multiple waiting gen_tcp:accepts

Tony Rogvall <>
Sat Jul 21 01:02:08 CEST 2001


Willem Broekema wrote:

> Is there a reason why there can not be two waiting gen_tcp:accept()'s 
> at the same time? In the code below,  when "start()" is called, the 
> *second* spawned do_accept will write "{error, einval}" as soon as it 
> is started, while I would expect this to behave as:

The simple answer is that the driver (erlang-c interface) is not coded 
to handle multiple accepts on the the same listen socket. Of course this 
should be possible, but first there must be yet an other iteration over 
the inet_drv implementation. The next inet_drv/gen_tcp implementation 
will hopefully handle this.

The more complicated answer is that the problem is that with 
non-blocking IO you have to listen/poll on the listen file descriptor to 
see when you can do the accept without blocking the entire erlang node. 
This means that the inet_drv driver must handle the accept queue it 
self, and this is what is missing (or the queue length is max 1).

>
> - A1 and A2 are put in a queue waiting for incoming connections
> - A1 gets the first incoming one
> - A2 gets the second
>
> start() ->
>    {ok, LSock} = gen_tcp:listen(5568,
>         [binary, {packet, 0}, {active, true}, {reuseaddr, true}]),
>    A1 = spawn_link(?MODULE, do_accept, [LSock, 1]),
>    A2 = spawn_link(?MODULE, do_accept, [LSock, 2]),
>    ready.

I would not bet on A1 getting the first and A2 getting the second, but 
may be it does not make any difference in this case ?

>
> do_accept(LSock, N) ->
>    io:write( gen_tcp:accept(LSock) ).
>
>
> Another question:
>
> "get_func()" below returns a function. Immediatly calling 
> "get_func()(3)" gives a syntax error, while "A = get_func(), A(3)" 
> behaves ok. What's the reason for that? I guess it has to do with that 
> funs are a late addition to Erlang, and the syntax parser didn't have 
> to deal with double brackets - "...(..)(..)" - before that?
>
> triple(X) -> 3*X.
>
> get_func() ->
>    fun triple/1.
>
> exec_func() ->
>    %% this gives an error: 

Try this:
        (get_func())(3).

>
>    % get_func()(3).
>
>    %% while this works:
>    A = get_func(),
>    A(3).
>
>
> TIA
>
> - Willem

/Tony






More information about the erlang-questions mailing list