How to Implement a Driver — erts v16.3

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# How to Implement a Driver

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#### Note

This section was written a long time ago. Most of it is still valid, as it explains important concepts, but this was written for an older driver interface so the examples do not work anymore. The reader is encouraged to read the `erl_driver` and `driver_entry` documentation also.

## Introduction

This section describes how to build your own driver for Erlang.

A driver in Erlang is a library written in C, which is linked to the Erlang emulator and called from Erlang. Drivers can be used when C is more suitable than Erlang, to speed up things, or to provide access to OS resources not directly accessible from Erlang.

A driver can be dynamically loaded, as a shared library (known as a DLL on Windows), or statically loaded, linked with the emulator when it is compiled and linked. Only dynamically loaded drivers are described here, statically linked drivers are beyond the scope of this section.

#### Warning

When a driver is loaded it is executed in the context of the emulator, shares the same memory and the same thread. This means that all operations in the driver must be non-blocking, and that any crash in the driver brings the whole emulator down. In short, be careful.

## Sample Driver

This section describes a simple driver for accessing a postgres database using the libpq C client library. Postgres is used because it is free and open source. For information on postgres, see www.postgres.org.

The driver is synchronous, it uses the synchronous calls of the client library. This is only for simplicity, but not good, as it halts the emulator while waiting for the database. This is improved below with an asynchronous sample driver.

The code is straightforward: all communication between Erlang and the driver is done with `port_control/3`, and the driver returns data back using the `rbuf`.

An Erlang driver only exports one function: the driver entry function. This is defined with a macro, `DRIVER_INIT`, which returns a pointer to a C `struct` containing the entry points that are called from the emulator. The `struct` defines the entries that the emulator calls to call the driver, with a `NULL` pointer for entries that are not defined and used by the driver.

The `start` entry is called when the driver is opened as a port with `open_port/2`. Here we allocate memory for a user data structure. This user data is passed every time the emulator calls us. First we store the driver handle, as it is needed in later calls. We allocate memory for the connection handle that is used by LibPQ. We also set the port to return allocated driver binaries, by setting flag `PORT_CONTROL_FLAG_BINARY`, calling `set_port_control_flags`. (This is because we do not know if our data will fit in the result buffer of `control`, which has a default size, 64 bytes, set up by the emulator.)

An entry `init` is called when the driver is loaded. However, we do not use this, as it is executed only once, and we want to have the possibility of several instances of the driver.

The `stop` entry is called when the port is closed.

The `control` entry is called from the emulator when the Erlang code calls `port_control/3`, to do the actual work. We have defined a simple set of commands: `connect` to log in to the database, `disconnect` to log out, and `select` to send a SQL-query and get the result. All results are returned through `rbuf`. The library `ei` in `erl_interface` is used to encode data in binary term format. The result is returned to the emulator as binary terms, so `binary_to_term` is called in Erlang to convert the result to term form.

The code is available in `pg_sync.c` in the `sample` directory of `erts`.

The driver entry contains the functions that will be called by the emulator. In this example, only `start`, `stop`, and `control` are provided:

    /* Driver interface declarations */
    static ErlDrvData start(ErlDrvPort port, char *command);
    static void stop(ErlDrvData drv_data);
    static int control(ErlDrvData drv_data, unsigned int command, char *buf,
                       int len, char **rbuf, int rlen);

    static ErlDrvEntry pq_driver_entry = {
        NULL,                        /* init */
        start,
        stop,
        NULL,                        /* output */
        NULL,                        /* ready_input */
        NULL,                        /* ready_output */
        "pg_sync",                   /* the name of the driver */
        NULL,                        /* finish */
        NULL,                        /* handle */
        control,
        NULL,                        /* timeout */
        NULL,                        /* outputv */
        NULL,                        /* ready_async */
        NULL,                        /* flush */
        NULL,                        /* call */
        NULL                         /* event */
    };

We have a structure to store state needed by the driver, in this case we only need to keep the database connection:

    typedef struct our_data_s {
        PGconn* conn;
    } our_data_t;

The control codes that we have defined are as follows:

    /* Keep the following definitions in alignment with the
     * defines in erl_pq_sync.erl
     */

    #define DRV_CONNECT             'C'
    #define DRV_DISCONNECT          'D'
    #define DRV_SELECT              'S'

This returns the driver structure. The macro `DRIVER_INIT` defines the only exported function. All the other functions are static, and will not be exported from the library.

