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 usingregister/2.
whereis(Name) | Returns the pid registered under Name, orundefinedif 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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