TUHS

tuhs@tuhs.org

16 participants
6536 discussions

by norman＠oclsc.org

Every time someone starts spouting about how unsafe C is, and how all the world's problems would be solved if only people would stop using it, I think of Flon's Axiom, for 35 years my favourite one-liner about programming and languages: There does not now, nor will there ever, exist a programming language in which it is the least bit hard to write bad programs. Flon's Axiom comes from a short note On Research in Structured Programming, published in SIGPLAN Notices in October 1975. It's just as true today. Over the years I've seen people misinterpret the Axiom as an argument against looking for better programming languages at all, but that's not what it means. (Read the original note--it's a page and a half--for full context; it is, alas, behind ACM's Digital Library paywall.) There are certainly languages that make certain sorts of mistakes easier or harder, or are easier or harder to read, but in the end most of that really is up to the programmer. Programming well requires a lot of thought and care and careful rereading, and often throwing half the code out and re-doing it better, and until we can have a programming community the majority of whom are up to those challenges, we will continue to have crashes and security vulnerabilities and other embarrassing bugs aplenty, no matter what language is used. Norman Wilson Toronto ON

8 years, 10 months

[tuhs] the origins of fork and join

by Nelson H. F. Beebe

The latest issue of the IEEE Annals of Computing was published electronically today, and it has an article that I expect many TUHS list readers will enjoy reading: Notes on the History of Fork and Join http://dx.doi.org/10.1109/MAHC.2016.34 ------------------------------------------------------------------------------- - Nelson H. F. Beebe Tel: +1 801 581 5254 - - University of Utah FAX: +1 801 581 4148 - - Department of Mathematics, 110 LCB Internet e-mail: beebe(a)math.utah.edu - - 155 S 1400 E RM 233 beebe(a)acm.org beebe(a)computer.org - - Salt Lake City, UT 84112-0090, USA URL: http://www.math.utah.edu/~beebe/ - -------------------------------------------------------------------------------

8 years, 10 months

fstat(2) on pipes?

by Warren Toomey

All, sorry this is slightly off-topic. I'm trying to find out what fstat(2) returns when the file descriptor is a pipe. The POSIX/Open Group documentation doesn't really specify what should be returned. Does anybody have any pointers? Thanks, Warren P.S. Why? xv6 has fstat() but returns an error if the file descriptor isn't associated with an i-node. I'm trying to work out if/how to fix it.

8 years, 11 months

Re: [TUHS] fstat(2) on pipes?

by Norman Wilson

I remember once, long ago--probably in the early 1980s--writing a program that expected fstat on a pipe to return the amount of data buffered in the pipe. It worked on the system on which I wrote the code. Then I tried it on another, related but different UNIX, and it didn't work. So if POSIX/SUS don't prescribe a standard, I don't think one should pretend there is one, and (as I learned back then) it's unwise to depend on the result, except I think it's fair not to expect fstat to fail on any valid file descriptor. I'm pretty sure that in 7/e and earlier, fstat on a pipe reported a regular file with zero links. There was a reason for this: the kernel in fact allocated an i-node from a designated pipe device (pipedev) file system, usually the root. So the excuse that `there's no i-node' was just wrong. In last-generation Research systems, when pipes were streams (and en passant became full duplex, which caused no trouble at all but simplified life elsewhere--I think I was the one who realized that meant we didn't need pseudo-ttys any more), the system allocated a pair of in-core i-nodes when a pipe was created. As long as such an i-node cannot be accidentally confused with one belonging to any disk file system, this causes no trouble at all, and since it is possible to have more than one disk file system this is trivially possible just by reserving a device number. (In fact by then our in-core i-nodes were marked with a file system type as well, and pipes just became their own file system.) stat always returned size 0 for (Research) stream pipes, partly because nobody cared enough, partly because the implementation of streams didn't keep an exact count of all the buffered data all along the stream, just a rough one sufficient for flow control. Besides, with a full-duplex pipe, which direction's data should be counted? Returning to the original question, I'd suggest that: -- fstat(fd) where fd is a pipe should succeed -- the file should be reported to have zero links, since that is the case for a pipe (unless a named pipe, but if you support those you probably have something else to stat anyway) -- the file type should be IFIFO if that type exists in xv6 (which it wouldn't were it a real emulation of 6/e, but I gather that's not the goal), IFREG otherwise -- permissions probably don't matter much, but for sanity's sake should be some reasonable constant. Norman Wilson Toronto ON