    /* INITIALIZATION AFTER LOADING */

    /*
     * This is the init function called after this driver has been loaded.
     * It must *not* be declared static. Must return the address to
     * the driver entry.
     */

    DRIVER_INIT(pq_drv)
    {
        return &pq_driver_entry;
    }

Here some initialization is done, `start` is called from `open_port/2`. The data will be passed to `control` and `stop`.

    /* DRIVER INTERFACE */
    static ErlDrvData start(ErlDrvPort port, char *command)
    {
        our_data_t* data;

        data = (our_data_t*)driver_alloc(sizeof(our_data_t));
        data->conn = NULL;
        set_port_control_flags(port, PORT_CONTROL_FLAG_BINARY);
        return (ErlDrvData)data;
    }

We call disconnect to log out from the database. (This should have been done from Erlang, but just in case.)

    static int do_disconnect(our_data_t* data, ei_x_buff* x);

    static void stop(ErlDrvData drv_data)
    {
        our_data_t* data = (our_data_t*)drv_data;

        do_disconnect(data, NULL);
        driver_free(data);
    }

We use the binary format only to return data to the emulator; input data is a string parameter for `connect` and `select`. The returned data consists of Erlang terms.

The functions `get_s` and `ei_x_to_new_binary` are utilities that are used to make the code shorter. `get_s` duplicates the string and zero-terminates it, as the postgres client library wants that. `ei_x_to_new_binary` takes an `ei_x_buff` buffer, allocates a binary, and copies the data there. This binary is returned in `*rbuf`. (Notice that this binary is freed by the emulator, not by us.)

    static char* get_s(const char* buf, int len);
    static int do_connect(const char *s, our_data_t* data, ei_x_buff* x);
    static int do_select(const char* s, our_data_t* data, ei_x_buff* x);

    /* As we are operating in binary mode, the return value from control
     * is irrelevant, as long as it is not negative.
     */
    static int control(ErlDrvData drv_data, unsigned int command, char *buf,
                       int len, char **rbuf, int rlen)
    {
        int r;
        ei_x_buff x;
        our_data_t* data = (our_data_t*)drv_data;
        char* s = get_s(buf, len);
        ei_x_new_with_version(&x);
        switch (command) {
            case DRV_CONNECT:    r = do_connect(s, data, &x);  break;
            case DRV_DISCONNECT: r = do_disconnect(data, &x);  break;
            case DRV_SELECT:     r = do_select(s, data, &x);   break;
            default:             r = -1;        break;
        }
        *rbuf = (char*)ei_x_to_new_binary(&x);
        ei_x_free(&x);
        driver_free(s);
        return r;
    }

`do_connect` is where we log in to the database. If the connection was successful, we store the connection handle in the driver data, and return `'ok'`. Otherwise, we return the error message from postgres and store `NULL` in the driver data.

    static int do_connect(const char *s, our_data_t* data, ei_x_buff* x)
    {
        PGconn* conn = PQconnectdb(s);
        if (PQstatus(conn) != CONNECTION_OK) {
            encode_error(x, conn);
            PQfinish(conn);
            conn = NULL;
        } else {
            encode_ok(x);
        }
        data->conn = conn;
        return 0;
    }

If we are connected (and if the connection handle is not `NULL`), we log out from the database. We need to check if we should encode an `'ok'`, as we can get here from function `stop`, which does not return data to the emulator:

    static int do_disconnect(our_data_t* data, ei_x_buff* x)
    {
        if (data->conn == NULL)
            return 0;
        PQfinish(data->conn);
        data->conn = NULL;
        if (x != NULL)
            encode_ok(x);
        return 0;
    }

We execute a query and encode the result. Encoding is done in another C module, `pg_encode.c`, which is also provided as sample code.

    static int do_select(const char* s, our_data_t* data, ei_x_buff* x)
    {
       PGresult* res = PQexec(data->conn, s);
        encode_result(x, res, data->conn);
        PQclear(res);
        return 0;
    }

