aboutsummaryrefslogblamecommitdiffstats
path: root/erts/doc/src/driver.xml
blob: c396ee0b909dc228420f846c2ff52ff58df6fcdf (plain) (tree)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812











































































































































































































































































































































































































































































































































































































































































































































































































































                                                                                                   
<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2001</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>How to implement a driver</title>
    <prepared>Jakob C</prepared>
    <docno></docno>
    <date>2000-11-28</date>
    <rev>PA1</rev>
    <file>driver.xml</file>
  </header>

  <note><p>This document was written a long time ago. A lot of it is still
           valid, but some things have changed since it was first written.
	   Updates of this document are planned for the future. The reader
	   is encouraged to also read the
	   <seealso marker="erl_driver">erl_driver</seealso>, and the
	   <seealso marker="erl_driver">driver_entry</seealso> documentation.
   </p></note>

  <section>
    <title>Introduction</title>
    <p>This chapter tells you how to build your own driver for erlang.</p>
    <p>A driver in Erlang is a library written in C, that is linked to
      the Erlang emulator and called from erlang. Drivers can be used
      when C is more suitable than Erlang, to speed things up, or to
      provide access to OS resources not directly accessible from
      Erlang.</p>
    <p>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 chapter.</p>
    <p>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 will bring the whole emulator down. In
      short: you have to be extremely careful!</p>
    <p></p>
  </section>

  <section>
    <title>Sample driver</title>
    <p>This is a simple driver for accessing a postgres
      database using the libpq C client library. Postgres
      is used because it's free and open source. For information
      on postgres, refer to the website www.postgres.org.</p>
    <p>The driver is synchronous, it uses the synchronous calls of
      the client library. This is only for simplicity, and is
      generally not good, since it will
      halt the emulator while waiting for the database.
      This will be improved on below with an asynchronous
      sample driver.</p>
    <p>The code is quite straight-forward: all
      communication between Erlang and the driver
      is done with <c><![CDATA[port_control/3]]></c>, and the
      driver returns data back using the <c><![CDATA[rbuf]]></c>.</p>
    <p>An Erlang driver only exports one function: the driver
      entry function. This is defined with a macro,
      <c><![CDATA[DRIVER_INIT]]></c>, and returns a pointer to a
      C <c><![CDATA[struct]]></c> containing the entry points that are
      called from the emulator. The <c><![CDATA[struct]]></c> defines the
      entries that the emulator calls to call the driver, with
      a <c><![CDATA[NULL]]></c> pointer for entries that are not defined
      and used by the driver.</p>
    <p>The <c><![CDATA[start]]></c> entry is called when the driver
      is opened as a port with <c><![CDATA[open_port/2]]></c>. Here
      we allocate memory for a user data structure.
      This user data will be passed every time the emulator
      calls us. First we store the driver handle, because it
      is needed in subsequent 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 the flag <c><![CDATA[PORT_CONTROL_FLAG_BINARY]]></c>, calling
      <c><![CDATA[set_port_control_flags]]></c>. (This is because
      we don't know whether our data will fit in the
      result buffer of <c><![CDATA[control]]></c>, which has a default size
      set up by the emulator, currently 64 bytes.)</p>
    <p>There is an entry <c><![CDATA[init]]></c> which is called when
      the driver is loaded, but we don't use this, since
      it is executed only once, and we want to have the
      possibility of several instances of the driver.</p>
    <p>The <c><![CDATA[stop]]></c> entry is called when the port
      is closed.</p>
    <p>The <c><![CDATA[control]]></c> entry is called from the emulator
      when the Erlang code calls <c><![CDATA[port_control/3]]></c>,
      to do the actual work. We have defined a simple set of
      commands: <c><![CDATA[connect]]></c> to login to the database, <c><![CDATA[disconnect]]></c>
      to log out and <c><![CDATA[select]]></c> to send a SQL-query and get the result.
      All results are returned through <c><![CDATA[rbuf]]></c>.
      The library <c><![CDATA[ei]]></c> in <c><![CDATA[erl_interface]]></c> is used
      to encode data in binary term format. The result is returned
      to the emulator as binary terms, so <c><![CDATA[binary_to_term]]></c>
      is called in Erlang to convert the result to term form.</p>
    <p>The code is available in <c><![CDATA[pg_sync.c]]></c> in the <c><![CDATA[sample]]></c>
      directory of <c><![CDATA[erts]]></c>.</p>
    <p>The driver entry contains the functions that
      will be called by the emulator. In our simple
      example, we only provide <c><![CDATA[start]]></c>, <c><![CDATA[stop]]></c>
      and <c><![CDATA[control]]></c>.</p>
    <code type="none"><![CDATA[
/* 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 */
};
    ]]></code>
    <p>We have a structure to store state needed by the driver,
      in this case we only need to keep the database connection.</p>
    <code type="none"><![CDATA[
typedef struct our_data_s {
    PGconn* conn;
} our_data_t;
    ]]></code>
    <p>These are control codes we have defined.</p>
    <code type="none"><![CDATA[
/* 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'
    ]]></code>
    <p>This just returns the driver structure. The macro
      <c><![CDATA[DRIVER_INIT]]></c> defines the only exported function.
      All the other functions are static, and will not be exported
      from the library.</p>
    <code type="none"><![CDATA[
/* 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;
}
    ]]></code>
    <p>Here we do some initialization, <c><![CDATA[start]]></c> is called from
      <c><![CDATA[open_port]]></c>. The data will be passed to <c><![CDATA[control]]></c>
      and <c><![CDATA[stop]]></c>.</p>
    <code type="none"><![CDATA[
/* 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;
}
    ]]></code>
    <p>We call disconnect to log out from the database.
      (This should have been done from Erlang, but just in case.)</p>
    <code type="none"><![CDATA[
 static int do_disconnect(our_data_t* data, ei_x_buff* x);

