summaryrefslogtreecommitdiffstats
path: root/doc/afb-plugin-writing.html
blob: 20e25972c493cd3d0fcbbcc679b74f058df8e12b (plain)
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
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
<html>
<head>
  <link rel="stylesheet" type="text/css" href="doc.css">
  <meta charset="UTF-8">
</head>
<body>
<a name="HOWTO.WRITE.a.PLUGIN.for.AFB-DAEMON"></a>
<h1>HOWTO WRITE a PLUGIN for AFB-DAEMON</h1>

<pre><code>version: 1
Date:    27 mai 2016
Author:  José Bollo
</code></pre>

<p><ul>
 <li><a href="#HOWTO.WRITE.a.PLUGIN.for.AFB-DAEMON">HOWTO WRITE a PLUGIN for AFB-DAEMON</a>
 <ul>
  <li><a href="#Summary">Summary</a>
  <ul>
   <li><a href="#Nature.of.a.plugin">Nature of a plugin</a></li>
   <li><a href="#Kinds.of.plugins">Kinds of plugins</a>
   <ul>
    <li><a href="#Application.plugins">Application plugins</a></li>
    <li><a href="#Service.plugins">Service plugins</a></li>
   </ul>
   </li>
   <li><a href="#Live.cycle.of.a.plugin.within.afb-daemon">Live cycle of a plugin within afb-daemon</a></li>
   <li><a href="#Content.of.a.plugin">Content of a plugin</a>
   <ul>
    <li><a href="#The.name.of.the.plugin">The name of the plugin</a></li>
    <li><a href="#Names.of.verbs">Names of verbs</a></li>
    <li><a href="#The.initialisation.function">The initialisation function</a></li>
    <li><a href="#Functions.implementing.verbs">Functions implementing verbs</a>
</li>
   </ul>
   </li>
  </ul>
  </li>
  <li><a href="#The.Tic-Tac-Toe.example">The Tic-Tac-Toe example</a></li>
  <li><a href="#Dependencies.when.compiling">Dependencies when compiling</a></li>
  <li><a href="#Header.files.to.include">Header files to include</a></li>
  <li><a href="#Choosing.names">Choosing names</a>
  <ul>
   <li><a href="#Names.for.API..plugin.">Names for API (plugin)</a></li>
   <li><a href="#Names.for.verbs">Names for verbs</a></li>
   <li><a href="#Names.for.arguments">Names for arguments</a></li>
   <li><a href="#Forging.names.widely.available">Forging names widely available</a></li>
  </ul>
  </li>
  <li><a href="#Writing.a.synchronous.verb.implementation">Writing a synchronous verb implementation</a>
  <ul>
   <li><a href="#The.incoming.request">The incoming request</a></li>
   <li><a href="#Associating.a.context.to.the.session">Associating a context to the session</a></li>
   <li><a href="#Sending.the.reply.to.a.request">Sending the reply to a request</a></li>
  </ul>
  </li>
  <li><a href="#Getting.argument.of.invocation">Getting argument of invocation</a>
  <ul>
   <li><a href="#Basic.functions.for.querying.arguments">Basic functions for querying arguments</a></li>
   <li><a href="#Arguments.for.received.files">Arguments for received files</a></li>
   <li><a href="#Arguments.as.a.JSON.object">Arguments as a JSON object</a></li>
  </ul>
  </li>
  <li><a href="#Initialisation.of.the.plugin.and.declaration.of.verbs">Initialisation of the plugin and declaration of verbs</a></li>
  <li><a href="#Sending.messages.to.the.log.system">Sending messages to the log system</a>
  <ul>
   <li><a href="#Verbs.for.logging.messages">Verbs for logging messages</a></li>
   <li><a href="#Managing.verbosity">Managing verbosity</a></li>
   <li><a href="#Output.format.and.destination">Output format and destination</a></li>
  </ul>
  </li>
  <li><a href="#Sending.events">Sending events</a></li>
  <li><a href="#Writing.an.asynchronous.verb.implementation">Writing an asynchronous verb implementation</a></li>
  <li><a href="#How.to.build.a.plugin">How to build a plugin</a>
  <ul>
   <li><a href="#Example.for.cmake.meta.build.system">Example for cmake meta build system</a></li>
   <li><a href="#Exporting.the.function.pluginAfbV1Register">Exporting the function pluginAfbV1Register</a></li>
   <li><a href="#Building.within.yocto">Building within yocto</a></li>
  </ul>
  </li>
 </ul>
 </li>
</ul></p>

<a name="Summary"></a>
<h2>Summary</h2>

<p>The binder afb-daemon serves files through
the HTTP protocol and offers access to API&rsquo;s through
HTTP or WebSocket protocol.</p>

<p>The plugins are used to add API&rsquo;s to afb-daemon.
This part describes how to write a plugin for afb-daemon.
Excepting this summary, this part is intended to be read
by developpers.</p>

<p>Before going into details, through a tiny example,
a short overview plugins basis is needed.</p>

<a name="Nature.of.a.plugin"></a>
<h3>Nature of a plugin</h3>

<p>A plugin is a separate piece of code made of a shared library.
The plugin is loaded and activated by afb-daemon when afb-daemon
starts.</p>

<p>Technically, a plugin is not linked to any library of afb-daemon.</p>

<a name="Kinds.of.plugins"></a>
<h3>Kinds of plugins</h3>

<p>There is two kinds of plugins: application plugins and service
plugins.</p>

<a name="Application.plugins"></a>
<h4>Application plugins</h4>

<p>Application plugins are intended to be instanciated for each
application: when an application using that plugin is started,
its binder starts a new instance of the plugin.</p>

<p>It means that the application plugins mainly have only one
context to manage for one client.</p>

<a name="Service.plugins"></a>
<h4>Service plugins</h4>

<p>Service plugins are intended to be instanciated only one time
only and connected to many clients.</p>

<p>So either it does not manage context at all or otherwise,
if it manages context, it should be able to manage one context
per client.</p>

<p>In details, it may be useful to have service plugins at a user
level.</p>

<a name="Live.cycle.of.a.plugin.within.afb-daemon"></a>
<h3>Live cycle of a plugin within afb-daemon</h3>

<p>The plugins are loaded and activated when afb-daemon starts.</p>

<p>At start, the plugin initialise itself.
If it fails to initialise then afb-daemon stops.</p>

<p>Conversely, if it success to initialize, it must declare
a name, that must be unique, and a list of API&rsquo;s verbs.</p>

<p>When initialized, the functions implementing the API&rsquo;s verbs
of the plugin are activated on call.</p>

<p>At the end, nothing special is done by afb-daemon.
Consequently, developpers of plugins should use &lsquo;atexit&rsquo;
or &lsquo;on_exit&rsquo; during initialisation if they need to
perform specific actions when stopping.</p>

<a name="Content.of.a.plugin"></a>
<h3>Content of a plugin</h3>

<p>For afb-daemon, a plugin contains 2 different
things: names and functions.</p>

