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erlangerlang-shell

Erlang: where does the unmatched message go?

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?
Solution

  • 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.