[TUHS] History of select(2)
clemc at ccc.com
Thu Jan 12 13:54:44 AEST 2017
Paul -- this is a great stuff and fills in some pieces of my memories.
FYI: CMU had the the Rand Ports code including empty() in some
implementations of kernels in use in the mid 1970s. The later UNIX cu(1)
program did not exist, and a couple of people hacked together a program
called "connect" which was used to connect between serial ports for
downloading PPT images to microprocessors such as a KIM-1 and communicating
with it. I say a "couple of people," because I do not remember who wrote
the original version. It could have been a number of us. I know I hacked
on it a good bit, as did others over the course of time (and we did not use
source control in those days). The sources were there for all to work
with and we all added stuff as we though of things that would help us out.
Anyway, besides microprocessor support, we also used connect(1) to
"front-end" the serial lines from the PDP-10's in CS and were doing some
rudimentary networking stuff over parallel DR-11C's, in the pre-TCP/IP
world [a discussion a couple of us have had separately - more in a minute].
But the point is that the dates line up with my being there and what we
were doing at the time. One version of connect(1) used the empty() system
call and Rand ports to do its thing. I also remember reading the
connect(1) code as an early education in network technology and concepts.
We would later take hunks of into a TCP stack [3-4 years later Phil Karn
would write the infamous KA9Q TCP stack and a few years later, Stan Smith
and I would write the VMS TCP etc in BLISS but it model on some C network
stuff from CMU... -- amazing circle].
Anyway, around the same time (either sept 76 or 77 would be my guess) the
late Ted Kowalski came to CMU for his OYOC time (I've forgotten which it
was to be honest) and brought a pre-UNIX/TS system with him - TS begat V7.
Ted brought that system up on the 11/34 in EE and I remember connect(1) was
the most important program that immediately broke. I also remember a
large argument between Ted and one of the other hackers (I've forgotten
whom), Ted saying we did not need it, it was wasteful, etc... and not in
the official editions. I remember he re-implemented the connect(1)
program one night with multiple process and EE systems were from them on
based on Ted's system [although, I would later get to know Chesson and Greg
would give me the mpx() code a year or two later for some networking stuff
I would work on but that is a different story].
The point is that while I have no memory of capac(), but I can confirm that
I definitely programmed with the empty() system call and Rand ports on a v6
based kernel in the mid-1970s and that it was definitely at places besides
Another thing I want thank you for it confirming something I have been
saying for few years and some people have had a hard time believing. The
specifications for what would become IP and TCP were kicking around the
ARPAnet in the late 1970s. We definitely had them at CMU and that's where
I first was introduced to them, long before the planned cut over in the
early 1980s. I probably was not aware of the global politics involved
outside of the ARPA community because I certainly thought at the time IP
was we were headed and it was what we were thinking about and considering
how to implement.
Anyway - thanks again for a great piece of hunt up some good stuff.
On Mon, Jan 9, 2017 at 2:36 AM, Paul Ruizendaal <pnr at planet.nl> wrote:
> On 9 Jan 2017, at 3:35 , Warren Toomey wrote:
> > Also, I came across this history of select(2) a while back:
> > https://idea.popcount.org/2016-11-01-a-brief-history-of-select2/
> > Cheers, Warren
> That is an interesting blog post, but I think it is a bit short on the
> history of things before 4.2BSD. Below my current understanding of what
> came before select().
> In March 1975 the first networked Unix was created at the University of
> Illinois, initially based on 5th edition, but soon ported to 6th edition.
> It is described in RFC681 and a paper by Greg Chesson. Note that UoI was
> the very first Unix licensee. Its primary authors were Steve Holmgren,
> Steve Bunch and Gary Grossman. Greg Chesson was also involved. Grossman had
> already done two earlier Arpanet implementations (the ANTS and ANTS II
> systems) on bare metal and had a deep understanding of what a good
> implementation needed.
> Their implementation was compact (about a thousand lines added to the
> kernel, and another thousand in the connection daemon) and - I'm my opinion
> at least - conceptually well integrated into the existing file API. It
> became the leading Unix Arpanet implementation with wide use from 1975 to
> 1981. Two things stand out: (i) no accept(); and (ii) no select(). The
> original authors are still with us, with the exception of Greg, and I asked
> for their input as well.
