> ## 10 Processes

> ### 10.1 Processes

> This chapter is under construction.

> ### 10.2 Process Creation

> A process is created by calling `spawn`:

>     spawn(Module, Name, Args) -> pid()
>       Module = Name = atom()
>       Args = [Arg1,...,ArgN]
>         ArgI = term()

> `spawn` creates a new process and returns the pid.

> The new process will start executing in `Module:Name(Arg1,...,ArgN)` where the arguments is the elements of the (possible empty) `Args` argument list.

> There exist a number of other `spawn` BIFs, for example `spawn/4` for spawning a process at another node. See `erlang(3)`.

> ### 10.3 Registered Processes

> Besides addressing a process by using its pid, it are also BIFs for registering a process under a name. The name must be an atom and is automatically unregistered if the process terminates:

> register(Name, Pid) | Associates the name `Name`, an atom, with the process `Pid`.
> registered() | Returns a list of names which have been registered using `register/2`.
> whereis(Name) | Returns the pid registered under `Name`, or `undefined` if the name is not registered.
> Name Registration BIFs.

> ### 10.4 Process Termination

> When a process terminates, it always terminates with an `exit reason` set. The reason may be any term.

> A process terminates normally, that is with exit reason `normal` when there is no more code to evaluate.

> A process terminates with exit reason `{Reason,Stack}` when a run-time error occurs. See Error and Error Handling.

> A process can terminate itself by calling one of the BIFs `exit(Reason)` or `fault(Reason)`. The process then terminates with reason `Reason` or `{Reason,Stack}`, respectively.

> A process may also be terminated if it receives an exit signal with another reason than `normal`, see Error Handling below.

> ### 10.5 Message Sending

> Processes communicate by sending and receiving messages. Messages are sent by using the send operator ! and received by calling receive.

> Message sending is asynchronous and safe, the message is guaranteed to eventually reach the recipient, provided that the recipient exists.

> ### 10.6 Error Handling

> Erlang has a built-in feature for error handling between processes. Terminating processes will emit exit signals to all linked processes, which may terminate as well or handle the exit in some way. This feature can be used to build hierarchical program structures where some processes are supervising other processes, for example restarting them if they terminate abnormally.

> Refer to OTP Design Principles for more information about OTP supervision trees, which uses this feature.

> #### 10.6.1 Emitting Exit Signals

> When a process terminates, it will terminate with an exit reason as explained in Process Termination above. This exit reason is emitted in an exit signal to all linked processes.

> A process can also call the function `exit(Pid,Reason)`. This will result in an exit signal with exit reason `Reason` being emitted to `Pid`, but does not affect the calling process.

> #### 10.6.2 Receiving Exit Signals

> The default behaviour when a process receives an exit signal with an exit reason other than `normal`, is to terminate and in turn emit exit signals with the same exit reason to its linked processes. An exit signal with reason `normal` is ignored.

> A process can be set to trap exit signals by calling:

>     process_flag(trap_exit, true)

> When a process is trapping exits, it will not terminate when an exit signal is received. Instead, the signal is transformed into a message `{'EXIT',FromPid,Reason}` which is put into the mailbox of the process just like a regular message.

> An exception to the above is if the exit reason is `kill`, that is if `exit(Pid,kill)` has been killed. This will unconditionally terminate the process, regardless of if it is trapping exit signals or not.

> ### 10.7 Monitoring Processes

> Processes can monitor another process by calling `erlang:monitor/2`. Monitoring is unidirectional. If the monitored process terminates, the monitoring process will receive a 'DOWN' message.

> ### 10.8 Process Dictionary

> Each process has its own process dictionary, accessed by calling the following BIFs:

>     put(Key, Value)
>     get(Key)
>     get()
>     get_keys(Value)
>     erase(Key)
>     erase()

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