Processes

## 10 Processes

### 10.1 Processes

Erlang is designed for massive concurrency. Erlang processes are light-weight (grow and shrink dynamically) with small memory footprint, fast to create and terminate and the scheduling overhead is low.

### 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.

### 10.3 Registered Processes

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

Name Registration BIFs.
`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.

### 10.4 Process Termination

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

A process is said to terminate normally, if the exit reason is the atom `normal`. A process with no more code to execute terminates normally.

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)`, `erlang:error(Reason)`, `erlang:error(Reason, Args)`, `erlang:fault(Reason)` or `erlang:fault(Reason, Args)`. The process then terminates with reason `Reason` for `exit/1` or `{Reason,Stack} for the others`.

A process may also be terminated if it receives an exit signal with another exit 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 Links

Two processes can be linked to each other. A link between two processes `Pid1` and `Pid2` is created by `Pid1` calling the BIF `link(Pid2)` (or vice versa). There also exists a number a `spawn_link` BIFs, which spawns and links to a process in one operation.

Links are bidirectional and there can only be one link between two processes. Repeated calls to `link(Pid)` have no effect.

A link can be removed by calling the BIF `unlink(Pid)`.

Links are used to monitor the behaviour of other processes, see Error Handling below.

### 10.7 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.

#### 10.7.1 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.

#### 10.7.2 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 called. This will unconditionally terminate the process, regardless of if it is trapping exit signals or not.

### 10.8 Monitors

An alternative to links are monitors. A process `Pid1` can create a monitor for `Pid2` by calling the BIF `erlang:monitor(process, Pid2)`. The function returns a reference `Ref`.

If `Pid2` terminates with exit reason `Reason`, a 'DOWN' message is sent to `Pid1`:

    {'DOWN', Ref, process, Pid2, Reason}

If `Pid2` does not exist, the 'DOWN' message is sent immediately with `Reason` set to `noproc`.

Monitors are unidirectional. Repeated calls to `erlang:monitor(process, Pid)` will create several, independent monitors and each one will send a 'DOWN' message when `Pid` terminates.

A monitor can be removed by calling `erlang:demonitor(Ref)`.

It is possible to create monitors for processes with registered names, also at other nodes.

### 10.9 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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