summaryrefslogtreecommitdiffstats
path: root/bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml
blob: 45366d5caa913fb51144b03e28724ea22366488d (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
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<chapter id="bitbake-user-manual-metadata">
    <title>Syntax and Operators</title>

    <para>
        Bitbake files have their own syntax.
        The syntax has similarities to several
        other languages but also has some unique features.
        This section describes the available syntax and operators
        as well as provides examples.
    </para>

    <section id='basic-syntax'>
        <title>Basic Syntax</title>

        <para>
            This section provides some basic syntax examples.
        </para>

        <section id='basic-variable-setting'>
            <title>Basic Variable Setting</title>

            <para>
                The following example sets <filename>VARIABLE</filename> to
                "value".
                This assignment occurs immediately as the statement is parsed.
                It is a "hard" assignment.
                <literallayout class='monospaced'>
     VARIABLE = "value"
                </literallayout>
                As expected, if you include leading or trailing spaces as part of
                an assignment, the spaces are retained:
                <literallayout class='monospaced'>
     VARIABLE = " value"
     VARIABLE = "value "
                </literallayout>
                Setting <filename>VARIABLE</filename> to "" sets it to an empty string,
                while setting the variable to " " sets it to a blank space
                (i.e. these are not the same values).
                <literallayout class='monospaced'>
     VARIABLE = ""
     VARIABLE = " "
                </literallayout>
            </para>
        </section>

        <section id='variable-expansion'>
            <title>Variable Expansion</title>

            <para>
                BitBake supports variables referencing one another's
                contents using a syntax that is similar to shell scripting.
                Following is an example that results in <filename>A</filename>
                containing "aval" and <filename>B</filename> evaluating to
                "preavalpost" based on that current value of
                <filename>A</filename>.
                <literallayout class='monospaced'>
     A = "aval"
     B = "pre${A}post"
                </literallayout>
                You should realize that whenever <filename>B</filename> is
                referenced, its evaluation will depend on the state of
                <filename>A</filename> at that time.
                Thus, later evaluations of <filename>B</filename> in the
                previous example could result in different values
                depending on the value of <filename>A</filename>.
            </para>
        </section>

        <section id='setting-a-default-value'>
            <title>Setting a default value (?=)</title>

            <para>
                You can use the "?=" operator to achieve a "softer" assignment
                for a variable.
                This type of assignment allows you to define a variable if it
                is undefined when the statement is parsed, but to leave the
                value alone if the variable has a value.
                Here is an example:
                <literallayout class='monospaced'>
     A ?= "aval"
                </literallayout>
                If <filename>A</filename> is set at the time this statement is parsed,
                the variable retains its value.
                However, if <filename>A</filename> is not set,
                the variable is set to "aval".
                <note>
                    This assignment is immediate.
                    Consequently, if multiple "?=" assignments
                    to a single variable exist, the first of those ends up getting
                    used.
                </note>
            </para>
        </section>

        <section id='setting-a-weak-default-value'>
            <title>Setting a weak default value (??=)</title>

            <para>
                It is possible to use a "weaker" assignment than in the
                previous section by using the "??=" operator.
                This assignment behaves identical to "?=" except that the
                assignment is made at the end of the parsing process rather
                than immediately.
                Consequently, when multiple "??=" assignments exist, the last
                one is used.
                Also, any "=" or "?=" assignment will override the value set with
                "??=".
                Here is an example:
                <literallayout class='monospaced'>
     A ??= "somevalue"
     A ??= "someothervalue"
                </literallayout>
                If <filename>A</filename> is set before the above statements are parsed,
                the variable retains its value.
                If <filename>A</filename> is not set,
                the variable is set to "someothervalue".
            </para>

            <para>
                Again, this assignment is a "lazy" or "weak" assignment
                because it does not occur until the end
                of the parsing process.
            </para>
        </section>

        <section id='immediate-variable-expansion'>
            <title>Immediate variable expansion (:=)</title>