<p>There is two kind of names:
 - the name of the plugin,
 - the names of the verbs.</p>

<p>There is two kind of functions:
 - the initialisation function
 - functions implementing verbs</p>

<p>Afb-daemon translates the name of the method that is
invoked to a pair of API and verb names. For example,
the method named <strong>foo/bar</strong> translated to the API
name <strong>foo</strong> and the verb name <strong>bar</strong>.
To serve it, afb-daemon search the plugin that record
the name <strong>foo</strong> and if it also recorded the verb <strong>bar</strong>,
it calls the implementation function declared for this verb.</p>

<p>Afb-daemon make no distinction between lower case
and upper case when searching for a method.
Thus, The names <strong>TicTacToe/Board</strong> and <strong>tictactoe/borad</strong>
are equals.</p>

<a name="The.name.of.the.plugin"></a>
<h4>The name of the plugin</h4>

<p>The name of the plugin is also known as the name
of the API that defines the plugin.</p>

<p>This name is also known as the prefix.</p>

<p>The name of a plugin MUST be unique within afb-daemon.</p>

<p>For example, when a client of afb-daemon
calls a method named <strong>foo/bar</strong>. Afb-daemon
extracts the prefix <strong>foo</strong> and the suffix <strong>bar</strong>.
<strong>foo</strong> is the API name and must match a plugin name,
the plugin that implements the verb <strong>bar</strong>.</p>

<a name="Names.of.verbs"></a>
<h4>Names of verbs</h4>

<p>Each plugin exposes a set of verbs that can be called
by client of afb-daemon.</p>

<p>The name of a verb MUST be unique within a plugin.</p>

<p>Plugins link verbs to functions that are called
when clients emit requests for that verb.</p>

<p>For example, when a client of afb-daemon
calls a method named <strong>foo/bar</strong>.</p>

<a name="The.initialisation.function"></a>
<h4>The initialisation function</h4>

<p>The initialisation function serves several purposes.</p>

<ol>
<li><p>It allows afb-daemon to check the version
of the plugin using the name of the initialisation
functions that it found. Currently, the initialisation
function is named <strong>pluginAfbV1Register</strong>. It identifies
the first version of plugins.</p></li>
<li><p>It allows the plugin to initialise itself.</p></li>
<li><p>It serves to the plugin to declare names, descriptions,
requirements and implmentations of the verbs that it exposes.</p></li>
</ol>


<a name="Functions.implementing.verbs"></a>
<h4>Functions implementing verbs</h4>

<p>When a method is called, afb-daemon constructs a request
object and pass it to the implementation function for verb
within the plugin of the API.</p>

<p>An implementation function receives a request object that
is used to get arguments of the request, to send
answer, to store session data.</p>

<p>A plugin MUST send an answer to the request.</p>

<p>But it is not mandatory to send the answer
before to return from the implementing function.
This behaviour is important for implementing
asynchronous actions.</p>

<p>Implementation functions that always reply to the request
before returning are named <em>synchronous implementations</em>.
Those that don&rsquo;t always reply to the request before
returning are named <em>asynchronous implementations</em>.</p>

<p>Asynchronous implementations typically initiate an
asynchronous action and record to send the reply
on completion of this action.</p>

<a name="The.Tic-Tac-Toe.example"></a>
<h2>The Tic-Tac-Toe example</h2>

<p>This part explains how to write an afb-plugin.
For the sake of being practical we will use many
examples from the tic-tac-toe example.
This plugin example is in <em>plugins/samples/tic-tac-toe.c</em>.</p>

<p>This plugin is named <strong><em>tictactoe</em></strong>.</p>

<a name="Dependencies.when.compiling"></a>
<h2>Dependencies when compiling</h2>

<p>Afb-daemon provides a configuration file for <em>pkg-config</em>.
Typing the command</p>

<pre><code>pkg-config --cflags afb-daemon
</code></pre>

<p>will print the flags to use for compiling, like this:</p>

<pre><code>$ pkg-config --cflags afb-daemon
-I/opt/local/include -I/usr/include/json-c 
</code></pre>

<p>For linking, you should use</p>

<pre><code>$ pkg-config --libs afb-daemon
-ljson-c
</code></pre>

<p>As you see, afb-daemon automatically includes dependency to json-c.
This is done through the <strong>Requires</strong> keyword of pkg-config
because almost all plugin will use <strong>json-c</strong>.</p>

<p>If this behaviour is a problem, let us know.</p>

<p>Internally, afb-daemon uses <strong>libsystemd</strong> for its event loop
and for its binding to D-Bus.
Plugins developpers are encouraged to also use this library.
But it is a matter of choice.
Thus there is no dependency to <strong>libsystemd</strong>.</p>

<blockquote><p>Afb-daemon provides no library for plugins.
The functions that the plugin need to have are given
to the plugin at runtime through pointer using read-only
memory.</p></blockquote>

<a name="Header.files.to.include"></a>
<h2>Header files to include</h2>

<p>The plugin <em>tictactoe</em> has the following lines for its includes:</p>

<pre><code>#define _GNU_SOURCE
#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;json-c/json.h&gt;
#include &lt;afb/afb-plugin.h&gt;
</code></pre>

<p>The header <em>afb/afb-plugin.h</em> includes all the features that a plugin
needs except two foreign header that must be included by the plugin
if it needs it:</p>

<ul>
<li><em>json-c/json.h</em>: this header must be include to handle json objects;</li>
<li><em>systemd/sd-event.h</em>: this must be include to access the main loop;</li>
<li><em>systemd/sd-bus.h</em>: this may be include to use dbus connections.</li>
</ul>


<p>The <em>tictactoe</em> plugin does not use systemd features so it is not included.</p>

<p>When including <em>afb/afb-plugin.h</em>, the macro <strong>_GNU_SOURCE</strong> must be
defined.</p>

<a name="Choosing.names"></a>
<h2>Choosing names</h2>

<p>The designer of a plugin must defines names for its plugin
(or its API) and for the verbs of its API. He also
must defines names for arguments given by name.</p>

<p>While forging names, the designer should take into account
the rules for making valid names and some rules that make
the names easy to use across plaforms.</p>

<p>The names and strings used ALL are UTF-8 encoded.</p>

<a name="Names.for.API..plugin."></a>
<h3>Names for API (plugin)</h3>

<p>The names of the API are checked.
All characters are authorised except:</p>

<ul>
<li>the control characters (\u0000 .. \u001f)</li>
<li>the characters of the set { &lsquo; &rsquo;, &lsquo;&ldquo;&rsquo;, &lsquo;#&rsquo;, &lsquo;%&rsquo;, &lsquo;&amp;&rsquo;,
&lsquo;&rsquo;&lsquo;, &rsquo;/&lsquo;, &rsquo;?&lsquo;, &rsquo;`&lsquo;, &rsquo;\x7f' }</li>
</ul>


