On 11/04/2015 20:23, Steve Litt wrote:
if (fork()) for (;;) wait(&status);
I'm not sure why he repeatedly calls wait() when there's supposed to be
only one direct child.
It's the reaper routine, pid 1's main task. PID 1 must reap zombies
constantly, even if it doesn't spawn children itself, because orphaned
processes are reassigned to it all the time.
Your answers in the reddit reference answered some questions I've had
for a long time. Thank you!
You're welcome!
No. I read that at the time, IIRC, but it didn't help me. Keep in mind,
I was less knowledgeable about init systems back then. But still,
documentation is supposed to work for unknowledgeable people, as long
as they meet some minimum criteria of underlying knowledge.
I doubt you read that page "at the time", since I only added it 3 weeks
ago. ;)
Please read it now and tell me if it helps you.
1. Tell me exactly what executable to specify in the init= part of
the kernel line, in order to init to S6. Including path to that
executable.
You didn't find it for a good reason: there's no such executable!
The http://skarnet.org/software/s6/s6-svscan-1.html page explains why,
and what to do. There's an example of a "stage 1 init" script in
examples/ROOT/etc/s6-init.
A future version of s6 will provide an executable to run as init -
since it seems a major pain point for a lot of people (you are not
alone!)
2. Tell me how to compile the executable that is the answer to
question 1.
N/A. Typically, what you would use as process 1 is a script. The
script would execute into s6-svscan in the end.
3. Tell me where to put the executable that is the answer to
question 1. In this age of symlinks between /bin and /usr/bin, this
is not always a simple question.
s6 doesn't care. You would put it somewhere on your root filesystem
and that's it. /bin/init, /sbin/init and /etc/init are the usual
conventions.
4. Tell me the other necessary executables I need to compile, and tell
me how to compile them and where in the directory structure to
place them.
"./configure && make && sudo make install" does pretty much everything
here. :P
5. Tell me the name and location of the executable I need to use if I
want to use S6 just as a process manager (like daemontools).
This would be an excellent learning step.
The problem with daemontools' svscanboot is that it uses an external,
unsupervised process - readproctitle - to log svscan's output. I don't
like that approach, because it introduces unreliability - if something
kills readproctitle, you lose your ability to log the supervision tree's
error messages until you restart the whole tree.
As a first approximation, though, you can simply use s6-svscan as the
program to use in the instructions at
http://skarnet.org/software/s6/s6-svscan-not-1.html
6. Tell me how to set up S6 to act as everything but PID1, kind of like
OpenRC does. Tell me the name of the S6 command I need to put
in /etc/inittab (assuming I'm using sysvinit) in order to achieve
"everything but PID1", and whether it should be labeled respawn or
not in /etc/inittab.
s6-svscan, respawn. There's still the logging issue - SysVinit will
print all your supervision tree's error messages on the system console -
but it will work.
As I remember (I tried S6 in November 2014), your compile instructions
were reasonable enough that compile tweaks were discoverable and I
could compile the executables. What was sorely missing was, what is the
executable serving as S6's PID1.
You should give it a try now, with a configure/make/make install build
system (it changed in December 2014) and the overview documentation page.
The reason why you couldn't find anything about an executable serving as
s6's pid 1 is that there isn't one! And there isn't one because it's hard
to provide one that works in every situation - for one thing, it's not
possible to provide a *portable* one. Users who want to use s6 as process
1, for now, have to write a script to do so, and I can only provide
documentation to help them. But I understand this is not enough, and I'm
working on it, and will have a solution ready in a few months.
Yes. I'm not even sure what you mean by a command line. Do you mean a 1
liner command that, in bash, would have lots of semicolons and go out
to column 300 or so, or do you mean the command prompt, or something
else?
I mean a command line as in the argv argument given to the execve()
system call when a new process is created and executed. If you want
to run a simple command like "nice 10 echo blah" in bash, you type
it as is, and it's the same in execline. If you want to run a compound
command in bash, you put lots of semicolons and other barbaric symbols;
but in execline, you don't - you just run a simple command that has the
same effect as your bash compound command.
For instance, the following command:
{ echo blah; cat /etc/passwd; } | sort -u
is short and sweet, but requires a shell to run, to interpret the
sequence and the pipe. The following command does the same, but
does not need a shell:
pipeline foreground echo blah "" cat /etc/passwd "" sort -u
You can't run the former from a C program without invoking a
shell. You'd run /bin/sh -c "{ echo blah..."
You can run the latter from a C program without invoking a shell.
It's "pre-parsed", if you will.
The point of execline is that any script can be pre-parsed into a
similar form. That is exactly what the execlineb binary does: it
reads a script, turns it into a command line of the form seen above,
and execs into it.
3) This isn't strictly S6's business, but it would help if you could
document a way to discover the filename/location of the *current*
pid1, so I could move it away and put the S6 pid1 in its place,
which is easier than tweaking grub.
ls -l /proc/1/exe :)
--
Laurent
