[erlang-questions] How to get a supervisor to tell me a child's startfunc?

[Torben Hoffmann]

Jim,

Since this seems to receive little attention and I kind of wanted to know
this too, I digged around a bit...

I have not tried this out with dynamically created children, but for
standard children something along the lines of:

supervisor:which_children(SupRef) to get the list of children of a
supervisor. The first element of the tuples in the list is the name of the
child.
sys:get_status(ChildName) will then give you details about the child.

Example:
1> application:start(sasl).
.... loads of things
2> supervisor:which_children(sasl_safe_sup).
[{overload,<0.41.0>,worker,[overload]},
 {alarm_handler,<0.40.0>,worker,dynamic}]
3> sys:get_status(overload).
{status,<0.41.0>,
        {module,gen_server},
        [[{'$ancestors',[sasl_safe_sup,sasl_sup,<0.37.0>]},
          {'$initial_call',{overload,init,1}}],
         running,<0.39.0>,[],
         [overload,
          {state,0,0,0.8,5244,0.1,{0,0},clear},
          overload,infinity]]}

The '$initial_call' tag seems to provide at least which function was called.

But it turns out that calling this on the supervisor is the solution!

11> sys:get_status(sasl_safe_sup).
{status,<0.39.0>,
        {module,gen_server},
        [[{'$ancestors',[sasl_sup,<0.37.0>]},
          {'$initial_call',{supervisor,sasl,1}}],
         running,<0.38.0>,[],
         [sasl_safe_sup,
          {state,{local,sasl_safe_sup},
                 one_for_one,
                 [{child,<0.41.0>,overload,
                         {overload,start_link,[]},
                         permanent,2000,worker,
                         [overload]},
                  {child,<0.40.0>,alarm_handler,
                         {alarm_handler,start_link,[]},
                         permanent,2000,worker,dynamic}],

{dict,0,16,16,8,80,48,{[],[],[],[],[],...},{{[],[],[],...}}},
                 4,3600,[],sasl,safe},
          supervisor,infinity]]}

Fouth element of the child tagged tuples reveals how he children were
started.

Sorry about dragging you through the whole investigation, but I think it
holds some value... at least as to how I think ;-)

Cheers,
Torben


On Fri, Oct 30, 2009 at 17:45, Jim McCoy <jim.mccoy@REDACTED> wrote:

> I need to discover the startfunc being used by a variety of processes
> started dynamically by a supervisor and I am having a hard time
> figuring out how to dig this information out of the system.  Process A
> starts a bunch of children for supervisor S over its lifetime, process
> B needs to do some maintenance or checking of which of these children
> are running at various points in time, to do this B needs to
> specifically know the start function that A used when it passed the
> child spec for any particular child process in to S.  This information
> does not seem to be available directly from the supervisor, so I am
> guessing that there is some other place I need to look but my various
> search queries seem to be missing the magic keyword that will not lead
> me back to the standard supervisor docs...
>
> Any hints?
>
> Thanks,
> jim
>
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