<p>In other words the set of forbidden characters is
{ \u0000..\u0020, \u0022, \u0023, \u0025..\u0027,
  \u002f, \u003f, \u0060, \u007f }.</p>

<p>Afb-daemon make no distinction between lower case
and upper case when searching for an API by its name.</p>

<a name="Names.for.verbs"></a>
<h3>Names for verbs</h3>

<p>The names of the verbs are not checked.</p>

<p>However, the validity rules for verb&rsquo;s names are the
same as for API&rsquo;s names except that the dot (.) character
is forbidden.</p>

<p>Afb-daemon make no distinction between lower case
and upper case when searching for an API by its name.</p>

<a name="Names.for.arguments"></a>
<h3>Names for arguments</h3>

<p>The names for arguments are not restricted and can be
anything.</p>

<p>The arguments are searched with the case sensitive
string comparison. Thus the names &ldquo;index&rdquo; and &ldquo;Index&rdquo;
are not the same.</p>

<a name="Forging.names.widely.available"></a>
<h3>Forging names widely available</h3>

<p>The key names of javascript object can be almost
anything using the arrayed notation:</p>

<pre><code>object[key] = value
</code></pre>

<p>That is not the case with the dot notation:</p>

<pre><code>object.key = value
</code></pre>

<p>Using the dot notation, the key must be a valid javascript
identifier.</p>

<p>For this reason, the chosen names should better be
valid javascript identifier.</p>

<p>It is also a good practice, even for arguments, to not
rely on the case sensitivity and to avoid the use of
names different only by the case.</p>

<a name="Writing.a.synchronous.verb.implementation"></a>
<h2>Writing a synchronous verb implementation</h2>

<p>The verb <strong>tictactoe/board</strong> is a synchronous implementation.
Here is its listing:</p>

<pre><code>/*
 * get the board
 */
static void board(struct afb_req req)
{
        struct board *board;
        struct json_object *description;

        /* retrieves the context for the session */
        board = board_of_req(req);
        INFO(afbitf, "method 'board' called for boardid %d", board-&gt;id);

        /* describe the board */
        description = describe(board);

        /* send the board's description */
        afb_req_success(req, description, NULL);
}
</code></pre>

<p>This examples show many aspects of writing a synchronous
verb implementation. Let summarize it:</p>

<ol>
<li><p>The function <strong>board_of_req</strong> retrieves the context stored
for the plugin: the board.</p></li>
<li><p>The macro <strong>INFO</strong> sends a message of kind <em>INFO</em>
to the logging system. The global variable named <strong>afbitf</strong>
used represents the interface to afb-daemon.</p></li>
<li><p>The function <strong>describe</strong> creates a json_object representing
the board.</p></li>
<li><p>The function <strong>afb_req_success</strong> sends the reply, attaching to
it the object <em>description</em>.</p></li>
</ol>


<a name="The.incoming.request"></a>
<h3>The incoming request</h3>

<p>For any implementation, the request is received by a structure of type
<strong>struct afb_req</strong>.</p>

<blockquote><p>Note that this is a PLAIN structure, not a pointer to a structure.</p></blockquote>

<p>The definition of <strong>struct afb_req</strong> is:</p>

<pre><code>/*
 * Describes the request by plugins from afb-daemon
 */
struct afb_req {
        const struct afb_req_itf *itf;  /* the interfacing functions */
        void *closure;          /* the closure for functions */
};
</code></pre>

<p>It contains two pointers: one, <em>itf</em>, points to the functions needed
to handle the internal request represented by the second pointer, <em>closure</em>.</p>

<blockquote><p>The structure must never be used directly.
Insted, use the intended functions provided
by afb-daemon and described here.</p></blockquote>

<p><em>req</em> is used to get arguments of the request, to send
answer, to store session data.</p>

<p>This object and its interface is defined and documented
in the file names <em>afb/afb-req-itf.h</em></p>

<p>The above example uses 2 times the request object <em>req</em>.</p>

<p>The first time, it is used for retrieving the board attached to
the session of the request.</p>

<p>The second time, it is used to send the reply: an object that
describes the current board.</p>

<a name="Associating.a.context.to.the.session"></a>
<h3>Associating a context to the session</h3>

<p>When the plugin <em>tic-tac-toe</em> receives a request, it musts regain
the board that describes the game associated to the session.</p>

<p>For a plugin, having data associated to a session is a common case.
This data is called the context of the plugin for the session.
For the plugin <em>tic-tac-toe</em>, the context is the board.</p>

<p>The requests <em>afb_req</em> offer four functions for
storing and retrieving the context associated to the session.</p>

<p>These functions are:</p>

<ul>
<li><p><strong>afb_req_context_get</strong>:
retrieves the context data stored for the plugin.</p></li>
<li><p><strong>afb_req_context_set</strong>:
store the context data of the plugin.</p></li>
<li><p><strong>afb_req_context</strong>:
retrieves the context data of the plugin,
if needed, creates the context and store it.</p></li>
<li><p><strong>afb_req_context_clear</strong>:
reset the stored data.</p></li>
</ul>


<p>The plugin <em>tictactoe</em> use a convenient function to retrieve
its context: the board. This function is <em>board_of_req</em>:</p>

<pre><code>/*
 * retrieves the board of the request
 */
static inline struct board *board_of_req(struct afb_req req)
{
        return afb_req_context(req, (void*)get_new_board, (void*)release_board);
}
</code></pre>

<p>The function <strong>afb_req_context</strong> ensure an existing context
for the session of the request.
Its two last arguments are functions. Here, the casts are required
to avoid a warning when compiling.</p>

<p>Here is the definition of the function <strong>afb_req_context</strong></p>

<pre><code>/*
 * Gets the pointer stored by the plugin for the session of 'req'.
 * If the stored pointer is NULL, indicating that no pointer was
 * already stored, afb_req_context creates a new context by calling
 * the function 'create_context' and stores it with the freeing function
 * 'free_context'.
 */
static inline void *afb_req_context(struct afb_req req, void *(*create_context)(), void (*free_context)(void*))
{
        void *result = afb_req_context_get(req);
        if (result == NULL) {
                result = create_context();
                afb_req_context_set(req, result, free_context);
        }
        return result;
}
</code></pre>

<p>The second argument if the function that creates the context.
For the plugin <em>tic-tac-toe</em> it is the function <strong>get_new_board</strong>.
The function <strong>get_new_board</strong> creates a new board and set its
count of use to 1. The boards are counting their count of use
to free there ressources when no more used.</p>

<p>The third argument if the function that frees the context.
For the plugin <em>tic-tac-toe</em> it is the function <strong>release_board</strong>.
The function <strong>release_board</strong> decrease the the count of use of
the board given as argument. If the use count decrease to zero,
the board data are freed.</p>

<p>The definition of the other functions for dealing with contexts are:</p>

<pre><code>/*
 * Gets the pointer stored by the plugin for the session of 'req'.
 * When the plugin has not yet recorded a pointer, NULL is returned.
 */
void *afb_req_context_get(struct afb_req req);

