On Fri, 02 May 2008 14:38:47 -0400, Roy Smith <[EMAIL PROTECTED]> wrote:
In article
<[EMAIL PROTECTED]>,
mp <[EMAIL PROTECTED]> wrote:
Thanks Roy. I was just trying to understand someone else's code, but
in the end it turns out that this was just a bug.
What weirded me out was how injecting a print statement preventing the
error from occurring, but now I get it. Without blocking, the
connection handshake occurs in parallel after the connect_exc method
is called. In my example, my processor reaches the send call before
the connection manages to complete in the background. However, if you
stick in a print statement after the connect_exc call and before the
send call, it delays processing just long enough for the connection to
complete, thus no exception is thrown by the send call.
I don't really understand that last part. There shouldn't be any
background processing going on, unless there's a lot more code than you
showed.
The three-way handshake *can't* happen in the background, because
connect_ex() has no way to know what value to return until the handshake is
completed. Let's say I send a SYN, and 15 seconds later, you send a RST
(indicating that the connection has been refused). The connect_ex() call
has to have waited the 15 seconds for this to happen so it knows to return
an appropriate error code.
I do not understand why sticking a print statement in there should make any
difference.
The background processing is going on in the kernel, where the TCP/IP
implementation is. After userspace has indicated the desire to establish
a connection, the kernel doesn't need any further input in order to set
up that connection. So non-blocking connect_ex starts the process and the
process will complete on its own (successfully or with an error) without
any further socket calls from the userspace application.
The right way to know when the connect attempt has actually completed
(successfully or with an error) is to use a mechanism like select() to
check for writeability. Once the socket is writeable, if it has a 0
SO_ERROR, the connection was successful, otherwise it failed.
Jean-Paul
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