Re: what is a reasonable socket select() wait time?

I do not think it fails.
TCP does not guarantee that you always receive data in the same chunks as
it was sent.
When server sends long stream of data, it is transmitted in segments
(roughly of MTU size each).
It takes time to receive segments and reassemble the stream.
In your example, select() does not see any data ready for reading. Maybe
data will be ready if you check several milliseconds later. This is normal
behaviour in TCP.
You may try to use non-zero timeout. But I think usually one needs some
other indication of the end of transmission in TCP (length control or an
end flag).
Consider this sequence:
- server sends amount of data, which translates to 10 segments
- 5 of them are received and can be read
- 6th is lost in a network, which causes delays for loss detection and
retransmission
- when you read form socket, you receive data from segments 1 to 5
- select() returns 0 - because there is no data yet
- segments 6 to 10 finally arrive several seconds later

    Alexei Pytel
speaking only for myself





                      Gene_Gaunt@Review
                      Works.com                To:       mi400@midrange.com
                      Sent by:                 cc:
                      mi400-admin@midra        Subject:  Re: [MI400] what is a 
reasonable socket select()
                      nge.com                   wait time?


                      04/22/2002 09:55
                      AM
                      Please respond to
                      mi400






Thanks gentlemen.

Sometimes select() fails to detect when more data is truly available.  I'm
not sure why select() fails.  My client receive buffer size is huge; I know
the server never completely fills it.  I know the client sets the fd_set
bits correctly.

The server basically loops like this:

DO FOREVER;
  RC = READ( client );
  RC = WRITE( client );
ENDDO;

and the client basically talks like this:

RC = WRITE( server );
DOU X = 0;
  RC = READ( server );  // size available is huge
  X = SELECT( read_set, wait time=0 );
ENDDO;

If the server sends more than our CHGTCPA TCPRCVBUF(8192) value, the client
loops until X=0.  This works fine---99.9% of the time.  But about once
every thousandth time, select() fails to detect when more data is truly
available, and the DOU ends after one iteration with X=0.

Why does the select() periodically fail?  How can my client truly, TRULY
determine if more data is available from the server?  Do I need a non-zero
wait time?  What is a reasonable value?

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