/*
 * Stores for the plugin the pointer 'context' to the session of 'req'.
 * The function 'free_context' will be called when the session is closed
 * or if plugin stores an other pointer.
 */
void afb_req_context_set(struct afb_req req, void *context, void (*free_context)(void*));

/*
 * Frees the pointer stored by the plugin for the session of 'req'
 * and sets it to NULL.
 *
 * Shortcut for: afb_req_context_set(req, NULL, NULL)
 */
static inline void afb_req_context_clear(struct afb_req req)
{
        afb_req_context_set(req, NULL, NULL);
}
</code></pre>

<a name="Sending.the.reply.to.a.request"></a>
<h3>Sending the reply to a request</h3>

<p>Two kinds of replies can be made: successful replies and
failure replies.</p>

<blockquote><p>Sending a reply to a request must be done at most one time.</p></blockquote>

<p>The two functions to send a reply of kind &ldquo;success&rdquo; are
<strong>afb_req_success</strong> and <strong>afb_req_success_f</strong>.</p>

<pre><code>/*
 * Sends a reply of kind success to the request 'req'.
 * The status of the reply is automatically set to "success".
 * Its send the object 'obj' (can be NULL) with an
 * informationnal comment 'info (can also be NULL).
 *
 * For conveniency, the function calls 'json_object_put' for 'obj'.
 * Thus, in the case where 'obj' should remain available after
 * the function returns, the function 'json_object_get' shall be used.
 */
void afb_req_success(struct afb_req req, struct json_object *obj, const char *info);

/*
 * Same as 'afb_req_success' but the 'info' is a formatting
 * string followed by arguments.
 *
 * For conveniency, the function calls 'json_object_put' for 'obj'.
 * Thus, in the case where 'obj' should remain available after
 * the function returns, the function 'json_object_get' shall be used.
 */
void afb_req_success_f(struct afb_req req, struct json_object *obj, const char *info, ...);
</code></pre>

<p>The two functions to send a reply of kind &ldquo;failure&rdquo; are
<strong>afb_req_fail</strong> and <strong>afb_req_fail_f</strong>.</p>

<pre><code>/*
 * Sends a reply of kind failure to the request 'req'.
 * The status of the reply is set to 'status' and an
 * informationnal comment 'info' (can also be NULL) can be added.
 *
 * Note that calling afb_req_fail("success", info) is equivalent
 * to call afb_req_success(NULL, info). Thus even if possible it
 * is strongly recommanded to NEVER use "success" for status.
 *
 * For conveniency, the function calls 'json_object_put' for 'obj'.
 * Thus, in the case where 'obj' should remain available after
 * the function returns, the function 'json_object_get' shall be used.
 */
void afb_req_fail(struct afb_req req, const char *status, const char *info);

/*
 * Same as 'afb_req_fail' but the 'info' is a formatting
 * string followed by arguments.
 *
 * For conveniency, the function calls 'json_object_put' for 'obj'.
 * Thus, in the case where 'obj' should remain available after
 * the function returns, the function 'json_object_get' shall be used.
 */
void afb_req_fail_f(struct afb_req req, const char *status, const char *info, ...);
</code></pre>

<blockquote><p>For conveniency, these functions call <strong>json_object_put</strong> to release the object <strong>obj</strong>
that they send. Then <strong>obj</strong> can not be used after calling one of these reply functions.
When it is not the expected behaviour, calling the function <strong>json_object_get</strong> on the object <strong>obj</strong>
before cancels the effect of <strong>json_object_put</strong>.</p></blockquote>

<a name="Getting.argument.of.invocation"></a>
<h2>Getting argument of invocation</h2>

<p>Many verbs expect arguments. Afb-daemon let plugins
retrieve their arguments by name not by position.</p>

<p>Arguments are given by the requests either through HTTP
or through WebSockets.</p>

<p>For example, the verb <strong>join</strong> of the plugin <strong>tic-tac-toe</strong>
expects one argument: the <em>boardid</em> to join. Here is an extract:</p>

<pre><code>/*
 * Join a board
 */
static void join(struct afb_req req)
{
        struct board *board, *new_board;
        const char *id;

        /* retrieves the context for the session */
        board = board_of_req(req);
        INFO(afbitf, "method 'join' called for boardid %d", board-&gt;id);

        /* retrieves the argument */
        id = afb_req_value(req, "boardid");
        if (id == NULL)
                goto bad_request;
        ...
</code></pre>

<p>The function <strong>afb_req_value</strong> search in the request <em>req</em>
for an argument whose name is given. When no argument of the
given name was passed, <strong>afb_req_value</strong> returns NULL.</p>

<blockquote><p>The search is case sensitive. So the name <em>boardid</em> is not the
same name than <em>BoardId</em>. But this must not be assumed so two
expected names of argument should not differ only by case.</p></blockquote>

<a name="Basic.functions.for.querying.arguments"></a>
<h3>Basic functions for querying arguments</h3>

<p>The function <strong>afb_req_value</strong> is defined as below:</p>

<pre><code>/*
 * Gets from the request 'req' the string value of the argument of 'name'.
 * Returns NULL if when there is no argument of 'name'.
 * Returns the value of the argument of 'name' otherwise.
 *
 * Shortcut for: afb_req_get(req, name).value
 */
static inline const char *afb_req_value(struct afb_req req, const char *name)
{
        return afb_req_get(req, name).value;
}
</code></pre>

<p>It is defined as a shortcut to call the function <strong>afb_req_get</strong>.
That function is defined as below:</p>

<pre><code>/*
 * Gets from the request 'req' the argument of 'name'.
 * Returns a PLAIN structure of type 'struct afb_arg'.
 * When the argument of 'name' is not found, all fields of result are set to NULL.
 * When the argument of 'name' is found, the fields are filled,
 * in particular, the field 'result.name' is set to 'name'.
 *
 * There is a special name value: the empty string.
 * The argument of name "" is defined only if the request was made using
 * an HTTP POST of Content-Type "application/json". In that case, the
 * argument of name "" receives the value of the body of the HTTP request.
 */
struct afb_arg afb_req_get(struct afb_req req, const char *name);
</code></pre>

<p>That function takes 2 parameters: the request and the name
of the argument to retrieve. It returns a PLAIN structure of
type <strong>struct afb_arg</strong>.</p>

<p>There is a special name that is defined when the request is
of type HTTP/POST with a Content-Type being application/json.
This name is <strong>&ldquo;&rdquo;</strong> (the empty string). In that case, the value
of this argument of empty name is the string received as a body
of the post and is supposed to be a JSON string.</p>

<p>The definition of <strong>struct afb_arg</strong> is:</p>

<pre><code>/*
 * Describes an argument (or parameter) of a request
 */
struct afb_arg {
        const char *name;   /* name of the argument or NULL if invalid */
        const char *value;  /* string representation of the value of the argument */
                                /* original filename of the argument if path != NULL */
        const char *path;   /* if not NULL, path of the received file for the argument */
                                /* when the request is finalized this file is removed */
};
</code></pre>

