Errata

or

^C doesn’t kill processes, SIGINT does

On the LUV mailing list recently, the perils of tty handling when booting with “init=/bin/bash” came up, and somebody asked why running ‘ping’ (without remembering to run stty or openvt first) will result in you having to reboot.

i.e. “why doesn’t ^C work in emergency mode”?

I wrote a fairly detailed, but comprehensible to a novice, answer and a friend suggested i should have written it as a blog post – which was the point of me setting up a blog in the first place.

So here’s a slightly edited version of it:

> Couldn’t you just open another console, run “top” and find the PID for
> ping and kill it? Or ps aux |grep ping and kill it? Is a reboot the
> only way to kill a ping where ^C is not available?

there’s nothing special about ^C except that by long convention it is mapped to the interrupt signal aka intr aka SIGINT. So, ^C itself doesn’t kill a program, the fact that it generates an interrupt signal does.

If you’ve rebooted for emergency maintenance (e.g. “init=/bin/bash” on the LILO or GRUB command line) then all you get is ONE instance of /bin/bash. Nothing else is running and nothing else has been run (because you’ve overriden the usual init with /bin/bash). No setup has been done on the terminal, and no other virtual terminals have been opened. The emergency environment is minimal (and deliberately so – you asked for it, you got it). It’s up to you to do whatever is needed to set up the environment so that you can do the required maintenance/repair work.

So if you run ping *before* remembering to set up the terminal (’stty sane’) then ^C doesn’t work because ^C doesn’t generate SIGINT so there’s no way of telling ping to quit, and if you haven’t opened another virtual terminal with openvt (aka “open” – it got renamed some time ago) then there is no other shell available to run top or ps or kill.

BTW, it’s not just ping. You’ll see the same problem in the same environment with ANY program that runs continuously until it gets a signal to terminate it. ping’s just the most obvious and common example.

Most programs, especially simple programs like ping, don’t have a ^C handler. They don’t need one. They have a signal handler which, amongst other things, handles SIGINT however it’s generated – it can be generated by pressing ^C in a normal tty environment, or by sending the program a SIGINT (e.g. “kill -2″).

That, BTW, is a core difference between a SIGTERM (just plain ‘kill’ or ‘kill -15′) and SIGKILL (‘kill -9′).

SIGTERM is usually handled by the program by setting up a signal handler for it, and it voluntarily terminates when it receives the signal (usually cleaning up, closing files, etc before it does so). If the program hasn’t set up a handler for SIGTERM then the default action is to just terminate. A program can set up a handler which does nothing on SIGTERM, which effectively just ignores the signal.

SIGKILL is handled by the kernel and it just kills the process immediately – it can not be blocked or intercepted by the program.

(By common convention, SIGHUP or ‘kill -1′ tells a long running daemon process to just reload it’s config files. It’s also generated when the tty that the program is running on is terminated – e.g. you log out or the modem hangs up or the ssh session dies, hence the name “HUP” aka “HANGUP”. That’s why you need to use something like nohup or screen if you want a program to continue running after you log out)

In a way, ‘kill’ is a bit of a misnomer. It’s actually a generic tool for sending signals to running processes, but the default action for most of those signals is to terminate the program. Here’s a list of the signals which can be sent.

$ kill -l
 1) SIGHUP	 2) SIGINT	 3) SIGQUIT	 4) SIGILL
 5) SIGTRAP	 6) SIGABRT	 7) SIGBUS	 8) SIGFPE
 9) SIGKILL	10) SIGUSR1	11) SIGSEGV	12) SIGUSR2
13) SIGPIPE	14) SIGALRM	15) SIGTERM	16) SIGSTKFLT
17) SIGCHLD	18) SIGCONT	19) SIGSTOP	20) SIGTSTP
21) SIGTTIN	22) SIGTTOU	23) SIGURG	24) SIGXCPU
25) SIGXFSZ	26) SIGVTALRM	27) SIGPROF	28) SIGWINCH
29) SIGIO	30) SIGPWR	31) SIGSYS	34) SIGRTMIN
35) SIGRTMIN+1	36) SIGRTMIN+2	37) SIGRTMIN+3	38) SIGRTMIN+4
39) SIGRTMIN+5	40) SIGRTMIN+6	41) SIGRTMIN+7	42) SIGRTMIN+8
43) SIGRTMIN+9	44) SIGRTMIN+10	45) SIGRTMIN+11	46) SIGRTMIN+12
47) SIGRTMIN+13	48) SIGRTMIN+14	49) SIGRTMIN+15	50) SIGRTMAX-14
51) SIGRTMAX-13	52) SIGRTMAX-12	53) SIGRTMAX-11	54) SIGRTMAX-10
55) SIGRTMAX-9	56) SIGRTMAX-8	57) SIGRTMAX-7	58) SIGRTMAX-6
59) SIGRTMAX-5	60) SIGRTMAX-4	61) SIGRTMAX-3	62) SIGRTMAX-2
63) SIGRTMAX-1	64) SIGRTMAX

