[erlang-questions] Why is setting socket options by setopts()better than by listen()?

Valentin Micic <>
Thu Nov 27 08:26:27 CET 2008


I wouldn't say better... I think it is more a question of semantic: one is
used to set/change default parameters, whilst another to inform driver about
specific connection behavior. Relying on a (changed) default behavior may
not be always desirable, as it may make program less readable -
gen_tcp:listen/2 may be placed anywhere in a code, whereas the person that
reads the code may want to know about the socket behavior right there, where
the connection has been accepted.

I would say that options related to gen_tcp:listen/2, therefore not
influencing session socket directly, should be placed alongside listen, say:

    gen_tcp:listen(Port, [{reuseaddr, true}])

OTOH, options related to the socket should be explicitly mentioned after the
connection has been accepted:

    inet:setopts(Socket, [binary, {active, once}])

IMO, one exception to this rule should be "active" flag, which I also use as
a part of gen_tcp:listen/2 argument:

        gen_tcp:listen(Port, [{reuseaddr, true}, {active, false}])

in order to prevent socket receiving any traffic without solicitation. But I
think this doesn't break the rule, as the call to inet:setopts/2 may change
that the way particular process requires it.

V.

-----Original Message-----
From: 
[mailto:] On Behalf Of Sergey A.
Sent: 26 November 2008 10:45 PM
To: 
Subject: [erlang-questions] Why is setting socket options by setopts()better
than by listen()?

Hello.

I've read the following in the "Programming Erlang":

-------------------8<-------------------
After we have accepted a connection, it's a good idea to explicitly
set the required socket options, like this:
{ok, Socket} = gen_tcp:accept(Listen),
inet:setopts(Socket, [{packet,4},binary, {nodelay,true},{active, true}]),
loop(Socket)
-------------------8<-------------------

And I have a small question. Why that is considered better solution
than using the same options, but passed to gen_tcp:listen/2 function:

gen_tcp:listen(2345, [{packet, 4}, binary, {nodelay, true}, {active,
true}, {reuseaddr, true}])

As I know, all the sockets obtained by calling gen_tcp:accept/1
inherit options listed in gen_tcp:listen.

Thanks.

--
Sergey.
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