On 24 September 2017 at 19:37, Vinnie Falco via Boost-users <boost-users@lists.boost.org> wrote:

>On Sun, Sep 24, 2017 at 1:15 AM, martin doc via Boost-users <boost-users@lists.boost.org> wrote:
> As boost::asio::connect() closes an open socket passed to it before making
> the remote connection, it's not possible to pass connect() a prebaked
> socket.

What makes you think that? My reading of the implementation is that
the socket is only closed when:

1. The range of addresses passed to connect() has more than one element, and
2. The connection attempt for the first address fails.

See:

<https://github.com/boostorg/asio/blob/b002097359f246b7b1478775251dfb153ab3ff4b/include/boost/asio/impl/connect.hpp#L108

You can avoid the socket closure by only attempting to connect to one address.

​Judging from that code, the socket is always closed before attempting a connect. The `s.close()`​ is right above the `s.connect()`.

My guess is this is because the iterated endpoints may be using different protocols (TCP/IPv4 or TCP/IPv6 for example). Things become a bit hairy when you take that into account. You can't use a local TCP/IPv4 endpoint and connect to a TCP/IPv6 peer. What should `asio::connect` do in that case? Ignore the IPv6 endpoint? Or just not bind to a specific local endpoint? Or should you give it an iterator with at least one local endpoint per protocol in the iterated remote endpoints? And in that case, what should `asio::connect` do if there isn't a matching local endpoint? Return an error? Or not bind to a local endpoint? Personally I'm not sure which behaviour makes the most sense here. It seems like ASIO decided to sidestep the whole issue.

You can still use `socket.connect()` directly with a single endpoint. Alternatively, you can wrap an already connected socket by passing the file descriptor to asio:

http://www.boost.org/doc/libs/1_65_1/doc/html/boost_asio/reference/basic_stream_socket/basic_stream_socket/overload4.html

http://www.boost.org/doc/libs/1_65_1/doc/html/boost_asio/reference/basic_stream_socket/assign.html

​

Another option is to write your own `connect` function that does support binding to a local endpoint. If you have a nice way of dealing with different protocols in the iterated remote endpoints, you could even suggest your function for inclusion in ASIO.

 

--

​ Maarten​

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