Re: Question on socket non block read

Scott,
I was using the example from your iProDeveloper RPG Programming with
sockets article regarding timeouts.
The code I posted came directly from there.

The issue I was having was when there was no data to be read, the read
would just wait forever. I was not getting a timeout because the Select to
set the timeout had not executed yet.

I changed my procedure to issue the recv after the select() and now it
works fine.

Jeff Young
Sr. Programmer Analyst

On Fri, Sep 18, 2015 at 10:29 AM, Scott Klement <rpg400-l@xxxxxxxxxxxxxxxx>
wrote:

I've found that it's more robust to call read/recv first, and only call
select() if read/recv returns that it "would block".

There are three reasons why this approach is better:

1) The same pattern can be used with SSL calls. Calling select() first is
not valid in SSL because select waits on the raw (still encrypted) socket.
It's possible for select() to wait for the network even though there's data
in the SSL buffer available to return immediately. Data that has already
been removed from the TCP channel, and decrypted and SSL has it in a buffer
to be returned -- but since it's out of the TCP buffer, select does not
know about it. I realize that this example did not use SSL, but I prefer
to use the same pattern for both,

2) When data is available on the socket already, calling recv/read first
is a bit more efficient, since there's already data and so no need for the
call to select().

3) There used to be sporadic situations where select() would get stuck
waiting when there's data in the TCP read buffer. I think all of these
were fixed, they were bugs in the OS ages ago... but using this pattern
worked around the problem.

What was the reason for calling select() first? As long as you are using
non-blocking sockets, I have never seen a disadvantage to calling read/recv
first...

-SK


On 9/17/2015 2:03 PM, Bradley Stone wrote:

select() should be called before the read()/recv() to know if the socket
is
ready for the read (or write).

The timeout of 60 seconds seems quite big. This isn't a delay if that's
what you're thinking.

If you specify 60 seconds that means it will wait on the select() for up
to
60 seconds for "readiness" and then error if not. If it's found to be
ready right away, it will pass through without error.

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