Date: Mon, 13 Sep 2021 21:25:23 +0200 From: Michael Tuexen <michael.tuexen@macmic.franken.de> To: John Baldwin <jhb@freebsd.org> Cc: Mark Johnston <markj@FreeBSD.org>, "src-committers@freebsd.org" <src-committers@FreeBSD.org>, "dev-commits-src-all@freebsd.org" <dev-commits-src-all@FreeBSD.org>, "dev-commits-src-main@freebsd.org" <dev-commits-src-main@FreeBSD.org> Subject: Re: git: b864b67a0d19 - main - socket: Do not include control messages in FIONREAD return value Message-ID: <BFB7C96E-CB33-4EB8-804C-E9AACC4BCAB7@macmic.franken.de> In-Reply-To: <270ac6c6-1fdf-879a-cbea-ef43200ef12a@FreeBSD.org> References: <202109122039.18CKdpwW094531@gitrepo.freebsd.org> <270ac6c6-1fdf-879a-cbea-ef43200ef12a@FreeBSD.org>
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> On 13. Sep 2021, at 18:29, John Baldwin <jhb@freebsd.org> wrote: >=20 > On 9/12/21 1:39 PM, Mark Johnston wrote: >> The branch main has been updated by markj: >> URL: = https://cgit.FreeBSD.org/src/commit/?id=3Db864b67a0d197f59ecf6698940600956= ceee2cae >> commit b864b67a0d197f59ecf6698940600956ceee2cae >> Author: Mark Johnston <markj@FreeBSD.org> >> AuthorDate: 2021-09-12 20:05:49 +0000 >> Commit: Mark Johnston <markj@FreeBSD.org> >> CommitDate: 2021-09-12 20:39:44 +0000 >> socket: Do not include control messages in FIONREAD return value >> Some system software expects to be able to read at least the = number of >> bytes returned by FIONREAD. When control messages are counted in = this >> return value, this assumption is violated. Follow Linux and = OpenBSD >> here (as well as our own kevent(EVFILT_READ)) and only return the = number >> of data bytes available. >> Reported by: avg >> MFC after: 2 weeks >=20 > In a somewhat similar vein, it would be nice to eventually have new = ioctls > to know how much data and control are available in the next message = for > datagram-oriented sockets. Right now if you are working with a = datagram > socket with variable-sized messages there's no good way to know how = big the > next message is (to resize a read buffer) as FIONREAD can count = multiple > messages. There's also no way at all to cope with control messages = aside > from retrying with some naive algorithm like doubling the size if = MSG_CTRUNC > is set, but that also requires always using MSG_PEEK so that you = always > end up reading a message at least twice. That still requires two system calls. In SCTP, there is a CMSG you can receive with a messages, which contains the metadata of the next message you will receive. So it might be an idea to define a CMSG containing the length of the = next message (if available). Best regards Michael >=20 > --=20 > John Baldwin
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