static void stop(ErlDrvData drv_data)
{
    do_disconnect((our_data_t*)drv_data, NULL);
}
    ]]></code>
    <p>We use the binary format only to return data to the emulator;
      input data is a string paramater for <c><![CDATA[connect]]></c> and
      <c><![CDATA[select]]></c>. The returned data consists of Erlang terms.</p>
    <p>The functions <c><![CDATA[get_s]]></c> and <c><![CDATA[ei_x_to_new_binary]]></c> are
      utilities that is used to make the code shorter. <c><![CDATA[get_s]]></c>
      duplicates the string and zero-terminates it, since the
      postgres client library wants that. <c><![CDATA[ei_x_to_new_binary]]></c>
      takes an <c><![CDATA[ei_x_buff]]></c> buffer and allocates a binary and
      copies the data there. This binary is returned in <c><![CDATA[*rbuf]]></c>.
      (Note that this binary is freed by the emulator, not by us.)</p>
    <code type="none"><![CDATA[
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);

/* Since 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;
}
    ]]></code>
    <p>In <c><![CDATA[do_connect]]></c> is where we log in to the database. If the connection
      was successful we store the connection handle in our driver
      data, and return ok. Otherwise, we return the error message
      from postgres, and store <c><![CDATA[NULL]]></c> in the driver data.</p>
    <code type="none"><![CDATA[
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;
}
    ]]></code>
    <p>If we are connected (if the connection handle is not <c><![CDATA[NULL]]></c>),
      we log out from the database. We need to check if a we should
      encode an ok, since we might get here from the <c><![CDATA[stop]]></c>
      function, which doesn't return data to the emulator.</p>
    <code type="none"><![CDATA[
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;
}
    ]]></code>
    <p>We execute a query and encodes the result. Encoding is done
      in another C module, <c><![CDATA[pg_encode.c]]></c> which is also provided
      as sample code.</p>
    <code type="none"><![CDATA[
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;
}
    ]]></code>
    <p>Here we simply checks the result from postgres, and
      if it's data we encode it as lists of lists with
      column data. Everything from postgres is C strings,
      so we just use <c><![CDATA[ei_x_encode_string]]></c> to send
      the result as strings to Erlang. (The head of the list
      contains the column names.)</p>
    <code type="none"><![CDATA[
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;
    }
}
    ]]></code>
  </section>

  <section>
    <title>Compiling and linking the sample driver</title>
    <p>The driver should be compiled and linked to a shared
      library (DLL on windows). With gcc this is done
      with the link flags <c><![CDATA[-shared]]></c> and <c><![CDATA[-fpic]]></c>.
      Since we use the <c><![CDATA[ei]]></c> library we should include
      it too. There are several versions of <c><![CDATA[ei]]></c>, compiled
      for debug or non-debug and multi-threaded or single-threaded.
      In the makefile for the samples the <c><![CDATA[obj]]></c> directory
      is used for the <c><![CDATA[ei]]></c> library, meaning that we use
      the non-debug, single-threaded version.</p>
  </section>