Here we check the result from postgres. If it is data, we encode it as lists of lists with column data. Everything from postgres is C strings, so we use `ei_x_encode_string` to send the result as strings to Erlang. (The head of the list contains the column names.)

    void encode_result(ei_x_buff* x, PGresult* res, PGconn* conn)
    {
        int row, n_rows, col, n_cols;
        switch (PQresultStatus(res)) {
        case PGRES_TUPLES_OK:
            n_rows = PQntuples(res);
            n_cols = PQnfields(res);
            ei_x_encode_tuple_header(x, 2);
            encode_ok(x);
            ei_x_encode_list_header(x, n_rows+1);
            ei_x_encode_list_header(x, n_cols);
            for (col = 0; col < n_cols; ++col) {
                ei_x_encode_string(x, PQfname(res, col));
            }
            ei_x_encode_empty_list(x);
            for (row = 0; row < n_rows; ++row) {
                ei_x_encode_list_header(x, n_cols);
                for (col = 0; col < n_cols; ++col) {
                    ei_x_encode_string(x, PQgetvalue(res, row, col));
                }
                ei_x_encode_empty_list(x);
            }
            ei_x_encode_empty_list(x);
            break;
        case PGRES_COMMAND_OK:
            ei_x_encode_tuple_header(x, 2);
            encode_ok(x);
            ei_x_encode_string(x, PQcmdTuples(res));
            break;
        default:
            encode_error(x, conn);
            break;
        }
    }

## Compiling and Linking the Sample Driver

The driver is to be compiled and linked to a shared library (DLL on Windows). With gcc, this is done with link flags `-shared` and `-fpic`. As we use the `ei` library, we should include it too. There are several versions of `ei`, compiled for debug or non-debug and multi-threaded or single-threaded. In the makefile for the samples, the `obj` directory is used for the `ei` library, meaning that we use the non-debug, single-threaded version.

## Calling a Driver as a Port in Erlang

Before a driver can be called from Erlang, it must be loaded and opened. Loading is done using the `erl_ddll` module (the `erl_ddll` driver that loads dynamic driver is actually a driver itself). If loading is successful, the port can be opened with `open_port/2`. The port name must match the name of the shared library and the name in the driver entry structure.

When the port has been opened, the driver can be called. In the `pg_sync` example, we do not have any data from the port, only the return value from the `port_control/3`.

The following code is the Erlang part of the synchronous postgres driver, `pg_sync.erl`:

    -module(pg_sync).

    -define(DRV_CONNECT, 1).
    -define(DRV_DISCONNECT, 2).
    -define(DRV_SELECT, 3).

    -export([connect/1, disconnect/1, select/2]).

    connect(ConnectStr) ->
        case erl_ddll:load_driver(".", "pg_sync") of
            ok -> ok;
            {error, already_loaded} -> ok;
            E -> exit({error, E})
        end,
        Port = open_port({spawn, ?MODULE}, []),
        case binary_to_term(port_control(Port, ?DRV_CONNECT, ConnectStr)) of
            ok -> {ok, Port};
            Error -> Error
        end.

    disconnect(Port) ->
        R = binary_to_term(port_control(Port, ?DRV_DISCONNECT, "")),
        port_close(Port),
        R.

    select(Port, Query) ->
        binary_to_term(port_control(Port, ?DRV_SELECT, Query)).

The API is simple:

- `connect/1` loads the driver, opens it, and logs on to the database, returning the Erlang port if successful.
- `select/2` sends a query to the driver and returns the result.
- `disconnect/1` closes the database connection and the driver. (However, it does not unload it.)

The connection string is to be a connection string for postgres.

The driver is loaded with `erl_ddll:load_driver/2`. If this is successful, or if it is already loaded, it is opened. This will call the `start` function in the driver.

We use the `port_control/3` function for all calls into the driver. The result from the driver is returned immediately and converted to terms by calling `binary_to_term/1`. (We trust that the terms returned from the driver are well-formed, otherwise the `binary_to_term/1` calls could be contained in a `catch`.)

## Sample Asynchronous Driver

Sometimes database queries can take a long time to complete, in our `pg_sync` driver, the emulator halts while the driver is doing its job. This is often not acceptable, as no other Erlang process gets a chance to do anything. To improve on our postgres driver, we re-implement it using the asynchronous calls in LibPQ.

