RE: TCP/IP Sockets

> David's original question was "if an error occurs on the read(), will
> it return zero?".  To this, I still say NO.  It might return zero
> (based on your knowledge), but it will also return -1 according to the
> doc's and my knowledge.  The safe thing for David would be to 
> check for less than or equal to zero.

Actually, Bob, it wasn't a question at all... my statement was ACTUALLY in
reply to a discussion on the RPG400-L list (sorry, my fumble fingers)...
where someone was looking for a way to determine if the remote system had
disconnected.

I researched the results of the read() function and found that it returns
the number of bytes read, < 0 on error, OR ZERO IF AND END OF FILE CONDITION
EXISTS (NO MORE DATA CAN BE EXTRACTED FROM THE SOCKET) [this is from
Internetworking with TCP/IP V3 by Comers & Stevens].

Correlating this with the IBM Sockets manual, I figured that a dropped
connection would result in read() returning zero.

This was actually research done to confirm what I actually observed in
recent testing.  I had a program that initiated contact with a remote
server, but the server went casters up... the client was in a very tight
loop where the read() function was always returning zero.

Now my read routine tests for total byte count = zero and bytes read = 0, in
which case it will return an error indicating end of file (remote side
disconnected).

david

> 
> Bob
> 
> -----Original Message-----
> From: Patrick Townsend <townsend@patownsend.com>
> To: MIDRANGE-L@midrange.com <MIDRANGE-L@midrange.com>
> Date: Saturday, June 12, 1999 3:02 PM
> 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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>  >--
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>  >software and consulting services.
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