See the man pages for init(8), kill(1), and especially signal(7) for more info on this topic.

Really, who cares?

1. How much petrol do people actually use, anyway? People whinge and bitch and moan about petrol going up a few cents, and queue up for ages to try to buy petrol at the low point of the weekly price cycle – and for what? A tank will hold less than 100 litres. Typically around 50, but let’s say it’s 100 litres. That means that a price difference of 1 cent is worth a whole $1.00. A price difference of 5 cents is a measly $5.00, less than the price of a cheap bought lunch. For the average driver, they’ll buy 100 litres once or maybe twice per week.

Who gives a damn?

If $1.00 or even $5.00 makes such a difference to someone’s budget then they’re in serious financial trouble anyway – so much trouble that they shouldn’t even think about having a car, because they can’t afford it.

Here’s a novel thought: they could drive less, make their kids walk to school, catch public transport, ride a bike, walk to the milkbar rather than drive, make sandwiches to take to work/school, and so on. They could also buy less of the worthless crap they buy (like biscuits and cakes and bottles of coke and other rubbish). There are thousands of things people could do so that they don’t have to give a damn about a few cents per litre.

Even better, if people reduce demand enough, they can seriously hurt the speculator bastards who have driven oil prices up so much in recent months….at least a quarter of the current price per barrel of oil is due to commodity speculators driving the price up. Hit the bastards where it hurts, drop demand so much that they lose serious money. If the price of petrol bothers you, don’t whinge, get revenge.

Actually, I’m entirely unconvinced that the general public actually care all that much about petrol prices anyway. It looks and feels much more like an astro-turfing campaign by the oil companies and petrol distributors – especially the recent crap about reducing the excise by 5c/litre, anyone can see that if the government does that the petrol companies will, within weeks, increase prices so that THEY get the 5c/litre rather than the government, resulting in yet another transfer of wealth from the public purse to private corporations. Same for all the bullshit about the fuel price watch program – it’s obvious that the opposition to that comes directly from the petrol companies who don’t want buyers to have good information to base their purchasing decisions on.

It’s also a useful distraction and pretend-debate for the government and the opposition – they can rant and rave and score points over a trivial, bullshit non-issue like this, and nobody will notice that nothing of any real substance or worth ever gets debated in parliament…and the media are either colluding in this or are too stupid to avoid getting sucked in.

2. We have the fourth-cheapest petrol in the world. So all this moaning is not about petrol being more expensive than it should be, it’s about not being quite as unreasonably privileged as we were in the past.

3. Oil prices are *inevitably* going up, they’re *never* coming back down. Get used to it, it’s only going to get worse from here – much worse. Instead of endlessly whining about the inevitable, focus on alternatives – like *DEMANDING* that the government start paying more than just empty tokenistic lip-service to the idea of alternative/renewable energy sources.

4. IMO, the sooner petrol hits $5.00 per litre or more, the better. By then we’ll really have to have functional alternative energy sources in place. Because if we don’t, we’re completely screwed.

Twelve months ago, I stopped being a part-time cyborg.

Twelve months ago last Sunday (18th May), I had a kidney transplant. It basically gave me my life back, so Sunday was kind of like a birthday. The kidney was donated by my mother, so she not only gave birth to me back in 1967, she gave me my life again last year.

The operation went well, recovery was un-problematic, and aside from a few initial difficulties getting used to all the anti-rejection drugs I have to take, my health just keeps getting better and better.