  <section>
    <title>Calling a driver as a port in Erlang</title>
    <p>Before a driver can be called from Erlang, it must be
      loaded and opened. Loading is done using the <c><![CDATA[erl_ddll]]></c>
      module (the <c><![CDATA[erl_ddll]]></c> driver that loads dynamic
      driver, is actually a driver itself). If loading is ok
      the port can be opened with <c><![CDATA[open_port/2]]></c>. The port
      name must match the name of the shared library and
      the name in the driver entry structure.</p>
    <p>When the port has been opened, the driver can be called. In
      the <c><![CDATA[pg_sync]]></c> example, we don't have any data from
      the port, only the return value from the
      <c><![CDATA[port_control]]></c>.</p>
    <p>The following code is the Erlang part of the synchronous
      postgres driver, <c><![CDATA[pg_sync.erl]]></c>.</p>
    <code type="none"><![CDATA[
-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)).
    ]]></code>
    <p>The api is simple: <c><![CDATA[connect/1]]></c> loads the driver, opens it
      and logs on to the database, returning the Erlang port
      if successful, <c><![CDATA[select/2]]></c> sends a query to the driver,
      and returns the result, <c><![CDATA[disconnect/1]]></c> closes the
      database connection and the driver. (It does not unload it,
      however.) The connection string should be a connection
      string for postgres.</p>
    <p>The driver is loaded with <c><![CDATA[erl_ddll:load_driver/2]]></c>,
      and if this is successful, or if it's already loaded,
      it is opened. This will call the <c><![CDATA[start]]></c> function
      in the driver.</p>
    <p>We use the <c><![CDATA[port_control/3]]></c> function for all
      calls into the driver, the result from the driver is
      returned immediately, and converted to terms by calling
      <c><![CDATA[binary_to_term/1]]></c>. (We trust that the terms returned
      from the driver are well-formed, otherwise the
      <c><![CDATA[binary_to_term]]></c> calls could be contained in a
      <c><![CDATA[catch]]></c>.)</p>
  </section>

