netdev - Re: UDP does not autobind on recv

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Message-ID: <1477314180.7065.108.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Mon, 24 Oct 2016 06:03:00 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     ML netdev <netdev@...r.kernel.org>
Subject: Re: UDP does not autobind on recv

On Mon, 2016-10-24 at 14:54 +0200, Jiri Slaby wrote:
> Hello,
> 
> as per man 7 udp:
>   In order to receive packets, the socket can be bound to
>   a local  address first  by using bind(2).  Otherwise,
>   the socket layer will automatically assign a free local
>   port out of the range defined by /proc/sys/net/ipv4
>   /ip_local_port_range and bind the socket to INADDR_ANY.
> 
> I did not know that bind is unneeded, so I tried that. But it does not
> work with this piece of code:
> int main()
> {
>     char buf[128];
>     int fd = socket(AF_INET, SOCK_DGRAM, 0);
>     recv(fd, buf, sizeof(buf), 0);
> }

autobind makes little sense at recv() time really.

How an application could expect to receive a frame to 'some socket'
without even knowing its port ?

How useful would that be exactly ?

How TCP behaves ?

I would say, fix the documentation if it is not correct.