Practical uses of readv?

[Rainer Weikusat]

For this first time in sixteen years, I came into a situation where
readv (recvmsg) was actually useful: In order to dump the
'non-ESP marker' in front of an ISAKMP message when NAT-T is being
used for a particular 'IPsec session'. So far, I've assumed that
'scatter read' mainly exists for symmetry reasons. Anyone ever used
that for anything? And if so, for what?

[Johann Klammer]

Reading TS packets from a DVB device, before software demultiplexing.
Each packet 188 bytes and has to(possibly) go to a different place.

[Ersek, Laszlo]

I used recvmsg() in an UDP-based file transfer program. The data packet
format was something like 2 octets "sender slot number" (in network byte
order), 8 octets packet serial number (ditto), and payload. The ACK format
was the same, minus payload.

In the receiver, the first two fields were scattered into an u16 and an
u32[2]. The payload was scattered into a malloc()'d buffer and possibly
linked into a sliding window, based on the packet serial number. The first
two fields were reused for acking.

The sender kept iterating over its own sliding window, sending out data
packets at a predefined rate. The packet number was checked in any ACK
that came in (quickly, with the help of the reflected slot number), and if
it was a match, the ACK'd payload and the packet serial number were
evicted from the slot and the slot was refilled from stdin and the
incremented packet counter.

Or something like that.

I wrote it for SUSv1 and it even ran on OpenVMS. Nothing industrial, just
a hobby project that still comes in handy occasionally. If anyone wanted
to take a look, <http://freecode.com/projects/udp_copy2>.

... I'm not saying scatter read was "required", but it felt quite fitting
at the time.

Laszlo