Non-blocking sockets, connect(), and socket states

From: Bernard Blackham (bernard_at_blackham.com.au)
Date: 04/28/05

  • Next message: Mingming Cao: "Re: [Ext2-devel] [RFC] Adding multiple block allocation" 
    Date:	Thu, 28 Apr 2005 18:34:51 +0800
To: linux-kernel@vger.kernel.org

    [previously sent to the allegedly dead netdev@oss.sgi.com]

    Hi,

    Through playing with tcpcp[1], I've found out about a quirk in the
    kernel's handling of non-blocking connection-based sockets. The
    sk_socket->state value can take on one of SS_FREE, SS_UNCONNECTED,
    SS_CONNECTING, SS_CONNECTED or SS_DISCONNECTING. On a standard
    *blocking*, connection-oriented socket (eg, TCP), after connect()
    returns, sk_socket->state will be SS_CONNECTED.

    However, if the socket is placed into non-blocking mode before the
    connect() call, connect() returns immediately with EINPROGRESS, and
    the sk_socket->state is set to SS_CONNECTING. When the socket
    finally does connect, the application is notified via poll(), but
    the state remains as SS_CONNECTING (which causes issues for tcpcp,
    though doesn't appear to have any other externally visible
    implications).

    Werner, the author of tcpcp, suggests that the application should
    call connect() on the socket a second time, after the successful
    connection, to force the sk_socket->state value to SS_CONNECTED.

    Should it be the kernel's responsibility to set SS_CONNECTED when
    the connection is established? Or should I go file bugs and submit
    patches on all the applications that use non-blocking sockets and
    don't call connect() a second time?

    Thanks in advance,

    Bernard.

    [1] http://tcpcp.sf.net/ 

    -- 
 Bernard Blackham <bernard at blackham dot com dot au>
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  • Next message: Mingming Cao: "Re: [Ext2-devel] [RFC] Adding multiple block allocation"

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