<p>The structure returns the data arguments that are known for the
request. This data include a field named <strong>path</strong>. This <strong>path</strong>
can be accessed using the function <strong>afb_req_path</strong> defined as
below:</p>

<pre><code>/*
 * Gets from the request 'req' the path for file attached to the argument of 'name'.
 * Returns NULL if when there is no argument of 'name' or when there is no file.
 * Returns the path of the argument of 'name' otherwise.
 *
 * Shortcut for: afb_req_get(req, name).path
 */
static inline const char *afb_req_path(struct afb_req req, const char *name)
{
        return afb_req_get(req, name).path;
}
</code></pre>

<p>The path is only defined for HTTP/POST requests that send file.</p>

<a name="Arguments.for.received.files"></a>
<h3>Arguments for received files</h3>

<p>As it is explained just above, clients can send files using
HTTP/POST requests.</p>

<p>Received files are attached to a arguments. For example, the
following HTTP fragment (from test/sample-post.html)
will send an HTTP/POST request to the method
<strong>post/upload-image</strong> with 2 arguments named <em>file</em> and
<em>hidden</em>.</p>

<pre><code>&lt;h2&gt;Sample Post File&lt;/h2&gt;
&lt;form enctype="multipart/form-data"&gt;
    &lt;input type="file" name="file" /&gt;
    &lt;input type="hidden" name="hidden" value="bollobollo" /&gt;
    &lt;br&gt;
    &lt;button formmethod="POST" formaction="api/post/upload-image"&gt;Post File&lt;/button&gt;
&lt;/form&gt;
</code></pre>

<p>In that case, the argument named <strong>file</strong> has its value and its
path defined and not NULL.</p>

<p>The value is the name of the file as it was
set by the HTTP client and is generally the filename on the
client side.</p>

<p>The path is the path of the file saved on the temporary local storage
area of the application. This is a randomly generated and unic filename
not linked in any way with the original filename on the client.</p>

<p>The plugin can use the file at the given path the way that it wants:
read, write, remove, copy, rename&hellip;
But when the reply is sent and the query is terminated, the file at
this path is destroyed if it still exist.</p>

<a name="Arguments.as.a.JSON.object"></a>
<h3>Arguments as a JSON object</h3>

<p>Plugins can get all the arguments as one single object.
This feature is provided by the function <strong>afb_req_json</strong>
that is defined as below:</p>

<pre><code>/*
 * Gets from the request 'req' the json object hashing the arguments.
 * The returned object must not be released using 'json_object_put'.
 */
struct json_object *afb_req_json(struct afb_req req);
</code></pre>

<p>It returns a json object. This object depends on how the request was
made:</p>

<ul>
<li><p>For HTTP requests, this is an object whose keys are the names of the
arguments and whose values are either a string for common arguments or
an object like { &ldquo;file&rdquo;: &ldquo;&hellip;&rdquo;, &ldquo;path&rdquo;: &ldquo;&hellip;&rdquo; }</p></li>
<li><p>For WebSockets requests, the returned object is the object
given by the client transparently transported.</p></li>
</ul>


<blockquote><p>In fact, for Websockets requests, the function <strong>afb_req_value</strong>
can be seen as a shortcut to
<strong><em>json_object_get_string(json_object_object_get(afb_req_json(req), name))</em></strong></p></blockquote>

<a name="Initialisation.of.the.plugin.and.declaration.of.verbs"></a>
<h2>Initialisation of the plugin and declaration of verbs</h2>

<p>To be active, the verbs of the plugin should be declared to
afb-daemon. And even more, the plugin itself must be recorded.</p>

<p>The mechanism for doing this is very simple: when afb-need starts,
it loads the plugins that are listed in its argument or configuration.</p>

<p>Loading a plugin follows the following steps:</p>

<ol>
<li><p>It loads the plugin using <em>dlopen</em>.</p></li>
<li><p>It searchs for the symbol named <strong>pluginAfbV1Register</strong> using <em>dlsym</em>.
This symbol is assumed to be the exported initialisation function of the plugin.</p></li>
<li><p>It build an interface object for the plugin.</p></li>
<li><p>It calls the found function <strong>pluginAfbV1Register</strong> and pass it the pointer
to its interface.</p></li>
<li><p>The function <strong>pluginAfbV1Register</strong> setup the plugin, initialize it.</p></li>
<li><p>The function <strong>pluginAfbV1Register</strong> returns the pointer to a structure
that describes the plugin: its version, its name (prefix or API name), and the
list of its verbs.</p></li>
<li><p>Afb-daemon checks that the returned version and name can be managed.
If it can manage it, the plugin and its verbs are recorded and can be used
when afb-daemon finishes it initialisation.</p></li>
</ol>


<p>Here is the listing of the function <strong>pluginAfbV1Register</strong> of the plugin
<em>tic-tac-toe</em>:</p>

<pre><code>/*
 * activation function for registering the plugin called by afb-daemon
 */
const struct AFB_plugin *pluginAfbV1Register(const struct AFB_interface *itf)
{
   afbitf = itf;         // records the interface for accessing afb-daemon
   return &amp;plugin_description;  // returns the description of the plugin
}
</code></pre>

<p>This is a very small function because the <em>tic-tac-toe</em> plugin doesn&rsquo;t have initialisation step.
It merely record the daemon&rsquo;s interface and returns its descritption.</p>

<p>The variable <strong>afbitf</strong> is a variable global to the plugin. It records the
interface to afb-daemon and is used for logging and pushing events.
Here is its declaration:</p>

<pre><code>/*
 * the interface to afb-daemon
 */
const struct AFB_interface *afbitf;
</code></pre>

<p>The description of the plugin is defined as below.</p>

<pre><code>/*
 * array of the verbs exported to afb-daemon
 */
static const struct AFB_verb_desc_v1 plugin_verbs[] = {
   /* VERB'S NAME     SESSION MANAGEMENT          FUNCTION TO CALL  SHORT DESCRIPTION */
   { .name= "new",   .session= AFB_SESSION_NONE, .callback= new,   .info= "Starts a new game" },
   { .name= "play",  .session= AFB_SESSION_NONE, .callback= play,  .info= "Asks the server to play" },
   { .name= "move",  .session= AFB_SESSION_NONE, .callback= move,  .info= "Tells the client move" },
   { .name= "board", .session= AFB_SESSION_NONE, .callback= board, .info= "Get the current board" },
   { .name= "level", .session= AFB_SESSION_NONE, .callback= level, .info= "Set the server level" },
   { .name= "join",  .session= AFB_SESSION_CHECK,.callback= join,  .info= "Join a board" },
   { .name= "undo",  .session= AFB_SESSION_NONE, .callback= undo,  .info= "Undo the last move" },
   { .name= "wait",  .session= AFB_SESSION_NONE, .callback= wait,  .info= "Wait for a change" },
   { .name= NULL } /* marker for end of the array */
};

