Erlang: where does the unmatched message go?

Question

When tried to send unmatched message to a spawned process in erlang shell, I was expecting the message should remain in the mailbox, but it seemed like the mailbox is empty, why?

Erlang R15B02 (erts-5.9.2) [smp:2:2] [async-threads:0]
Eshell V5.9.2  (abort with ^G) 
1> Pid = spawn(fun()->receive stop->stop end end).
<0.33.0>
2> Pid ! msg.
msg
3> erlang:process_info(Pid, messages).
{messages,[]} %% where is the msg?

Answer 1

When the message can't be matched against a receive pattern it is moved from the mailbox to a save queue, see http://ndpar.blogspot.se/2010/11/erlang-explained-selective-receive.html for a detailed explanation of what happens.

The messages parameter to process_info/2 only shows the mailbox contents, AFAIK there is no way to inspect the contents of the save queue.

Answer 2

The message is of course there and it will be checked in subsequent receives. The fact that you can't see it with erlang:process_info(Pid, messages) is, in my opinion, weird.

Answer 3

(ppb2_bs6@esekilvxen245)1> self() ! a.
a
(ppb2_bs6@esekilvxen245)2> erlang:process_info(self(), messages).
{messages,[a]}