Re: How to properly daemonize?

[Robert Elz <kre%munnari.OZ.AU@localhost>]

    Date:        Fri, 14 Jul 2017 18:25:32 +0200
    From:        Edgar =?iso-8859-1?B?RnXf?= <ef%math.uni-bonn.de@localhost>
    Message-ID:  <20170714162532.GJ50210%trav.math.uni-bonn.de@localhost>

  | Perhaps I should state more clearly what my questions are
  | -- is the re-parenting to PPID 1 just to have someone wait()?

Yes.   But this is just how the system works, there's nothing magic
or special happening here, any process whose parent dies gets reparented
to init.

  | -- what exactly is the point (save sanity) of redirecting stdXXX to /dev/null?

sanity.

  | -- which flags to open() /dev/null with for stdXXX?

doesn't matter, the idea is that they don't get used - O_RDONLY for all
of them makes some sense, reading /dev/null simply gives EOF and the process
will (usually) read no more, writing to it would give an error, which is
probably the best outcome.

  | -- is opening /dev/null and dup2()ing to stdXXX just an optimization?

Yes.   Simpler for the process, and saves kernel data structs.

  | -- what's the point of some daemons assuring the outer process doesn't exit
  |    before the innermost actually runs?

Depends upon the needs of the processes.   Some creating processes want
to make sure it works, and try again if there was a failure.  In other
cases timing matters, want to avoid going to the next step (whatever that is)
before the daemon is ready to work.  In others it just doesn't matter.

  | And:
  | -- is there a way to detach in a shell script?

Not that I am aware of.   Most of it you can do, but not the setsid().

kre