Re: [patch] pipe: add support for shrinking and growing pipes
From: Michael Kerrisk
Date: Mon May 24 2010 - 11:44:01 EST
On Mon, May 24, 2010 at 4:51 PM, Brian Bloniarz <bmb@xxxxxxxxxxxx> wrote:
> On 05/24/2010 03:28 AM, Michael Kerrisk wrote:
>> Actually, SO_*BUF is pretty weird. It returns double what was
>> supplied. It's not simply a matter of rounding up: it always doubles
>> what was supplied.
>
> Rationale in net/core/sock.c:
>
> set_rcvbuf:
> sk->sk_userlocks |= SOCK_RCVBUF_LOCK;
> /*
> * We double it on the way in to account for
> * "struct sk_buff" etc. overhead. Applications
> * assume that the SO_RCVBUF setting they make will
> * allow that much actual data to be received on that
> * socket.
> *
> * Applications are unaware that "struct sk_buff" and
> * other overheads allocate from the receive buffer
> * during socket buffer allocation.
> *
> * And after considering the possible alternatives,
> * returning the value we actually used in getsockopt
> * is the most desirable behavior.
> */
> if ((val * 2) < SOCK_MIN_RCVBUF)
> sk->sk_rcvbuf = SOCK_MIN_RCVBUF;
> else
> sk->sk_rcvbuf = val * 2;
> break;
>
> I'm guessing pipes don't have this kind of wrinkle.
Yes, all of the above is understood. It's exposing these details to
userspace that's weird...
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface" http://blog.man7.org/
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