The asynchronous version of the driver is in the sample files `pg_async.c` and `pg_asyng.erl`.

    /* Driver interface declarations */
    static ErlDrvData start(ErlDrvPort port, char *command);
    static void stop(ErlDrvData drv_data);
    static int control(ErlDrvData drv_data, unsigned int command, char *buf,
                       int len, char **rbuf, int rlen);
    static void ready_io(ErlDrvData drv_data, ErlDrvEvent event);

    static ErlDrvEntry pq_driver_entry = {
        NULL,                     /* init */
        start,
        stop,
        NULL,                     /* output */
        ready_io,                 /* ready_input */
        ready_io,                 /* ready_output */
        "pg_async",               /* the name of the driver */
        NULL,                     /* finish */
        NULL,                     /* handle */
        control,
        NULL,                     /* timeout */
        NULL,                     /* outputv */
        NULL,                     /* ready_async */
        NULL,                     /* flush */
        NULL,                     /* call */
        NULL                      /* event */
    };

    typedef struct our_data_t {
        PGconn* conn;
        ErlDrvPort port;
        int socket;
        int connecting;
    } our_data_t;

Some things have changed from `pg_sync.c`: we use the entry `ready_io` for `ready_input` and `ready_output`, which is called from the emulator only when there is input to be read from the socket. (Actually, the socket is used in a `select` function inside the emulator, and when the socket is signaled, indicating there is data to read, the `ready_input` entry is called. More about this below.)

Our driver data is also extended, we keep track of the socket used for communication with postgres, and also the port, which is needed when we send data to the port with `driver_output`. We have a flag `connecting` to tell whether the driver is waiting for a connection or waiting for the result of a query. (This is needed, as the entry `ready_io` is called both when connecting and when there is a query result.)

    static int do_connect(const char *s, our_data_t* data)
    {
        PGconn* conn = PQconnectStart(s);
        if (PQstatus(conn) == CONNECTION_BAD) {
            ei_x_buff x;
            ei_x_new_with_version(&x);
            encode_error(&x, conn);
            PQfinish(conn);
            conn = NULL;
            driver_output(data->port, x.buff, x.index);
            ei_x_free(&x);
        }
        PQconnectPoll(conn);
        int socket = PQsocket(conn);
        data->socket = socket;
        driver_select(data->port, (ErlDrvEvent)socket, DO_READ, 1);
        driver_select(data->port, (ErlDrvEvent)socket, DO_WRITE, 1);
        data->conn = conn;
        data->connecting = 1;
        return 0;
    }

The `connect` function looks a bit different too. We connect using the asynchronous `PQconnectStart` function. After the connection is started, we retrieve the socket for the connection with `PQsocket`. This socket is used with the `driver_select` function to wait for connection. When the socket is ready for input or for output, the `ready_io` function is called.

Notice that we only return data (with `driver_output`) if there is an error here, otherwise we wait for the connection to be completed, in which case our `ready_io` function is called.

    static int do_select(const char* s, our_data_t* data)
    {
        data->connecting = 0;
        PGconn* conn = data->conn;
        /* if there's an error return it now */
        if (PQsendQuery(conn, s) == 0) {
            ei_x_buff x;
            ei_x_new_with_version(&x);
            encode_error(&x, conn);
            driver_output(data->port, x.buff, x.index);
            ei_x_free(&x);
        }
        /* else wait for ready_output to get results */
        return 0;
    }

The `do_select` function initiates a select, and returns if there is no immediate error. The result is returned when `ready_io` is called.

    static void ready_io(ErlDrvData drv_data, ErlDrvEvent event)
    {
        PGresult* res = NULL;
        our_data_t* data = (our_data_t*)drv_data;
        PGconn* conn = data->conn;
        ei_x_buff x;
        ei_x_new_with_version(&x);
        if (data->connecting) {
            ConnStatusType status;
            PQconnectPoll(conn);
            status = PQstatus(conn);
            if (status == CONNECTION_OK)
                encode_ok(&x);
            else if (status == CONNECTION_BAD)
                encode_error(&x, conn);
        } else {
            PQconsumeInput(conn);
            if (PQisBusy(conn))
                return;
            res = PQgetResult(conn);
            encode_result(&x, res, conn);
            PQclear(res);
            for (;;) {
                res = PQgetResult(conn);
                if (res == NULL)
                    break;
                PQclear(res);
            }
        }
        if (x.index > 1) {
            driver_output(data->port, x.buff, x.index);
            if (data->connecting)
                driver_select(data->port, (ErlDrvEvent)data->socket, DO_WRITE, 0);
        }
        ei_x_free(&x);
    }