            <para>
                The ":=" operator results in a variable's
                contents being expanded immediately,
                rather than when the variable is actually used:
                <literallayout class='monospaced'>
     T = "123"
     A := "${B} ${A} test ${T}"
     T = "456"
     B = "${T} bval"
     C = "cval"
     C := "${C}append"
                </literallayout>
                In this example, <filename>A</filename> contains
                "test 123" because <filename>${B}</filename> and
                <filename>${A}</filename> at the time of parsing are undefined,
                which leaves "test 123".
                And, the variable <filename>C</filename>
                contains "cvalappend" since <filename>${C}</filename> immediately
                expands to "cval".
            </para>
        </section>

        <section id='appending-and-prepending'>
            <title>Appending (+=) and prepending (=+) With Spaces</title>

            <para>
                Appending and prepending values is common and can be accomplished
                using the "+=" and "=+" operators.
                These operators insert a space between the current
                value and prepended or appended value.
            </para>

            <para>
                These operators take immediate effect during parsing.
                Here are some examples:
                <literallayout class='monospaced'>
     B = "bval"
     B += "additionaldata"
     C = "cval"
     C =+ "test"
                </literallayout>
                The variable <filename>B</filename> contains
                "bval additionaldata" and <filename>C</filename>
                contains "test cval".
            </para>
        </section>

        <section id='appending-and-prepending-without-spaces'>
            <title>Appending (.=) and Prepending (=.) Without Spaces</title>

            <para>
                If you want to append or prepend values without an
                inserted space, use the ".=" and "=." operators.
            </para>

            <para>
                These operators take immediate effect during parsing.
                Here are some examples:
                <literallayout class='monospaced'>
     B = "bval"
     B .= "additionaldata"
     C = "cval"
     C =. "test"
                </literallayout>
                The variable <filename>B</filename> contains
                "bvaladditionaldata" and
                <filename>C</filename> contains "testcval".
            </para>
        </section>

        <section id='appending-and-prepending-override-style-syntax'>
            <title>Appending and Prepending (Override Style Syntax)</title>

            <para>
                You can also append and prepend a variable's value
                using an override style syntax.
                When you use this syntax, no spaces are inserted.
            </para>

            <para>
                These operators differ from the ":=", ".=", "=.", "+=", and "=+"
                operators in that their effects are deferred
                until after parsing completes rather than being immediately
                applied.
                Here are some examples:
                <literallayout class='monospaced'>
     B = "bval"
     B_append = " additional data"
     C = "cval"
     C_prepend = "additional data "
     D = "dval"
     D_append = "additional data"
                </literallayout>
                The variable <filename>B</filename> becomes
                "bval additional data" and <filename>C</filename> becomes
                "additional data cval".
                The variable <filename>D</filename> becomes
                "dvaladditional data".
                <note>
                    You must control all spacing when you use the
                    override syntax.
                </note>
            </para>
        </section>

        <section id='removing-override-style-syntax'>
            <title>Removal (Override Style Syntax)</title>

            <para>
                You can remove values from lists using the removal
                override style syntax.
                Specifying a value for removal causes all occurrences of that
                value to be removed from the variable.
            </para>

            <para>
                When you use this syntax, BitBake expects one or more strings.
                Surrounding spaces are removed as well.
                Here is an example:
                <literallayout class='monospaced'>
     FOO = "123 456 789 123456 123 456 123 456"
     FOO_remove = "123"
     FOO_remove = "456"
     FOO2 = "abc def ghi abcdef abc def abc def"
     FOO2_remove = "abc def"
                </literallayout>
                The variable <filename>FOO</filename> becomes
                "789 123456" and <filename>FOO2</filename> becomes
                "ghi abcdef".
            </para>
        </section>

        <section id='variable-flag-syntax'>
            <title>Variable Flag Syntax</title>

            <para>
                Variable flags are BitBake's implementation of variable properties
                or attributes.
                It is a way of tagging extra information onto a variable.
                You can find more out about variable flags in general in the
                "<link linkend='variable-flags'>Variable Flags</link>"
                section.
            </para>

            <para>
                You can define, append, and prepend values to variable flags.
                All the standard syntax operations previously mentioned work
                for variable flags except for override style syntax
                (i.e. <filename>_prepend</filename>, <filename>_append</filename>,
                and <filename>_remove</filename>).
            </para>

