Re: minimum write size of SOCK_STREAM?

[Andrew Cagney <andrew.cagney%gmail.com@localhost>]

On Thu, 30 Oct 2025 at 21:17, Andrew Cagney <andrew.cagney%gmail.com@localhost> wrote:
>
> Hi,
>
> I've what I thought was some stock (if somewhat old) code that used a
> SOCK_STREAM for communication between a daemon and its control
> program.
>
> The daemon opened the SOCK_STREAM, passed it libevent, and then waits.
> The control program then opens the socket, sends a single message, and
> then waits for responses until the daemon closes the socket.
>
> Because the messages were all below the socket's buffer size (8K
> according to getsockopt()), my understanding is partial reads
> shouldn't happen.  The largest write theoretical write is ~3k.
>
> My problem is that the minimum size of the controller->daemon message
> has been shrinking.  It was >=1k, but is now closer to 770 bytes.  And
> when the smallest message is sent, the daemon finds that it has only,
> and exactly 400, bytes returned (larger messages are fine).
>
> So (I'm struggling to ask this with a straight face) is there a
> minimum size (around 800 bytes), below which a read from a SOCK_STREAM
> will only return part of the data?

857 bytes is good, 856 bytes is bad; and:

ipsec whack: ERROR: bumping up buffer from 736 to 856
[root@rise source 141]# uname -a
NetBSD rise 10.1 NetBSD 10.1 (GENERIC) #0: Mon Dec 16 13:08:11 UTC
2024  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC
amd64

> Andrew
>
> (Oh, the same code is happy on FreeBSD, OpenBSD, and Linux*)