/*
 * description of the plugin for afb-daemon
 */
static const struct AFB_plugin plugin_description =
{
   /* description conforms to VERSION 1 */
   .type= AFB_PLUGIN_VERSION_1,
   .v1= {               /* fills the v1 field of the union when AFB_PLUGIN_VERSION_1 */
      .prefix= "tictactoe",     /* the API name (or plugin name or prefix) */
      .info= "Sample tac-tac-toe game", /* short description of of the plugin */
      .verbs = plugin_verbs     /* the array describing the verbs of the API */
   }
};
</code></pre>

<p>The structure <strong>plugin_description</strong> describes the plugin.
It declares the type and version of the plugin, its name, a description
and a list of its verbs.</p>

<p>The list of verbs is an array of structures describing the verbs and terminated by a marker:
a verb whose name is NULL.</p>

<p>The description of the verbs for this version is made of 4 fields:</p>

<ul>
<li><p>the name of the verbs,</p></li>
<li><p>the session management flags,</p></li>
<li><p>the implementation function to be call for the verb,</p></li>
<li><p>a short description.</p></li>
</ul>


<p>The structure describing verbs is defined as follows:</p>

<pre><code>/*
 * Description of one verb of the API provided by the plugin
 * This enumeration is valid for plugins of type 1
 */
struct AFB_verb_desc_v1
{
       const char *name;                       /* name of the verb */
       enum AFB_session_v1 session;            /* authorisation and session requirements of the verb */
       void (*callback)(struct afb_req req);   /* callback function implementing the verb */
       const char *info;                       /* textual description of the verb */
};
</code></pre>

<p>For technical reasons, the enumeration <strong>enum AFB_session_v1</strong> is not exactly an
enumeration but the wrapper of constant definitions that can be mixed using bitwise or
(the C operator |).</p>

<p>The constants that can bit mixed are:</p>

<table>
<thead>
<tr>
<th>Constant name        </th>
<th> Meaning</th>
</tr>
</thead>
<tbody>
<tr>
<td><strong>AFB_SESSION_CREATE</strong>   </td>
<td> Equals to AFB_SESSION_LOA_EQ_0|AFB_SESSION_RENEW</td>
</tr>
<tr>
<td><strong>AFB_SESSION_CLOSE</strong>    </td>
<td> Closes the session after the reply and set the LOA to 0</td>
</tr>
<tr>
<td><strong>AFB_SESSION_RENEW</strong>    </td>
<td> Refreshes the token of authentification</td>
</tr>
<tr>
<td><strong>AFB_SESSION_CHECK</strong>    </td>
<td> Just requires the token authentification</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_LE_0</strong> </td>
<td> Requires the current LOA to be lesser then or equal to 0</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_LE_1</strong> </td>
<td> Requires the current LOA to be lesser then or equal to 1</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_LE_2</strong> </td>
<td> Requires the current LOA to be lesser then or equal to 2</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_LE_3</strong> </td>
<td> Requires the current LOA to be lesser then or equal to 3</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_GE_0</strong> </td>
<td> Requires the current LOA to be greater then or equal to 0</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_GE_1</strong> </td>
<td> Requires the current LOA to be greater then or equal to 1</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_GE_2</strong> </td>
<td> Requires the current LOA to be greater then or equal to 2</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_GE_3</strong> </td>
<td> Requires the current LOA to be greater then or equal to 3</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_EQ_0</strong> </td>
<td> Requires the current LOA to be equal to 0</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_EQ_1</strong> </td>
<td> Requires the current LOA to be equal to 1</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_EQ_2</strong> </td>
<td> Requires the current LOA to be equal to 2</td>
</tr>
<tr>
<td><strong>AFB_SESSION_LOA_EQ_3</strong> </td>
<td> Requires the current LOA to be equal to 3</td>
</tr>
</tbody>
</table>


<p>If any of this flags is set, afb-daemon requires the token authentification
as if the flag <strong>AFB_SESSION_CHECK</strong> had been set.</p>

<p>The special value <strong>AFB_SESSION_NONE</strong> is zero and can be used to avoid any check.</p>

<blockquote><p>Note that <strong>AFB_SESSION_CREATE</strong> and <strong>AFB_SESSION_CLOSE</strong> might be removed in later versions.</p></blockquote>

<a name="Sending.messages.to.the.log.system"></a>
<h2>Sending messages to the log system</h2>

<p>Afb-daemon provides 4 levels of verbosity and 5 verbs for logging messages.</p>

<p>The verbosity is managed. Options allow the change the verbosity of afb-daemon
and the verbosity of the plugins can be set plugin by plugin.</p>

<p>The verbs for logging messages are defined as macros that test the
verbosity level and that call the real logging function only if the
message must be output. This avoid evaluation of arguments of the
formatting messages if the message must not be output.</p>

<a name="Verbs.for.logging.messages"></a>
<h3>Verbs for logging messages</h3>

<p>The 5 logging verbs are:</p>

<table>
<thead>
<tr>
<th>Macro   </th>
<th style="text-align:center;"> Verbosity </th>
<th> Meaning                       </th>
<th style="text-align:center;"> syslog level</th>
</tr>
</thead>
<tbody>
<tr>
<td>ERROR   </td>
<td style="text-align:center;">     0     </td>
<td> Error conditions              </td>
<td style="text-align:center;">     3</td>
</tr>
<tr>
<td>WARNING </td>
<td style="text-align:center;">     1     </td>
<td> Warning conditions            </td>
<td style="text-align:center;">     4</td>
</tr>
<tr>
<td>NOTICE  </td>
<td style="text-align:center;">     1     </td>
<td> Normal but significant condition  </td>
<td style="text-align:center;">     5</td>
</tr>
<tr>
<td>INFO    </td>
<td style="text-align:center;">     2     </td>
<td> Informational                 </td>
<td style="text-align:center;">     6</td>
</tr>
<tr>
<td>DEBUG   </td>
<td style="text-align:center;">     3     </td>
<td> Debug-level messages          </td>
<td style="text-align:center;">     7</td>
</tr>
</tbody>
</table>


<p>You can note that the 2 verbs <strong>WARNING</strong> and <strong>INFO</strong> have the same level
of verbosity. But they don&rsquo;t have the same <em>syslog level</em>. It means that
they are output with a different level on the logging system.</p>

<p>All of these verbs have the same signature:</p>

<pre><code>void ERROR(const struct AFB_interface *afbitf, const char *message, ...);
</code></pre>

<p>The first argument <strong>afbitf</strong> is the interface to afb daemon that the
plugin received at its initialisation when <strong>pluginAfbV1Register</strong> was called.</p>

<p>The second argument <strong>message</strong> is a formatting string compatible with printf/sprintf.</p>

<p>The remaining arguments are arguments of the formating message like for printf.</p>

<a name="Managing.verbosity"></a>
<h3>Managing verbosity</h3>

<p>Depending on the level of verbosity, the messages are output or not.
The following table explains what messages will be output depending
ont the verbosity level.</p>