            <para>
                Here are some examples showing how to set variable flags:
                <literallayout class='monospaced'>
     FOO[a] = "abc"
     FOO[b] = "123"
     FOO[a] += "456"
                </literallayout>
                The variable <filename>FOO</filename> has two flags:
                <filename>a</filename> and <filename>b</filename>.
                The flags are immediately set to "abc" and "123", respectively.
                The <filename>a</filename> flag becomes "abc456".
            </para>

            <para>
                No need exists to pre-define variable flags.
                You can simply start using them.  
                One extremely common application
                is to attach some brief documentation to a BitBake variable as
                follows:
                <literallayout class='monospaced'>
    CACHE[doc] = "The directory holding the cache of the metadata."
                </literallayout>
            </para>
        </section>

        <section id='inline-python-variable-expansion'>
            <title>Inline Python Variable Expansion</title>

            <para>
                You can use inline Python variable expansion to
                set variables.
                Here is an example:
                <literallayout class='monospaced'>
     DATE = "${@time.strftime('%Y%m%d',time.gmtime())}"
                </literallayout>
                This example results in the <filename>DATE</filename>
                variable being set to the current date.
            </para>

            <para>
                Probably the most common use of this feature is to extract
                the value of variables from BitBake's internal data dictionary,
                <filename>d</filename>. 
                The following lines select the values of a package name 
                and its version number, respectively:
                <literallayout class='monospaced'>
    PN = "${@bb.parse.BBHandler.vars_from_file(d.getVar('FILE'),d)[0] or 'defaultpkgname'}"
    PV = "${@bb.parse.BBHandler.vars_from_file(d.getVar('FILE'),d)[1] or '1.0'}"
                </literallayout>
            </para>
        </section>

        <section id='providing-pathnames'>
            <title>Providing Pathnames</title>

            <para>
                When specifying pathnames for use with BitBake,
                do not use the tilde ("~") character as a shortcut
                for your home directory.
                Doing so might cause BitBake to not recognize the
                path since BitBake does not expand this character in
                the same way a shell would.
            </para>

            <para>
                Instead, provide a fuller path as the following
                example illustrates:
                <literallayout class='monospaced'>
     BBLAYERS ?= " \
       /home/scott-lenovo/LayerA \
       "
                </literallayout>
            </para>
        </section>
    </section>

    <section id='conditional-syntax-overrides'>
        <title>Conditional Syntax (Overrides)</title>

        <para>
            BitBake uses
            <link linkend='var-OVERRIDES'><filename>OVERRIDES</filename></link>
            to control what variables are overridden after BitBake
            parses recipes and configuration files.
            This section describes how you can use
            <filename>OVERRIDES</filename> as conditional metadata,
            talks about key expansion in relationship to
            <filename>OVERRIDES</filename>, and provides some examples
            to help with understanding.
        </para>

        <section id='conditional-metadata'>
            <title>Conditional Metadata</title>

            <para>
                You can use <filename>OVERRIDES</filename> to conditionally select
                a specific version of a variable and to conditionally
                append or prepend the value of a variable.
                <itemizedlist>
                    <listitem><para><emphasis>Selecting a Variable:</emphasis>
                        The <filename>OVERRIDES</filename> variable is
                        a colon-character-separated list that contains items
                        for which you want to satisfy conditions.
                        Thus, if you have a variable that is conditional on “arm”, and “arm”
                        is in <filename>OVERRIDES</filename>, then the “arm”-specific
                        version of the variable is used rather than the non-conditional
                        version.
                        Here is an example:
                        <literallayout class='monospaced'>
     OVERRIDES = "architecture:os:machine"
     TEST = "default"
     TEST_os = "osspecific"
     TEST_nooverride = "othercondvalue"
                        </literallayout>
                        In this example, the <filename>OVERRIDES</filename>
                        variable lists three overrides:
                        "architecture", "os", and "machine".
                        The variable <filename>TEST</filename> by itself has a default
                        value of "default".
                        You select the os-specific version of the <filename>TEST</filename>
                        variable by appending the "os" override to the variable
                        (i.e.<filename>TEST_os</filename>).
                        </para>

