RE: TCP/IP Sockets

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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