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Drivers

Bjorn Gustavsson
2009-11-16
drivers.xml

This section provides a brief overview on how to write efficient drivers.

It is assumed that you have a good understanding of drivers.

## Drivers and Concurrency

The runtime system always takes a lock before running any code in a driver.

By default, that lock is at the driver level, that is, if several ports have been opened to the same driver, only code for one port at the same time can be running.

A driver can be configured to have one lock for each port instead.

If a driver is used in a functional way (that is, holds no state, but only does some heavy calculation and returns a result), several ports with registered names can be opened beforehand, and the port to be used can be chosen based on the scheduler ID as follows:

`-define(PORT_NAMES(), {some_driver_01, some_driver_02, some_driver_03, some_driver_04, some_driver_05, some_driver_06, some_driver_07, some_driver_08, some_driver_09, some_driver_10, some_driver_11, some_driver_12, some_driver_13, some_driver_14, some_driver_15, some_driver_16}). client_port() -> element(erlang:system_info(scheduler_id) rem tuple_size(?PORT_NAMES()) + 1, ?PORT_NAMES()).`

As long as there are no more than 16 schedulers, there will never be any lock contention on the port lock for the driver.

## Avoiding Copying Binaries When Calling a Driver

There are basically two ways to avoid copying a binary that is sent to a driver:

If the Data argument for port_control/3 is a binary, the driver will be passed a pointer to the contents of the binary and the binary will not be copied. If the Data argument is an iolist (list of binaries and lists), all binaries in the iolist will be copied.

Therefore, if you want to send both a pre-existing binary and some extra data to a driver without copying the binary, you must call port_control/3 twice; once with the binary and once with the extra data. However, that will only work if there is only one process communicating with the port (because otherwise another process can call the driver in-between the calls).

Implement an outputv callback (instead of an output callback) in the driver. If a driver has an outputv callback, refc binaries passed in an iolist in the Data argument for port_command/2 will be passed as references to the driver.

## Returning Small Binaries from a Driver

The runtime system can represent binaries up to 64 bytes as heap binaries. They are always copied when sent in messages, but they require less memory if they are not sent to another process and garbage collection is cheaper.

If you know that the binaries you return are always small, you are advised to use driver API calls that do not require a pre-allocated binary, for example, driver_output() or erl_drv_output_term(), using the ERL_DRV_BUF2BINARY format, to allow the runtime to construct a heap binary.

## Returning Large Binaries without Copying from a Driver

To avoid copying data when a large binary is sent or returned from the driver to an Erlang process, the driver must first allocate the binary and then send it to an Erlang process in some way.

Use driver_alloc_binary() to allocate a binary.

There are several ways to send a binary created with driver_alloc_binary():

From the control callback, a binary can be returned if set_port_control_flags() has been called with the flag value PORT_CONTROL_FLAG_BINARY. A single binary can be sent with driver_output_binary(). Using erl_drv_output_term() or erl_drv_send_term(), a binary can be included in an Erlang term.