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4.4 Robustness
PongPID = spawn(tut20, pong, []),
spawn(Ping_Node, tut20, ping, [3, PongPID]).
(s1@bill)3> tut20:start(s2@kosken).
Pong received ping
<3820.41.0>
Ping received pong
Pong received ping
Ping received pong
Pong received ping
Ping received pong
This is a slight modification of the ping pong program where both processes are spawned from the same start/1
function, and the "ping" process can be spawned on a separate node. Notice the use of the link BIF. "Ping" calls
exit(ping) when it finishes and this causes an exit signal to be sent to "pong", which also terminates.
It is possible to modify the default behaviour of a process so that it does not get killed when it receives abnormal
exit signals. Instead, all signals are turned into normal messages on the format {'EXIT',FromPID,Reason} and
added to the end of the receiving process' message queue. This behaviour is set by:
process_flag(trap_exit, true)
There are several other process flags, see erlang(3). Changing the default behaviour of a process in this way is usually
not done in standard user programs, but is left to the supervisory programs in OTP. However, the ping pong program
is modified to illustrate exit trapping.
-module(tut21).
-export([start/1,
ping/2, pong/0]).
ping(N, Pong_Pid) ->
link(Pong_Pid),
ping1(N, Pong_Pid).
ping1(0, _) ->
exit(ping);
ping1(N, Pong_Pid) ->
Pong_Pid ! {ping, self()},
receive
pong ->
io:format("Ping received pong~n", [])
end,
ping1(N - 1, Pong_Pid).
pong() ->
process_flag(trap_exit, true),
pong1().
pong1() ->
receive
{ping, Ping_PID} ->
io:format("Pong received ping~n", []),
Ping_PID ! pong,
pong1();
{'EXIT', From, Reason} ->
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