                        <para>
                            To better understand this, consider a practical example
                            that assumes an OpenEmbedded metadata-based Linux 
                            kernel recipe file.
                            The following lines from the recipe file first set
                            the kernel branch variable <filename>KBRANCH</filename>
                            to a default value, then conditionally override that
                            value based on the architecture of the build:
                        <literallayout class='monospaced'>
    KBRANCH = "standard/base"
    KBRANCH_qemuarm  = "standard/arm-versatile-926ejs"
    KBRANCH_qemumips = "standard/mti-malta32"
    KBRANCH_qemuppc  = "standard/qemuppc"
    KBRANCH_qemux86  = "standard/common-pc/base"
    KBRANCH_qemux86-64  = "standard/common-pc-64/base"
    KBRANCH_qemumips64 = "standard/mti-malta64"
                        </literallayout>
                        </para></listitem>
                    <listitem><para><emphasis>Appending and Prepending:</emphasis>
                        BitBake also supports append and prepend operations to
                        variable values based on whether a specific item is
                        listed in <filename>OVERRIDES</filename>.
                        Here is an example:
                        <literallayout class='monospaced'>
     DEPENDS = "glibc ncurses"
     OVERRIDES = "machine:local"
     DEPENDS_append_machine = "libmad"
                        </literallayout>
                        In this example, <filename>DEPENDS</filename> becomes
                        "glibc ncurses libmad".
                        </para>

                        <para>
                            Again using a kernel recipe file as an example, the
                            following lines will conditionally append to the
                            <filename>KERNEL_FEATURES</filename> variable based
                            on the architecture:
                        <literallayout class='monospaced'>
    KERNEL_FEATURES_append = " ${KERNEL_EXTRA_FEATURES}"
    KERNEL_FEATURES_append_qemux86=" cfg/sound.scc cfg/paravirt_kvm.scc"
    KERNEL_FEATURES_append_qemux86-64=" cfg/sound.scc cfg/paravirt_kvm.scc"
                        </literallayout>
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='key-expansion'>
            <title>Key Expansion</title>

            <para>
                Key expansion happens when the BitBake datastore is finalized
                just before BitBake expands overrides.
                To better understand this, consider the following example:
                <literallayout class='monospaced'>
     A${B} = "X"
     B = "2"
     A2 = "Y"
                </literallayout>
                In this case, after all the parsing is complete, and
                before any overrides are handled, BitBake expands
                <filename>${B}</filename> into "2".
                This expansion causes <filename>A2</filename>, which was
                set to "Y" before the expansion, to become "X".
            </para>
        </section>

        <section id='variable-interaction-worked-examples'>
            <title>Examples</title>

            <para>
                Despite the previous explanations that show the different forms of
                variable definitions, it can be hard to work
                out exactly what happens when variable operators, conditional
                overrides, and unconditional overrides are combined.
                This section presents some common scenarios along
                with explanations for variable interactions that
                typically confuse users.
            </para>

            <para>
                There is often confusion concerning the order in which
                overrides and various "append" operators take effect.
                Recall that an append or prepend operation using "_append"
                and "_prepend" does not result in an immediate assignment
                as would "+=", ".=", "=+", or "=.".
                Consider the following example:
                <literallayout class='monospaced'>
     OVERRIDES = "foo"
     A = "Z"
     A_foo_append = "X"
                </literallayout>
                For this case, <filename>A</filename> is
                unconditionally set to "Z" and "X" is
                unconditionally and immediately appended to the variable
                <filename>A_foo</filename>.
                Because overrides have not been applied yet,
                <filename>A_foo</filename> is set to "X" due to the append
                and <filename>A</filename> simply equals "Z".
            </para>

            <para>
                Applying overrides, however, changes things.
                Since "foo" is listed in <filename>OVERRIDES</filename>,
                the conditional variable <filename>A</filename> is replaced
                with the "foo" version, which is equal to "X".
                So effectively, <filename>A_foo</filename> replaces <filename>A</filename>.
            </para>