The `ready_io` function is called when the socket we got from postgres is ready for input or output. Here we first check if we are connecting to the database. In that case, we check connection status and return OK if the connection is successful, or error if it is not. If the connection is not yet established, we simply return; `ready_io` is called again.

If we have a result from a connect, indicated by having data in the `x` buffer, we no longer need to select on output (`ready_output`), so we remove this by calling `driver_select`.

If we are not connecting, we wait for results from a `PQsendQuery`, so we get the result and return it. The encoding is done with the same functions as in the earlier example.

Error handling is to be added here, for example, checking that the socket is still open, but this is only a simple example.

The Erlang part of the asynchronous driver consists of the sample file `pg_async.erl`.

    -module(pg_async).

    -define(DRV_CONNECT, $C).
    -define(DRV_DISCONNECT, $D).
    -define(DRV_SELECT, $S).

    -export([connect/1, disconnect/1, select/2]).

    connect(ConnectStr) ->
        case erl_ddll:load_driver(".", "pg_async") of
            ok -> ok;
            {error, already_loaded} -> ok;
            _ -> exit({error, could_not_load_driver})
        end,
        Port = open_port({spawn, ?MODULE}, [binary]),
        port_control(Port, ?DRV_CONNECT, ConnectStr),
        case return_port_data(Port) of
            ok ->
                {ok, Port};
            Error ->
                Error
        end.

    disconnect(Port) ->
        port_control(Port, ?DRV_DISCONNECT, ""),
        R = return_port_data(Port),
        port_close(Port),
        R.

    select(Port, Query) ->
        port_control(Port, ?DRV_SELECT, Query),
        return_port_data(Port).

    return_port_data(Port) ->
        receive
            {Port, {data, Data}} ->
                binary_to_term(Data)
        end.

The Erlang code is slightly different, as we do not return the result synchronously from `port_control/3`, instead we get it from `driver_output` as data in the message queue. The function `return_port_data` above receives data from the port. As the data is in binary format, we use `binary_to_term/1` to convert it to an Erlang term. Notice that the driver is opened in binary mode (`open_port/2` is called with option `[binary]`). This means that data sent from the driver to the emulator is sent as binaries. Without option `binary`, they would have been lists of integers.

## An Asynchronous Driver Using driver_async

As a final example we demonstrate the use of `driver_async`. We also use the driver term interface. The driver is written in C++. This enables us to use an algorithm from STL. We use the `next_permutation` algorithm to get the next permutation of a list of integers. For large lists (> 100,000 elements), this takes some time, so we perform this as an asynchronous task.

The asynchronous API for drivers is complicated. First, the work must be prepared. In the example, this is done in `output`. We could have used `control`, but we want some variation in the examples. In our driver, we allocate a structure that contains anything that is needed for the asynchronous task to do the work. This is done in the main emulator thread. Then the asynchronous function is called from a driver thread, separate from the main emulator thread. Notice that the driver functions are not re-entrant, so they are not to be used. Finally, after the function is completed, the driver callback `ready_async` is called from the main emulator thread, this is where we return the result to Erlang. (We cannot return the result from within the asynchronous function, as we cannot call the driver functions.)

The following code is from the sample file `next_perm.cc`. The driver entry looks like before, but also contains the callback `ready_async`.

    static ErlDrvEntry next_perm_driver_entry = {
        NULL,                        /* init */
        start,
        NULL,                        /* stop */
        output,
        NULL,                        /* ready_input */
        NULL,                        /* ready_output */
        "next_perm",                 /* the name of the driver */
        NULL,                        /* finish */
        NULL,                        /* handle */
        NULL,                        /* control */
        NULL,                        /* timeout */
        NULL,                        /* outputv */
        ready_async,
        NULL,                        /* flush */
        NULL,                        /* call */
        NULL                         /* event */
    };

The `output` function allocates the work area of the asynchronous function. As we use C++, we use a struct, and stuff the data in it. We must copy the original data, it is not valid after we have returned from the `output` function, and the `do_perm` function is called later, and from another thread. We return no data here, instead it is sent later from the `ready_async` callback.

