Processes and message passing

spawn

The spawn function starts a new process and answers its process identifier, a pid. The process runs a fun and shares nothing with the caller.

The new process has its own heap and its own stack, so a crash inside it never corrupts the caller.

1> Pid = spawn(fun() -> io:format("hi~n") end).

hi

<0.84.0>

2> is_pid(Pid).

true

Send with !

The exclamation mark sends a message to a pid. The message is any term, and the sender never waits: send is a fire-and-forget step.

The message lands in the receiver's mailbox, in the order it arrived.

1> Pid ! {greet, "Ada"}.

{greet,"Ada"}

2> self().

<0.82.0>

receive ... end

The receive expression waits for a mailbox message that matches one of its patterns. The first match runs its body; the rest stay queued.

A timeout after the patterns answers after the milliseconds you give it, so receive never blocks forever.

loop() ->

receive

{greet, Who} ->

io:format("hello ~s~n", [Who]);

stop --> ok

end,

loop().

The mailbox

Every process owns a mailbox. Sending never fails, even when the receiver is busy or has crashed, and the message is copied to the receiver.

receive picks messages selectively, which means a process can skip over messages that do not match yet.

Next: Spawn and links

Article author: Arthur Isaev