Erlang and OTP in Action.pdf - Synrc

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specified. When the init/1 function returns it is then that the start or start_link function used
to start the behaviour process returns and unblocks the caller. This is to enable
deterministic startup of many processes such that when the start up function unblocks and a
process id is returned that the process is ready and fully initialized. A string of dependent
processes can be started up each being sure the processes it depends on are fully initialized.
These properties are requisite for developing coherent and predictable concurrent systems.
This start up and initialization is accomplished via a library called proc_lib. Below is an
implementation that blocks the startup function until the init function, executed by the newly
spawned process, has finished its execution.
Code Listing 14.4 – proc_lib example
-module(my_server).
-export([start_link/0]).
-export([init/1]).
start_link() ->
proc_lib:start_link(my_server, init, [self()]). #1
init(Parent) ->
register(MODULE, self()),
proc_lib:init_ack(Parent, {ok, self()}), #2
loop().
loop() ->
receive
Msg ->
handle_msg(Msg),
loop()
end.
At code annotation #1 proc_lib:start_link/3 is used to spawn a process. The function
takes the arguments Module, Function and Arguments which indicate which function to
spawn with and how to spawn that function. In this case my_server:init(self()) is called.
Self is the process id of the calling process, the one that is to be linked to, the one that is
doing the actual spawning. Passing this parent pid to the child process via the init function
allows the code at code annotation #2 to function. After all the registration steps have been
accomplished by the init/1 function the proc_lib:init_ack function is called to send a message
back to the parent process indicating that the proc_lib:start_link or start function can
unblock.
PROCESS LOOPS
As mentioned earlier processes are just functions that loop, which means loop recursively.
This means that they better be properly tail recursive or the stack will grow to consume
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