<table>
<thead>
<tr>
<th style="text-align:center;">Level of verbosity </th>
<th> Outputed macro</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:center;">0          </td>
<td> ERROR</td>
</tr>
<tr>
<td style="text-align:center;">1          </td>
<td> ERROR + WARNING + NOTICE</td>
</tr>
<tr>
<td style="text-align:center;">2          </td>
<td> ERROR + WARNING + NOTICE + INFO</td>
</tr>
<tr>
<td style="text-align:center;">3          </td>
<td> ERROR + WARNING + NOTICE + INFO + DEBUG</td>
</tr>
</tbody>
</table>


<a name="Output.format.and.destination"></a>
<h3>Output format and destination</h3>

<p>The syslog level is used for forging a prefix to the message.
The prefixes are:</p>

<table>
<thead>
<tr>
<th style="text-align:center;">syslog level </th>
<th> prefix</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:center;">0      </td>
<td> <0> EMERGENCY</td>
</tr>
<tr>
<td style="text-align:center;">1      </td>
<td> <1> ALERT</td>
</tr>
<tr>
<td style="text-align:center;">2      </td>
<td> <2> CRITICAL</td>
</tr>
<tr>
<td style="text-align:center;">3      </td>
<td> <3> ERROR</td>
</tr>
<tr>
<td style="text-align:center;">4      </td>
<td> <4> WARNING</td>
</tr>
<tr>
<td style="text-align:center;">5      </td>
<td> <5> NOTICE</td>
</tr>
<tr>
<td style="text-align:center;">6      </td>
<td> <6> INFO</td>
</tr>
<tr>
<td style="text-align:center;">7      </td>
<td> <7> DEBUG</td>
</tr>
</tbody>
</table>


<p>The message is issued to the standard error.
The final destination of the message depends on how the systemd service
was configured through the variable <strong>StandardError</strong>: It can be
journal, syslog or kmsg. (See man sd-daemon).</p>

<a name="Sending.events"></a>
<h2>Sending events</h2>

<p>Since version 0.5, plugins can broadcast events to any potential listener.
This kind of bradcast is not targeted. Event targeted will come in a future
version of afb-daemon.</p>

<p>The plugin <em>tic-tac-toe</em> broadcasts events when the board changes.
This is done in the function <strong>changed</strong>:</p>

<pre><code>/*
 * signals a change of the board
 */
static void changed(struct board *board, const char *reason)
{
        ...
        struct json_object *description;

        /* get the description */
        description = describe(board);

        ...

        afb_daemon_broadcast_event(afbitf-&gt;daemon, reason, description);
}
</code></pre>

<p>The description of the changed board is pushed via the daemon interface.</p>

<p>Within the plugin <em>tic-tac-toe</em>, the <em>reason</em> indicates the origin of
the change. For the function <strong>afb_daemon_broadcast_event</strong>, the second
parameter is the name of the broadcasted event. The third argument is the
object that is transmitted with the event.</p>

<p>The function <strong>afb_daemon_broadcast_event</strong> is defined as below:</p>

<pre><code>/*
 * Broadcasts widely the event of 'name' with the data 'object'.
 * 'object' can be NULL.
 * 'daemon' MUST be the daemon given in interface when activating the plugin.
 *
 * For conveniency, the function calls 'json_object_put' for 'object'.
 * Thus, in the case where 'object' should remain available after
 * the function returns, the function 'json_object_get' shall be used.
 */
void afb_daemon_broadcast_event(struct afb_daemon daemon, const char *name, struct json_object *object);
</code></pre>

<blockquote><p>Be aware, as for reply functions, the <strong>object</strong> is automatically released using
<strong>json_object_put</strong> by the function. Then call <strong>json_object_get</strong> before
calling <strong>afb_daemon_broadcast_event</strong> to keep <strong>object</strong> available
after the returning of the function.</p></blockquote>

<p>In fact the event name received by the listener is prefixed with
the name of the plugin. So when the change occurs after a move, the
reason is <strong>move</strong> and then the clients receive the event <strong>tictactoe/move</strong>.</p>

<blockquote><p>Note that nothing is said about the case sensitivity of event names.
However, the event is always prefixed with the name that the plugin
declared, with the same case, followed with a slash /.
Thus it is safe to compare event using a case sensitive comparison.</p></blockquote>

<a name="Writing.an.asynchronous.verb.implementation"></a>
<h2>Writing an asynchronous verb implementation</h2>

<p>The <em>tic-tac-toe</em> example allows two clients or more to share the same board.
This is implemented by the verb <strong>join</strong> that illustrated partly the how to
retrieve arguments.</p>

<p>When two or more clients are sharing a same board, one of them can wait
until the state of the board changes. (This coulded also be implemented using
events because an even is generated each time the board changes).</p>

<p>In this case, the reply to the wait is sent only when the board changes.
See the diagram below:</p>

<pre><code>CLIENT A       CLIENT B         TIC-TAC-TOE
   |              |                  |
   +--------------|-----------------&gt;| wait . . . . . . . .
   |              |                  |                     .
   :              :                  :                      .
   :              :                  :                      .
   |              |                  |                      .
   |              +-----------------&gt;| move . . .           .
   |              |                  |          V           .
   |              |&lt;-----------------+ success of move      .
   |              |                  |                    .
   |&lt;-------------|------------------+ success of wait  &lt;
</code></pre>

<p>Here, this is an invocation of the plugin by an other client that
unblock the suspended <em>wait</em> call.
But in general, this will be a timer, a hardware event, the sync with
a concurrent process or thread, &hellip;</p>

<p>So the case is common, this is an asynchronous implementation.</p>

<p>Here is the listing of the function <strong>wait</strong>:</p>

<pre><code>static void wait(struct afb_req req)
{
        struct board *board;
        struct waiter *waiter;

        /* retrieves the context for the session */
        board = board_of_req(req);
        INFO(afbitf, "method 'wait' called for boardid %d", board-&gt;id);

        /* creates the waiter and enqueues it */
        waiter = calloc(1, sizeof *waiter);
        waiter-&gt;req = req;
        waiter-&gt;next = board-&gt;waiters;
        afb_req_addref(req);
        board-&gt;waiters = waiter;
}
</code></pre>

<p>After retrieving the board, the function adds a new waiter to the
current list of waiters and returns without sending a reply.</p>

<p>Before returning, it increases the reference count of the
request <strong>req</strong> using the function <strong>afb_req_addref</strong>.</p>

<blockquote><p>When the implentation of a verb returns without sending a reply,
it <strong>MUST</strong> increment the reference count of the request
using <strong>afb_req_addref</strong>. If it doesn&rsquo;t bad things can happen.</p></blockquote>

<p>Later, when the board changes, it calls the function <strong>changed</strong>
of <em>tic-tac-toe</em> with the reason of the change.</p>