  <section>
    <title>Sample asynchronous driver</title>
    <p>Sometimes database queries can take long time to
      complete, in our <c><![CDATA[pg_sync]]></c> driver, the emulator
      halts while the driver is doing it's job. This is
      often not acceptable, since no other Erlang processes
      gets a chance to do anything. To improve on our
      postgres driver, we reimplement it using the asynchronous
      calls in LibPQ.</p>
    <p>The asynchronous version of the driver is in the
      sample files <c><![CDATA[pg_async.c]]></c> and <c><![CDATA[pg_asyng.erl]]></c>.</p>
    <code type="none"><![CDATA[
/* 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;
    ]]></code>
    <p>Here some things have changed from <c><![CDATA[pg_sync.c]]></c>: we use the
      entry <c><![CDATA[ready_io]]></c> for <c><![CDATA[ready_input]]></c> and
      <c><![CDATA[ready_output]]></c> which will be called from the emulator only
      when there is input to be read from the socket. (Actually, the
      socket is used in a <c><![CDATA[select]]></c> function inside
      the emulator, and when the socket is signalled,
      indicating there is data to read, the <c><![CDATA[ready_input]]></c> entry
      is called. More on this below.)</p>
    <p>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
      <c><![CDATA[driver_output]]></c>. We have a flag <c><![CDATA[connecting]]></c> to tell
      whether the driver is waiting for a connection or waiting
      for the result of a query. (This is needed since the entry
      <c><![CDATA[ready_io]]></c> will be called both when connecting and
      when there is query result.)</p>
    <code type="none"><![CDATA[
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;
}
    ]]></code>
    <p>The <c><![CDATA[connect]]></c> function looks a bit different too. We connect
      using the asynchronous <c><![CDATA[PQconnectStart]]></c> function. After the
      connection is started, we retrieve the socket for the connection
      with <c><![CDATA[PQsocket]]></c>. This socket is used with the
      <c><![CDATA[driver_select]]></c> function to wait for connection. When
      the socket is ready for input or for output, the <c><![CDATA[ready_io]]></c>
      function will be called.</p>
    <p>Note that we only return data (with <c><![CDATA[driver_output]]></c>) if there
      is an error here, otherwise we wait for the connection to be completed,
      in which case our <c><![CDATA[ready_io]]></c> function will be called.</p>
    <code type="none"><![CDATA[
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) {
\011ei_x_buff x;
\011ei_x_new_with_version(&x);
\011encode_error(&x, conn);
\011driver_output(data->port, x.buff, x.index);
\011ei_x_free(&x);
    }
    /* else wait for ready_output to get results */
    return 0;
}
    ]]></code>
    <p>The <c><![CDATA[do_select]]></c> function initiates a select, and returns
      if there is no immediate error. The actual result will be returned
      when <c><![CDATA[ready_io]]></c> is called.</p>
    <code type="none"><![CDATA[
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) {
\011ConnStatusType status;
\011PQconnectPoll(conn);
\011status = PQstatus(conn);
\011if (status == CONNECTION_OK)
\011    encode_ok(&x);
\011else if (status == CONNECTION_BAD)
\011    encode_error(&x, conn);
    } else {
\011PQconsumeInput(conn);
\011if (PQisBusy(conn))
\011    return;
\011res = PQgetResult(conn);
\011encode_result(&x, res, conn);
\011PQclear(res);
\011for (;;) {
\011    res = PQgetResult(conn);
\011    if (res == NULL)
\011\011break;
\011    PQclear(res);
\011}
    }
    if (x.index > 1) {
\011driver_output(data->port, x.buff, x.index);
\011if (data->connecting) 
\011    driver_select(data->port, (ErlDrvEvent)data->socket, DO_WRITE, 0);
    }
    ei_x_free(&x);
}
    ]]></code>
    <p>The <c><![CDATA[ready_io]]></c> function will be 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's not. If the
      connection is not yet established, we simply return; <c><![CDATA[ready_io]]></c>
      will be called again.</p>
    <p>If we have result from a connect, indicated that we have data in
      the <c><![CDATA[x]]></c> buffer, we no longer need to select on
      output (<c><![CDATA[ready_output]]></c>), so we remove this by calling
      <c><![CDATA[driver_select]]></c>.</p>
    <p>If we're not connecting, we're waiting for results from a
      <c><![CDATA[PQsendQuery]]></c>, so we get the result and return it. The
      encoding is done with the same functions as in the earlier
      example.</p>
    <p>We should add error handling here, for instance checking
      that the socket is still open, but this is just a simple
      example.</p>
    <p>The Erlang part of the asynchronous driver consists of the
      sample file <c><![CDATA[pg_async.erl]]></c>.</p>
    <code type="none"><![CDATA[
-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
\011ok -> ok;
\011{error, already_loaded} -> ok;
\011_ -> 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
\011ok -> 
\011    {ok, Port};
\011Error ->
\011    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
\011{Port, {data, Data}} ->
\011    binary_to_term(Data)
    end.
    ]]></code>
    <p>The Erlang code is slightly different, this is because we
      don't return the result synchronously from <c><![CDATA[port_control]]></c>,
      instead we get it from <c><![CDATA[driver_output]]></c> as data in the
      message queue. The function <c><![CDATA[return_port_data]]></c> above
      receives data from the port. Since the data is in
      binary format, we use <c><![CDATA[binary_to_term/1]]></c> to convert
      it to Erlang term. Note that the driver is opened in
      binary mode, <c><![CDATA[open_port/2]]></c> is called with the option
      <c><![CDATA[[binary]]]></c>. This means that data sent from the driver
      to the emulator is sent as binaries. Without the <c><![CDATA[binary]]></c>
      option, they would have been lists of integers.</p>
  </section>