8 years, 11 months

Re: [TUHS] fstat(2) on pipes?

by jnc＠mercury.lcs.mit.edu

> From: Warren Toomey > xv6 is a Unix-like OS written for teaching purposes. I'm fairly well-aware of Xv6; I too am planning to use it in a project. But back to the original topic, it sounds like there's a huge amount of variance in the semantics of doing fstat() on a pipe. V6 doesn't special-case it in any way, but it sounds as if other systems do. What V6 does (to complete the list) is grow the temporary file being used to buffer the pipe contents up to a certain maximum size, whereupon it halts the writer, and waits for the reader to catch up - at which point it truncates the file, and adjusts the read and write pointers back to 0. So fstat() on V6, which doesn't special-case pipes in any way for fstat(), apparently returns 'waiting_to_be_read' plus 'already_read'. >>> xv6 has fstat() but returns an error if the file descriptor isn't >>> associated with an i-node. >> ?? All pipe file descriptors should have an inode? To answer my own question, after a quick look at the Xv6 sources (on my desktop ;-); it turns out that Xv6 handles pipes completely differently; instead of borrowing an inode, they have special 'pipe' structures. Hence the error return in fstat() on Xv6. (That difference also limits the amount of buffered data in a pipe to 512 bytes. So don't expect high throughput from a pipe on Xv6! :-) So I guess you get to pick which semantics you want fstat() on a pipe to have there: V6's, V7's (see below), or something else! :-) > 7th Ed seems to return the amount of free space in the pipe, if I read > the code correctly: I'm not sure of that (see below), but I think it would make more sense to return the amount of un-read data (which is what I think it does do), as the closest semantics to fstat() on a file. It might also make sense to return the amount of free space (to a writer), and the amount of data available to read (to a reader), since those are the numbers users will care about. (Although then fstat() on the write side of a pipe will have semantics which are inconsistent with fstat() on files. And if the user code knows the maximum amount of buffering in a pipe, it could work out the available write space from that, and the amount currently un-read.) > fstat() > { > ... > /* Call stat1() with the current offset in the pipe */ > stat1(fp->f_inode, uap->sb, fp->f_flag&FPIPE? fp->f_un.f_offset: 0); > } > stat1() > { > ... > ds.st_size = ip->i_size - pipeadj; I'm too lazy to go read the code (even though I already have it :-), but V7 seems to usually be very similar to V6. So, what I suspect this code does is pass the expression: ((fp->f_flag & FPIPE) ? fp->f_un.f_offset : 0) as 'pipeadj' (to account for the amount that's already been read), and then returns (ip->i_size - pipeadj), i.e. the amount remaining un-read, as the size. Noel

8 years, 11 months

Re: [TUHS] fstat(2) on pipes?

by jnc＠mercury.lcs.mit.edu

> From: Warren Toomey > I'm trying to find out what fstat(2) returns when the file descriptor > is a pipe. In V6, it returns information about the file (inode) used as a temporary storage area for data which has been written into the pipe, but not yet read; i.e. it's an un-named file with a length which varies between 0 and 4KB. > xv6 has fstat() but returns an error if the file descriptor isn't > associated with an i-node. ?? All pipe file descriptors should have an inode? Noel

8 years, 11 months

Help with a Unix-ish project?

