systemd (Was Re: tmpfs for strategic directories)

Nils Philippsen nils at
Tue Jun 1 09:49:17 UTC 2010

On Tue, 2010-06-01 at 02:52 +0200, Lennart Poettering wrote:
> On Wed, 26.05.10 14:47, Simo Sorce (ssorce at wrote:
> > > environment variables are normally inherited when forking/execing. We
> > > want to make sure that only the process we actually start ourselves
> > > parses and handles LISTEN_FDS. We want to avoid that if this daemon
> > > might spawn some other process, it might get confused into handling
> > > LISTEN_FDS, although that env var wasn't actually intended for it.
> > > 
> > > And hence we say that LISTEN_PID should be verified first, and only if
> > > it matches LISTEN_FDS should be handled.
> > 
> > If you are mandating behavior in daemons, wouldn't it be simpler to
> > mandate the daemon unsets LISTEN_FDS ?
> Yes, our reference implementation actually does that. But we didn't want
> to rely on that, simply because handling the environ[] array is so messy,
> since memory allocation is not clear and the whole thing is not
> thread-safe. In many case environ[] should probably be considered a
> read-only data structure during runtime.

I beg to differ, at least if we're talking about languages where you are
able to just let a single thread run (not sure about Java): Simply
document that the daemon should read and unset LISTEN_FDS (using
unsetenv() instead of messing with environ manually) while there is only
a single thread.

That is, if you really must use environment variables for passing that
information... Others have mentioned it already: The environment is for
information that is meant to be long-lived and shared between processes,
it should be feasible to pass this information via a command line option
instead. Even more so if handling of the environment is so messy.

Nils Philippsen      "Those who would give up Essential Liberty to purchase 
Red Hat               a little Temporary Safety, deserve neither Liberty
nils at       nor Safety."  --  Benjamin Franklin, 1759
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