  <section>
    <title>An asynchronous driver using driver_async</title>
    <p>As a final example we demonstrate the use of <c><![CDATA[driver_async]]></c>.
      We also use the driver term interface. The driver is written
      in C++. This enables us to use an algorithm from STL. We will
      use the <c><![CDATA[next_permutation]]></c> algorithm to get the next permutation
      of a list of integers. For large lists (more than 100000
      elements), this will take some time, so we will perform this
      as an asynchronous task.</p>
    <p>The asynchronous api for drivers are quite complicated. First
      of all, the work must be prepared. In our example we do this
      in <c><![CDATA[output]]></c>. We could have used <c><![CDATA[control]]></c> just as well,
      but we want some variation in our examples. In our driver, we allocate
      a structure that contains all 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. Note that the driver-
      functions are not reentrant, so they shouldn't be used.
      Finally, after the function is completed, the driver callback
      <c><![CDATA[ready_async]]></c> is called from the main emulator thread,
      this is where we return the result to Erlang. (We can't
      return the result from within the asynchronous function, since
      we can't call the driver-functions.)</p>
    <p>The code below is from the sample file <c><![CDATA[next_perm.cc]]></c>.</p>
    <p>The driver entry looks like before, but also contains the
      call-back <c><![CDATA[ready_async]]></c>.</p>
    <code type="none"><![CDATA[
static ErlDrvEntry next_perm_driver_entry = {
    NULL,\011\011\011/* init */
    start,
    NULL, \011\011\011/* stop */
    output,\011\011\011
    NULL,\011\011\011/* ready_input */
    NULL,\011\011\011/* ready_output */ 
    "next_perm",                /* the name of the driver */
    NULL,\011\011\011/* finish */
    NULL,\011\011\011/* handle */
    NULL,\011\011\011/* control */
    NULL,\011\011\011/* timeout */
    NULL,\011\011\011/* outputv */
    ready_async,
    NULL,\011\011\011/* flush */
    NULL,\011\011\011/* call */
    NULL\011\011\011/* event */
};
    ]]></code>
    <p>The <c><![CDATA[output]]></c> function allocates the work-area of the
      asynchronous function. Since we use C++, we use a struct,
      and stuff the data in it. We have to copy the original data,
      it is not valid after we have returned from the <c><![CDATA[output]]></c>
      function, and the <c><![CDATA[do_perm]]></c> function will be called later,
      and from another thread. We return no data here, instead it will
      be sent later from the <c><![CDATA[ready_async]]></c> call-back.</p>
    <p>The <c><![CDATA[async_data]]></c> will be passed to the <c><![CDATA[do_perm]]></c> function.
      We do not use a <c><![CDATA[async_free]]></c> function (the last argument to
      <c><![CDATA[driver_async]]></c>, it's only used if the task is cancelled
      programmatically.</p>
    <code type="none"><![CDATA[
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,
\011\011\011       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);
}
    ]]></code>
    <p>In the <c><![CDATA[do_perm]]></c> we simply do the work, operating
      on the structure that was allocated in <c><![CDATA[output]]></c>.</p>
    <code type="none"><![CDATA[
static void do_perm(void* async_data)
{
    our_async_data* d = reinterpret_cast<our_async_data*>(async_data);
    if (d->prev)
\011prev_permutation(d->data.begin(), d->data.end());
    else
\011next_permutation(d->data.begin(), d->data.end());
}
    ]]></code>
    <p>In the <c><![CDATA[ready_async]]></c> function, the output is sent back to the
      emulator. We use the driver term format instead of <c><![CDATA[ei]]></c>.
      This is the only way to send Erlang terms directly to a driver,
      without having the Erlang code to call <c><![CDATA[binary_to_term/1]]></c>. In
      our simple example this works well, and we don't need to use
      <c><![CDATA[ei]]></c> to handle the binary term format.</p>
    <p>When the data is returned we deallocate our data.</p>
    <code type="none"><![CDATA[
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();
\011 i != d->data.end(); ++i) {
\011*rp++ = ERL_DRV_INT;
\011*rp++ = *i;
    }
    *rp++ = ERL_DRV_NIL;
    *rp++ = ERL_DRV_LIST;
    *rp++ = n+1;
    driver_output_term(port, result, result_n);    
    delete[] result;
    delete d;
}
    ]]></code>
    <p>This driver is called like the others from Erlang, however, since
      we use <c><![CDATA[driver_output_term]]></c>, there is no need to call
      binary_to_term. The Erlang code is in the sample file
      <c><![CDATA[next_perm.erl]]></c>.</p>
    <p>The input is changed into a list of integers and sent to
      the driver.</p>
    <code type="none"><![CDATA[
-module(next_perm).

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

load() ->
    case whereis(next_perm) of
\011undefined ->
\011    case erl_ddll:load_driver(".", "next_perm") of
\011\011ok -> ok;
\011\011{error, already_loaded} -> ok;
\011\011E -> exit(E)
\011    end,
\011    Port = open_port({spawn, "next_perm"}, []),
\011    register(next_perm, Port);
\011_ ->
\011    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
\011Result ->
\011    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]).
    ]]></code>
  </section>
</chapter>