by Warren Toomey

Hi all, I'm working on a Unix-related project, and I thought I'd ask if anybody here might help. There's a pared-down Unix-like system, xv6, which is inspired by 6th Edition Unix and the Lions Commentary. Its purpose is to teach OS principles. The website and book are here: https://pdos.csail.mit.edu/6.828/2014/xv6.html https://pdos.csail.mit.edu/6.828/2014/xv6/book-rev8.pdf Unfortunately, while the kernel is nice, they don't provide much of a run-time environment, so it feels too much of a toy to use. I had the idea of porting a small set of libraries and commands over to get it to the point where it feels a bit like 7th Edition. I've made a start by using the Minix 2.0 libraries and commands, see https://github.com/DoctorWkt/xv6-minix2 and the NOTES file. I now realise that bringing up a libc plus associated commands will involve a fair bit of work. So, if anybody is interested in helping, let me know. Thanks in advance, Warren

8 years, 11 months

Re: [TUHS] History repeating itself (was: Unix v6 problem with /tmp)

by Norman Wilson

Dave Horsfall: Not Henry Spencer, perchance? ===== Since the Canadian in question had been working in the US since 1964 or so, he must by now be pushing 70 years old. I haven't seen Henry for some years, but I don't think he has aged that much. Norman Wilson Toronto ON

8 years, 11 months

Re: [TUHS] History repeating itself (was: Unix v6 problem with /tmp)

by Rudi Blom

> Date: Sat, 30 Jul 2016 15:30:36 +0000 > From: Michael Kjörling <michael(a)kjorling.se> > To: tuhs(a)tuhs.org > Subject: Re: [TUHS] History repeating itself > Message-ID: <20160730153036.GI3375(a)yeono.kjorling.se> > Content-Type: text/plain; charset=utf-8 > > On 30 Jul 2016 10:15 -0400, from cowan(a)mercury.ccil.org (John Cowan): >>> Who needs FedEx? >> >> Well, latency counts for something too, as does radius: if I want to >> send bulk data from New York to London (a very normal thing to do), >> your station wagon isn't going to count for much. > > You could, however, get an economy class flight ticket and load up > your suitcase with either HDDs or SDXCs (I suspect SDXCs would be > better per amount of data from the perspective of both volume and > weight, and would take better to handling). Given FedEx's prices, > _once you have the infrastructure set up_ (which you'll need whether > you have someone travel with the media, by air or by stationwagon, or > FedEx it), that _might_ even compare favorably in terms of bytes > transferred per second per dollar. (Now that's a measurement of > throughput I don't think I've seen before; B/s/$.) Of course, you'd > need someone who can babysit the suitcase, which potentially adds to > the cost, but the stationwagon traditionally hasn't been self-driving > either, and most of a transatlantic flight isn't active time on part > of the person travelling with the suitcase so you could go with an > overnight flight and allow the person to sleep. > > If you want to reduce the risk of the bag getting handled roughly or > lost in handling, reduce the above to carry-on luggage; it will still > provide a quite respectable throughput. > > ... ... > > It might not be the absolute cheapest approach, but it seems rather > hard to beat in terms of throughput per dollar for bulk data transfer, > especially if you already have someone who would travel anyway and can > be convinced to take a company-approved suitcase in return for having > their ticket paid for. > > -- > Michael Kjörling • https://michael.kjorling.se • michael(a)kjorling.se > “People who think they know everything really annoy > those of us who know we don’t.” (Bjarne Stroustrup) > To setup the 'infrastructure might be the tricky part. Many years ago I flew from Montreal to Amsterdam and had two stacks of 5-1/4" diskettes with me. No papers, confiscated in Amsterdam. Cheers, Rudi

8 years, 11 months

Unix v6 problem with /tmp

by Mark Longridge

Hi folks, My root partition for Unix v6 is almost full and /dev/rk0 only has 83 blocks. The trouble is I wanted to compile bc.y and I think it needs around 300 blocks of temporary space. I was wondering if there was a way to set up Unix v6 so that it could use one of the other drives for tmp space. I tried to set up a link using ln but it seems I can't link across filesystems. The exact error is "26: Intermediate file error". I managed to rearrange things so that /dev/rk0 had over 300 blocks of free space and it fixed the problem, but I'm curious if there was another solution. Mark

8 years, 11 months

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