And although the twice-daily ‘pill-salad’ I have to take kind of sucks, it sure as hell beats the shit out of spending about 6 hours trapped in a chair (comfy as it was), hooked up to a dialysis machine, three times per week. Dialysis added up to about 20 or 25 hours a week including preparation time and cleaning up afterwards, equivalent to a half-time job for both me and my partner (I dialysed at home, thanks to the dialysis machine and supplies provided by the hospital. I am so glad that I live in a civilised country with a public health system for all. I’d probably be dead if I lived somewhere barbaric like the U.S. where access to decent health care is dependent on your income or wealth)

As well as the enormous improvement the transplant has made to my daily life, I’m looking forward to travelling again. Travel was basically impossibly impractical on dialysis. Post-transplant, it’s quite possible as long as I’m careful and take appropriate precautions (the anti-rejection drugs partially suppress my immune system, so I’m at higher risk for catching some infectious diseases)

Best of all, the greatest risk of losing a transplanted organ is within the first twelve months – if the transplant survives that long, there’s an excellent chance that it will survive ten or twenty years or more.

And I’m hoping that, if I ever need another kidney, in five or ten or twenty years time they’ll be able to grow me one “to order” from my own DNA, so no risk of rejection.

See this article, Baby boom link to rise in child-welfare cases at The Age web site.

What a great legacy for John Howard – his “baby bonus”, a grubby and cynical vote-buying exercise for the 2004 Federal Election has resulted in a mini- baby-boom.

It has also resulted in a more than 66% increase in interventions by the Child Protection Agency (from 600 in 2000-2001 to over 1000 last financial year). And that’s just from the first few batches of baby-bonus babies.

Perhaps the people who think a $3000 “bonus” (as introduced in 2004, now increased to $5000) is a good reason to have a baby are precisely the kind of people who shouldn’t be having babies.

“Cool, $5000 if i have a baby”. or worse, “Cool, $5000 if i make my girlfriend have a baby”. Great reasons to have a baby. Thank you, Mr Howard.

At least we now all know the answer to the question “How many abused and neglected babies is a vote worth”?

I’m getting more and more annoyed by web sites that have style-sheets with tiny little fonts and widths specified in pixels rather than percentages (or ‘em’ units).

Almost every web site i visit these days seems to have a style sheet written by some idiot who thinks “if it looks good on my screen using my eyes then it’s perfect for everyone”.

WRONG

Specifying widths in pixels is NOT for text. It’s for images, you morons.

Continue Reading »

see point #4 of this .

words of wisdom.

There was a discussion today on the LUV-main mailing list about apache virtual hosting. Someone suggested mod_vhost_alias which is OK for a simple solution, but pretty limited.

IMO, there are much better ways of doing it. Specifically, generating config fragments for each virtual host.

I wrote such a system years ago, and have always thought “I should package it up for debian”. There’s a moderate amount of work involved in doing so, and probably an even larger amount of support work afterwards. Worst of all, I’d have to write – or con someone else into writing – documentation for it (it’s easy enough to use that you can be SHOWN how to use it in 10 minutes, but that isn’t good enough for a distributed package).

I guess the point of this blog post is to find out if there’s sufficient interest to make it worth the effort. Feel free to comment or email me about it.

Continue Reading »

In this post, Steven Hanley wonders how to keep /var/cache/apt/archives empty on a machine that has a full mirror of debian on it.

That’s actually the wrong question.

The right question is ‘Why is /var/cache/apt/archives even relevant on a machine with a full mirror of debian?’

/etc/apt/sources.list has always supported file:/ URLs. for example, sources.list on my local debian mirror looks like this:

deb file:/home/ftp/debian unstable main contrib non-free
deb file:/home/ftp/debian-multimedia/ unstable main

With a file URL, apt-get doesn’t need to download any packages, and thus doesn’t need to cache anything in /var/cache/apt/archives. No need to keep it empty, it just is empty.

In a related post, Andrew Pollock suggests adding DPkg::Post-Invoke { “apt-get clean”; }; to /etc/apt/apt.conf.

That would be fine if you ONLY ever ran “apt-get update ; apt-get dist-upgrade”.

It’s not fine if you run “apt-get -d” to download packages first, then install a few packages with “apt-get install package”, and then run “apt-get dist-upgrade”.

If you try that, then the DPKG::Post-Invoke rule will clean out /var/cache/apt/archives when you install the individual package(s). When you then run the dist-upgrade, apt-get will have to download all the packages again.

Better just to get into the habit of running apt-get clean or autoclean occasionally when you want to reclaim some disk space.