The `async_data` is passed to the `do_perm` function. We do not use a `async_free` function (the last argument to `driver_async`), it is only used if the task is cancelled programmatically.

    struct our_async_data {
        bool prev;
        vector<int> data;
        our_async_data(ErlDrvPort p, int command, const char* buf, int len);
    };

    our_async_data::our_async_data(ErlDrvPort p, int command,
                                   const char* buf, int len)
        : prev(command == 2),
          data((int*)buf, (int*)buf + len / sizeof(int))
    {
    }

    static void do_perm(void* async_data);

    static void output(ErlDrvData drv_data, char *buf, int len)
    {
        if (*buf < 1 || *buf > 2) return;
        ErlDrvPort port = reinterpret_cast<ErlDrvPort>(drv_data);
        void* async_data = new our_async_data(port, *buf, buf+1, len);
        driver_async(port, NULL, do_perm, async_data, do_free);
    }

In the `do_perm` we do the work, operating on the structure that was allocated in `output`.

    static void do_perm(void* async_data)
    {
        our_async_data* d = reinterpret_cast<our_async_data*>(async_data);
        if (d->prev)
            prev_permutation(d->data.begin(), d->data.end());
        else
            next_permutation(d->data.begin(), d->data.end());
    }

In the `ready_async` function the output is sent back to the emulator. We use the driver term format instead of `ei`. This is the only way to send Erlang terms directly to a driver, without having the Erlang code to call `binary_to_term/1`. In the simple example this works well, and we do not need to use `ei` to handle the binary term format.

When the data is returned, we deallocate our data.

    static void ready_async(ErlDrvData drv_data, ErlDrvThreadData async_data)
    {
        ErlDrvPort port = reinterpret_cast<ErlDrvPort>(drv_data);
        our_async_data* d = reinterpret_cast<our_async_data*>(async_data);
        int n = d->data.size(), result_n = n*2 + 3;
        ErlDrvTermData *result = new ErlDrvTermData[result_n], *rp = result;
        for (vector<int>::iterator i = d->data.begin();
             i != d->data.end(); ++i) {
            *rp++ = ERL_DRV_INT;
            *rp++ = *i;
        }
        *rp++ = ERL_DRV_NIL;
        *rp++ = ERL_DRV_LIST;
        *rp++ = n+1;
        driver_output_term(port, result, result_n);
        delete[] result;
        delete d;
    }

This driver is called like the others from Erlang. However, as we use `driver_output_term`, there is no need to call `binary_to_term/1`. The Erlang code is in the sample file `next_perm.erl`.

The input is changed into a list of integers and sent to the driver.

    -module(next_perm).

    -export([next_perm/1, prev_perm/1, load/0, all_perm/1]).

    load() ->
        case whereis(next_perm) of
            undefined ->
                case erl_ddll:load_driver(".", "next_perm") of
                    ok -> ok;
                    {error, already_loaded} -> ok;
                    E -> exit(E)
                end,
                Port = open_port({spawn, "next_perm"}, []),
                register(next_perm, Port);
            _ ->
                ok
        end.

    list_to_integer_binaries(L) ->
        [<<I:32/integer-native>> || I <- L].

    next_perm(L) ->
        next_perm(L, 1).

    prev_perm(L) ->
        next_perm(L, 2).

    next_perm(L, Nxt) ->
        load(),
        B = list_to_integer_binaries(L),
        port_control(next_perm, Nxt, B),
        receive
            Result ->
                Result
        end.

    all_perm(L) ->
        New = prev_perm(L),
        all_perm(New, L, [New]).

    all_perm(L, L, Acc) ->
        Acc;
    all_perm(L, Orig, Acc) ->
        New = prev_perm(L),
        all_perm(New, Orig, [New | Acc]).

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