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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