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Date: Wed, 05 Nov 2014 14:31:12 -0500
From: Austin S Hemmelgarn <ahferroin7@gmail.com>
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To: Martin Tournoij <martin@arp242.net>, linux-kernel@vger.kernel.org
Subject: Re: [RFC] The SIGINFO signal from BSD
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On 2014-11-05 10:17, Martin Tournoij wrote:
> Hi there,
>
> As a long-time BSD user, I have become quite used to the SIGINFO (sent =
with ^t)
> feature; after switching to Linux as my desktop a few months ago, I ver=
y much
> miss this.
>
> SIGINFO prints the status of the process to the terminal; BSD cp, for e=
xample,
> shows show much data it's copied:
>
>      $ cp large_file /dev/null
>      <press ^t>
>      load: 1.39  cmd: cp 85837 [running] 3.91r 0.00u 0.98s 8% 2340k
>      large_file -> /dev/null  15%
>
> As you see, it shows the current load, pid, process status, memory usag=
e, as
> well as how much of the file has been copied. Many other BSD tools prin=
t similar
> statistics (mv, tar, dd, sleep, fetch, etc.).
>
> On Linux, sometimes SIGUSR1 is used for similar purposes, the problem w=
ith this
> is that SIGUSR{1,2} will terminate a program if a program doesn't have =
a signal
> handler installed(!)
> SIGUSR1 also has no defined meaning, and may do something radically dif=
ferent;
> nginx, for example, reopens the logfiles on SIGUSR1, and SIGUSR2 upgrad=
es the
> nginx executable on-the-fly.
>
> So you need to carefully inspect the documentation, hope it's not out o=
f date,
> and then send SIGUSR1 and pray.
>
> This is different from SIGINFO, which does nothing when it's not instal=
led,
> which is safe. You can send SIGINFO to any process, and not be afraid y=
ou kill
> it.
>
> In addition, it's also not easy to send SIGUSR1, you need to open a new=

> terminal, find the pid, and use a kill command (you could also use
> pkill/killall, with the risk of sending the signal to other processes).=

>
> SIGINFO is, AFAIK, supported since 4.4BSD & descendants (ie. all modern=
 BSD
> systems), as well as MacOS X. Perhaps other systems as well (but did no=
t
> investigate).
>
> Why don't we have SIGINFO on Linux? Would a patch to implement this be =
accepted?
>
> SIGINFO is not defined in any standard, but Linux already implements ot=
her
> useful non-standard signals (most notably SIGWINCH). IMHO it's a very u=
seful
> feature.
>
> Thanks,
> Martin
>
> PS.
> I am *not* subscribed to this maillist; please cc me in replies!
Given a quick look at the signal(7) man page, it appears that on at=20
least alpha and sparc systems, SIGINFO is a synonym for SIGPWR, which=20
comes from SVR4, where it was used to indicate that the system had lost=20
power (I don't really understand this usage, but my guess was that UPS=20
monitoring software was supposed to send init a SIGPWR when the UPS=20
switched off AC to the backup power source).

You have to understand however, that the reason that SIGINFO works like=20
that on *BSD is that the kernel and core userspace are developed=20
together, whereas on Linux, they are maintained entirely separately.=20
Outside of core userspace components, using SIGINFO that way on *BSD is=20
just convention.  The people to talk to about that for the core=20
utilities on Linux would be the maintainers of the GNU coreutils, or=20
whatever your distribution might use in their place (I think it's very=20
unlikely that busybox or toybox would implement it however).



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