Ports and Port Drivers

## 13 Ports and Port Drivers

Examples of how to use ports and port drivers can be found in Interoperability Tutorial. The BIFs mentioned are as usual documented in `erlang(3)`.

### 13.1 Ports

Ports provide the basic mechanism for communication with the external world, from Erlang's point of view. They provide a byte-oriented interface to an external program. When a port has been created, Erlang can communicate with it by sending and receiving lists of bytes, including binaries.

The Erlang process which creates a port is said to be the port owner, or the connected process of the port. All communication to and from the port should go via the port owner. If the port owner terminates, so will the port (and the external program, if it is written correctly).

The external program resides in another OS process. By default, it should read from standard input (file descriptor 0) and write to standard output (file descriptor 1). The external program should terminate when the port is closed.

### 13.2 Port Drivers

It is also possible to write a driver in C according to certain principles and dynamically link it to the Erlang runtime system. The linked-in driver looks like a port from the Erlang programmer's point of view and is called a port driver.

| An erroneous port driver will cause the entire Erlang runtime system to leak memory, hang or crash.

Port drivers are documented in `erl_driver(4)`, `driver_entry(1)` and `erl_ddll(3)`.

### 13.3 Port BIFs

To create a port:

Port Creation BIF.
`open_port(PortName, PortSettings` | Returns a port identifier `Port` as the result of opening a new Erlang port. Messages can be sent to and received from a port identifier, just like a pid. Port identifiers can also be linked to or registered under a name using `link/1` and `register/2`.

`PortName` is usually a tuple `{spawn,Command}`, where the string `Command` is the name of the external program. The external program runs outside the Erlang workspace unless a port driver with the name `Command` is found. If found, that driver is started.

`PortSettings` is a list of settings (options) for the port. The list typically contains at least a tuple `{packet,N}` which specifies that data sent between the port and the external program are preceded by an N-byte length indicator. Valid values for N are 1, 2 or 4. If binaries should be used instead of lists of bytes, the option `binary` must be included.

The port owner `Pid` can communicate with the port `Port` by sending and receiving messages. (In fact, any process can send the messages to the port, but the messages from the port always go to the port owner).

Below, `Data` must be an I/O list. An I/O list is a binary or a (possibly deep) list of binaries or integers in the range 0..255.

Messages Sent To a Port.
`{Pid,{command,Data}}` | Sends `Data` to the port.
`{Pid,close}` | Closes the port. Unless the port is already closed, the port replies with `{Port,closed}` when all buffers have been flushed and the port really closes.
`{Pid,{connect,NewPid}}` | Sets the port owner of `Port` to `NewPid`. Unless the port is already closed, the port replies with `{Port,connected}` to the old port owner. Note that the old port owner is still linked to the port, but the new port owner is not.

Messages Received From a Port.
`{Port,{data,Data}}` | `Data` is received from the external program.
`{Port,closed}` | Reply to `Port ! {Pid,close}`.
`{Port,connected}` | Reply to `Port ! {Pid,{connect,NewPid}}`
`{'EXIT',Port,Reason}` | If the port has terminated for some reason.

Instead of sending and receiving messages, there are also a number of BIFs that can be used. These can be called by any process, not only the port owner.

Port BIFs.
`port_command(Port,Data)` | Sends `Data` to the port.
`port_close(Port)` | Closes the port.
`port_connect(Port,NewPid)` | Sets the port owner of `Port` to `NewPid`. The old port owner `Pid` stays linked to the port and have to call `unlink(Port)` if this is not desired.
`erlang:port_info(Port,Item)` | Returns information as specified by `Item`.
`erlang:ports()` | Returns a list of all ports on the current node.

There are some additional BIFs that only apply to port drivers: `port_control/3` and `erlang:port_call/3`.

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