Format of Pids

Vance Shipley <>
Tue Jun 3 21:53:56 CEST 2003


Thank you very much for your definitive response!

On Tue, Jun 03, 2003 at 02:51:34PM +0200, Rickard Green wrote:
}  Creation is an integer which identifies "the creation of a node".
}  Creation is incremented when a node is restarted. Currently creation
}  is a 2-bit unsigned integer. 0 is special and means something like:
}  actual creation used right now. Creation isn't shown when a pid is
}  printed.

That raises a few questions.  In ei (erl_interface) creation is a short.

I have been setting creation with get_pid().  Since 29607 isn't going
to fit in the 2-bit unsigned integer used for creation in the VM I 
wonder whether any of this is useful.

The documentation, in describing ei_connect_init(), has this to say
of creation:

	"creation identifies a specific instance of a C node. It can
	 help prevent the node from receiving messages sent to an 
	 earlier process with the same registered name."

This suggests that the creation will be included in the cnode's pid
sent to the VM so that if it replies to that received pid and the
nessage is received by a new instance of the cnode we will recognize
that the destination pid is not ours.  If this is the case how is
the short used in for creation in the cnode mapped to the 2-bit 
unsigned integer used in the VM?

I also missed this in the documentation:

	"A C node acting as a server will be assigned a creation
	 number when it calls erl_publish() or erl_xpublish()."

I don't see this happening.  At least it isn't overwriting the
creation I set (e.g. 29607).

}   > For instance if I want to quickly test if a Pid
}   > exists I could do if(pid->serial).  But is
}   > that right or is num the right field to check?
}   > Or do I really have to compare the whole
}   > structure?
}  Yes, when testing for equality you have to compare the hole
}  structure.
Sorry I wasn't clear.  I want to test if there is a valid pid 
stored in the erlang_pid struct.  Is it safe to say that if 
((erlang_pid *)->num == 0) that there is no valid pid stored
there?  That is to say that num is always greater than 0 in
a pid (if we ignore the otp_ring process which I will never
want to send to)?


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