptor
>> > > >>    points to a file or a socket.
>> > >
>> > > >>buffer
>> > > >>    (Output)  The pointer to a user-supplied buffer in which
the
>> > data
>> > > is
>> > > >>    to be stored.
>> > >
>> > > >>buffer_length
>> > > >>    (Input)  The length of the buffer
>> > >
>> > > >>Return Value
>> > >
>> > > >>read() returns an integer.  Possible values are:
>> > >
>> > > >>   -1 (unsuccessful)
>> > > >>   n  (successful), where n is the number of bytes returned.
>> > >
>> > > Regards,
>> > > Bob Crothers
>> > >
>> > > -----Original Message-----
>> > > From: Rich Duzenbury <rduz@aros.net>
>> > > To: MIDRANGE-L@midrange.com <MIDRANGE-L@midrange.com>
>> > > Date: Friday, June 11, 1999 11:16 PM
>> > > Subject: RE: TCP/IP Sockets
>> > >
>> > > >No, I think David is correct here.
>> > > >
>> > > >OS/400 Sockets programming manual:
>> > > >
>> > > >Usage notes:
>> > > >
>> > > >1. For sockets that use a connection-oriented transport service
>> > (for
>> > > example, sockets with a type of SOCK_STREAM), a returned value
of
>> > zero
>> > > indicates one of the following
>> > > >
>> > > >- The partner program has issues a close() for the socket.
>> > > >
>> > > >- The partner program has issued a shutdown() to disable
writing to
>> > > the socket.
>> > > >
>> > > >- The connection is broken and the error was retuened on a
>> > previously
>> > > issued socket function.
>> > > >
>> > > >- A shutdown() to disable readingwas previously done on the
socket
>> > > >
>> > > >So, all of my socket reads check for error codes (values less
than
>> > 0),
>> > > but also the special value of 0 bytes read, and assume the other
>> > side
>> > > unceremoniously dumped us off.
>> > > >
>> > > >> -----Original Message-----
>> > > >> From: owner-midrange-l@midrange.com
>> > > >> [mailto:owner-midrange-l@midrange.com]On Behalf Of Bob
Crothers
>> > > >> Sent: Friday, June 11, 1999 6:51 PM
>> > > >> To: MIDRANGE-L@midrange.com
>> > > >> Subject: Re: TCP/IP Sockets
>> > > >>
>> > > >>
>> > > >> David,
>> > > >>
>> > > >> Close, but not quite.  the read() will return a -1 to
indicate an
>> > > >> error.  At that point, the errno variable (defined in C
header
>> > file
>> > > >> errno.h).  Retrieving this value is very easy in ILE/C.  I
don't
>> > > know
>> > > >> how to get it with RPG.  But I could create a tiny service
pgm to
>> > > >> retrieve it if you would like and nobody comes up with a
better
>> > way.
>> > > >>
>> > > >> Also note that it depends on how the socket on the other end
is
>> > > closed.
>> > > >> If a close() is issued, then you will get the -1.  But, if
the
>> > other
>> > > >> side ends without actually closing the socket (eg:
ctl-alt-del on
>> > > >> windows), you might not get any indication of a problem.  The
>> > > >> SO_KEEPALIVE socket option is SUPPOSED to handle this
situation,
>> > but
>> > > I
>> > > >> have never gotten it to work on the AS/400 (at least not up
to
>> > > V3R7).
>> > > >>
>> > > >> Bob
>> > > >>
>> > > >> -----Original Message-----
>> > > >> From: David Gibbs <David.Gibbs@IL.US.MKS.com>
>> > > >> To: 'Midrange Mailing List' <MIDRANGE-L@midrange.com>
>> > > >> Date: Friday, June 11, 1999 4:10 PM
>> > > >> Subject: RE: TCP/IP Sockets
>> > > >>
>> > > >>
>> > > >> >Folks:
>> > > >> >
>> > > >> >I'm not 100% sure on this... but I think that, when the
remote
>> > site
>> > > of
>> > > >> a
>> > > >> >TCP/IP connection disconnects abnormally, a read() (on the
other
>> > > side)
>> > > >> will
>> > > >> >return zero.
>> > > >> >
>> > > >> >david
>> > > >> >+---
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>> > --
>> > IBM AS/400 communications, FTP automation, and network security
>> > software and consulting services.
>> >
>> > http://www.patownsend.com
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>
>http://www.patownsend.com
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