> (i) no accept()
> Listening sockets worked a bit different from today. If one opened a
> listening socket it would not return a descriptor but block instead; when a
> connection was made it would return with the listening socket now bound to
> the new connection. Server applications would open a listening socket and
> do a double fork for the client connection (i.e. getting process 1 as its
> parent); the main process would loop around and open a new listening socket
> (this can all be verified in surviving application sources). According to
> Steve Holmgren this was not perceived as a big problem at the time. Network
> speeds were still so low that the brief gap in listening did not matter
> much, and the double fork was just a few lines of code.
> This changed when the CSRG team moved from a long-haul, Arpanet, 56Kb/s
> context to a local, Ethernet, 3Mb/s context and Sam Leffler came up with
> the concept of accept(). In 4.1a BSD and 2.9BSD the queue of pending
> connections was fixed (possibly 1, I have to check). In 4.1c BSD listen()
> was introduced; before then whether a socket was active or listening was a
> flag to opening the socket. The second parameter to listen() specified the
> maximum number of pending connections [as an aside, note that I'm using
> 'socket' in the BSD sense; the term socket changed meaning several times
> between 1973 and 1983].
> (ii) no select()
> This was the real pain (Holmgren reconfirmed that). This is what Dennis
> must have referred to in his retrospect paper. Various solutions were
> thought of, but in Network Unix the model remained using separate processes
> for simultaneous reading and writing. Progress in this area came from two
> other places involved in Unix and Arpanet: Rand and BBN.
> In 1977 Rand was taking on this problem (see
> http://www.dtic.mil/dtic/tr/fulltext/u2/a044200.pdf and
> http://www.dtic.mil/dtic/tr/fulltext/u2/a044201.pdf). They considered a
> solution with a new system call 'empty()' that would tell if there was any
> data available on a file descriptor, a crude form of non-blocking I/O if
> you like. As this would consume precious CPU cycles it proved inadequate.
> Instead they came up with "ports". A port was a (possibly named) pipe with
> multiple writers and a single reader, and it was created with a 'port()'
> system call. The reader would see each write preceded by a header block
> identifying the reader. The implementation (see second PDF) was simple,
> apparently only taking 200 words of kernel code. Rand ports are a
> simplistic version of the 'mpx' facility done by Greg Chesson at Bell Labs
> (in 1978?). I am not sure whether this was independent invention or that
> Greg was aware of Rand ports. Unfortunately we cannot ask him anymore.
> Later in 1977, over at BBN, Jack Haverty was doing an experimental TCP/IP
> stack for Unix (this was TCP 2.5, not TCP 4). He had a working stack
> written in PDP11 assembler for a different OS and was making this run on
> Unix. He was using Rand ports to connect clients to the network stack, but
> still lacked the required primitives to make this work properly. So he came
> up with the await() system call, a direct precursor to select(). It is
> documented in BBN report 3911 (http://bit.ly/2iU1TNK), including man
> pages. With the awtenb() and awtdis() one would manage the monitored
> descriptors (like the bit vectors going into a select), and await() would
> then wait for an event or time out.
> Related to this was the capac() system call, to get the 'capacity' of a
> descriptor. This returns the amount of data that can safely be written to
> or read from a descriptor. I suppose it is an improved version of empty().
> There is no equivalent of that in the later BSD sockets, perhaps because
> non-blocking I/O in the current sense was about to arrive. With port(),
> await() and capac() it becomes possible to write single threaded network
> An example may be found here, the first TCP/IP (version 4) stack in C for
> Unix, from early 1979: http://digital2.library.ucla.e
> du/viewItem.do?ark=21198/zz002gvzqg (scroll down past IMP stuff). It's
> documented in IEN98 (https://www.rfc-editor.org/ien/ien98.txt). I'm
> currently retyping this source so that it can be better studied.
> The await() call is not in the TCP/IP code done for 4.1 BSD by BBN. I'm
> puzzled by this as it is evidently useful and Jack Haverty and Rob Gurwitz
> worked in the same corridor at BBN at the time. In 4.1a the select() call
> appears and it seems to be an improved version of await(), with the need
> for awtenb() en awtdis() replaced by the use of bit vectors. I am not sure
> if Bill Joy was aware of await() or whether it was independent invention.
> Here we can ask, but I have no contact details.
> Hope the above is of interest. I'm still learning new things about these
> topics every day, so please advise if my above understanding is wrong.
> As a side note, I am still looking for:
> - surviving copies of UoI "Network Unix" (I'm currently no further than
> papers and bits of source that lingered in other code bases)
> - surviving copies of the 4.1a BSD distribution tape (Kirk McKusick's tape
> was damaged)
> - surviving source of the kernel code of port(), await() and capac();
> (could possibly be recreated from documentation)
> Any and all help very much appreciated.
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