<p>Here is the full listing of the function <strong>changed</strong>:</p>

<pre><code>/*
 * signals a change of the board
 */
static void changed(struct board *board, const char *reason)
{
        struct waiter *waiter, *next;
        struct json_object *description;

        /* get the description */
        description = describe(board);

        waiter = board-&gt;waiters;
        board-&gt;waiters = NULL;
        while (waiter != NULL) {
                next = waiter-&gt;next;
                afb_req_success(waiter-&gt;req, json_object_get(description), reason);
                afb_req_unref(waiter-&gt;req);
                free(waiter);
                waiter = next;
        }

        afb_event_sender_push(afb_daemon_get_event_sender(afbitf-&gt;daemon), reason, description);
}
</code></pre>

<p>The list of waiters is walked and a reply is sent to each waiter.
After the sending the reply, the reference count of the request
is decremented using <strong>afb_req_unref</strong> to allow its resources to be freed.</p>

<blockquote><p>The reference count <strong>MUST</strong> be decremented using <strong>afb_req_unref</strong> because,
otherwise, there is a leak of resources.
It must be decremented <strong>AFTER</strong> the sending of the reply, because, otherwise,
bad things may happen.</p></blockquote>

<a name="How.to.build.a.plugin"></a>
<h2>How to build a plugin</h2>

<p>Afb-daemon provides a <em>pkg-config</em> configuration file that can be
queried by the name <strong>afb-daemon</strong>.
This configuration file provides data that should be used
for compiling plugins. Examples:</p>

<pre><code>$ pkg-config --cflags afb-daemon
$ pkg-config --libs afb-daemon
</code></pre>

<a name="Example.for.cmake.meta.build.system"></a>
<h3>Example for cmake meta build system</h3>

<p>This example is the extract for building the plugin <em>afm-main</em> using <em>CMAKE</em>.</p>

<pre><code>pkg_check_modules(afb afb-daemon)
if(afb_FOUND)
        message(STATUS "Creation afm-main-plugin for AFB-DAEMON")
        add_library(afm-main-plugin MODULE afm-main-plugin.c)
        target_compile_options(afm-main-plugin PRIVATE ${afb_CFLAGS})
        target_include_directories(afm-main-plugin PRIVATE ${afb_INCLUDE_DIRS})
        target_link_libraries(afm-main-plugin utils ${afb_LIBRARIES})
        set_target_properties(afm-main-plugin PROPERTIES
                PREFIX ""
                LINK_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/afm-main-plugin.export-map"
        )
        install(TARGETS afm-main-plugin LIBRARY DESTINATION ${plugin_dir})
else()
        message(STATUS "Not creating the plugin for AFB-DAEMON")
endif()
</code></pre>

<p>Let now describe some of these lines.</p>

<pre><code>pkg_check_modules(afb afb-daemon)
</code></pre>

<p>This first lines searches to the <em>pkg-config</em> configuration file for
<strong>afb-daemon</strong>. Resulting data are stored in the following variables:</p>

<table>
<thead>
<tr>
<th>Variable      </th>
<th> Meaning</th>
</tr>
</thead>
<tbody>
<tr>
<td>afb_FOUND     </td>
<td> Set to 1 if afb-daemon plugin development files exist</td>
</tr>
<tr>
<td>afb_LIBRARIES     </td>
<td> Only the libraries (w/o the &lsquo;-l&rsquo;) for compiling afb-daemon plugins</td>
</tr>
<tr>
<td>afb_LIBRARY_DIRS  </td>
<td> The paths of the libraries (w/o the &lsquo;-L&rsquo;) for compiling afb-daemon plugins</td>
</tr>
<tr>
<td>afb_LDFLAGS       </td>
<td> All required linker flags for compiling afb-daemon plugins</td>
</tr>
<tr>
<td>afb_INCLUDE_DIRS  </td>
<td> The &lsquo;-I&rsquo; preprocessor flags (w/o the &lsquo;-I&rsquo;) for compiling afb-daemon plugins</td>
</tr>
<tr>
<td>afb_CFLAGS    </td>
<td> All required cflags for compiling afb-daemon plugins</td>
</tr>
</tbody>
</table>


<p>If development files are found, the plugin can be added to the set of
target to build.</p>

<pre><code>add_library(afm-main-plugin MODULE afm-main-plugin.c)
</code></pre>

<p>This line asks to create a shared library having only the
source file afm-main-plugin.c (that is compiled).
The default name of the created shared object is
<strong>libafm-main-plugin.so</strong>.</p>

<pre><code>set_target_properties(afm-main-plugin PROPERTIES
        PREFIX ""
        LINK_FLAGS "-Wl,--version-script=${CMAKE_CURRENT_SOURCE_DIR}/afm-main-plugin.export-map"
)
</code></pre>

<p>This lines are doing two things:</p>

<ol>
<li><p>It renames the built library from <strong>libafm-main-plugin.so</strong> to <strong>afm-main-plugin.so</strong>
by removing the implicitely added prefix <em>lib</em>. This step is not mandatory
at all because afb-daemon doesn&rsquo;t check names of files when loading it.
The only convention that use afb-daemon is that extension is <strong>.so</strong>
but this convention is used only when afb-daemon discovers plugin
from a directory hierarchy.</p></li>
<li><p>It applies a version script at link to only export the conventional name
of the entry point: <strong>pluginAfbV1Register</strong>. See below. By default, the linker
that creates the shared object exports all the public symbols (C functions that
are not <strong>static</strong>).</p></li>
</ol>


<p>Next line are:</p>

<pre><code>target_include_directories(afm-main-plugin PRIVATE ${afb_INCLUDE_DIRS})
target_link_libraries(afm-main-plugin utils ${afb_LIBRARIES})
</code></pre>

<p>As you can see it uses the variables computed by <strong><em>pkg_check_modules(afb afb-daemon)</em></strong>
to configure the compiler and the linker.</p>

<a name="Exporting.the.function.pluginAfbV1Register"></a>
<h3>Exporting the function pluginAfbV1Register</h3>

<p>The function <strong>pluginAfbV1Register</strong> must be exported. This can be achieved
using a version script when linking. Here is the version script that is
used for <em>tic-tac-toe</em> (plugins/samples/export.map).</p>

<pre><code>{ global: pluginAfbV1Register; local: *; };
</code></pre>

<p>This sample <a href="https://sourceware.org/binutils/docs-2.26/ld/VERSION.html#VERSION">version script</a>
exports as global the symbol <em>pluginAfbV1Register</em> and hides any
other symbols.</p>

<p>This version script is added to the link options using the
option <strong>&ndash;version-script=export.map</strong> is given directly to the
linker or using th option <strong>-Wl,&ndash;version-script=export.map</strong>
when the option is given to the C compiler.</p>

<a name="Building.within.yocto"></a>
<h3>Building within yocto</h3>

<p>Adding a dependency to afb-daemon is enough. See below:</p>

<pre><code>DEPENDS += " afb-daemon "
</code></pre>
</body>
</html>