            <para>
                This next example changes the order of the override and
                the append:
                <literallayout class='monospaced'>
     OVERRIDES = "foo"
     A = "Z"
     A_append_foo = "X"
                </literallayout>
                For this case, before overrides are handled,
                <filename>A</filename> is set to "Z" and <filename>A_append_foo</filename>
                is set to "X".
                Once the override for "foo" is applied, however,
                <filename>A</filename> gets appended with "X".
                Consequently, <filename>A</filename> becomes "ZX".
                Notice that spaces are not appended.
            </para>

            <para>
                This next example has the order of the appends and overrides reversed
                back as in the first example:
                <literallayout class='monospaced'>
     OVERRIDES = "foo"
     A = "Y"
     A_foo_append = "Z"
     A_foo_append += "X"
                </literallayout>
                For this case, before any overrides are resolved,
                <filename>A</filename> is set to "Y" using an immediate assignment.
                After this immediate assignment, <filename>A_foo</filename> is set
                to "Z", and then further appended with
                "X" leaving the variable set to "Z X".
                Finally, applying the override for "foo" results in the conditional
                variable <filename>A</filename> becoming "Z X" (i.e.
                <filename>A</filename> is replaced with <filename>A_foo</filename>).
            </para>

            <para>
                This final example mixes in some varying operators:
                <literallayout class='monospaced'>
     A = "1"
     A_append = "2"
     A_append = "3"
     A += "4"
     A .= "5"
                </literallayout>
                For this case, the type of append operators are affecting the
                order of assignments as BitBake passes through the code
                multiple times.
                Initially, <filename>A</filename> is set to "1 45" because
                of the three statements that use immediate operators.
                After these assignments are made, BitBake applies the
                <filename>_append</filename> operations.
                Those operations result in <filename>A</filename> becoming "1 4523".
            </para>
        </section>
    </section>

    <section id='sharing-functionality'>
        <title>Sharing Functionality</title>

        <para>
            BitBake allows for metadata sharing through include files
            (<filename>.inc</filename>) and class files
            (<filename>.bbclass</filename>).
            For example, suppose you have a piece of common functionality
            such as a task definition that you want to share between
            more than one recipe.
            In this case, creating a <filename>.bbclass</filename>
            file that contains the common functionality and then using
            the <filename>inherit</filename> directive in your recipes to
            inherit the class would be a common way to share the task.
        </para>

        <para>
            This section presents the mechanisms BitBake provides to
            allow you to share functionality between recipes.
            Specifically, the mechanisms include <filename>include</filename>,
            <filename>inherit</filename>, <filename>INHERIT</filename>, and
            <filename>require</filename> directives.
        </para>

        <section id='locating-include-and-class-files'>
            <title>Locating Include and Class Files</title>

            <para>
                BitBake uses the
                <link linkend='var-BBPATH'><filename>BBPATH</filename></link>
                variable to locate needed include and class files.
                The <filename>BBPATH</filename> variable is analogous to
                the environment variable <filename>PATH</filename>.
            </para>

            <para>
                In order for include and class files to be found by BitBake,
                they need to be located in a "classes" subdirectory that can
                be found in <filename>BBPATH</filename>.
            </para>
        </section>

        <section id='inherit-directive'>
            <title><filename>inherit</filename> Directive</title>

            <para>
                When writing a recipe or class file, you can use the
                <filename>inherit</filename> directive to inherit the
                functionality of a class (<filename>.bbclass</filename>).
                BitBake only supports this directive when used within recipe
                and class files (i.e. <filename>.bb</filename> and
                <filename>.bbclass</filename>).
            </para>

            <para>
                The <filename>inherit</filename> directive is a rudimentary
                means of specifying what classes of functionality your
                recipes require.
                For example, you can easily abstract out the tasks involved in
                building a package that uses Autoconf and Automake and put
                those tasks into a class file that can be used by your recipe.
            </para>

            <para>
                As an example, your recipes could use the following directive
                to inherit an <filename>autotools.bbclass</filename> file.
                The class file would contain common functionality for using
                Autotools that could be shared across recipes:
                <literallayout class='monospaced'>
     inherit autotools
                </literallayout>
                In this case, BitBake would search for the directory
                <filename>classes/autotools.bbclass</filename>
                in <filename>BBPATH</filename>.
                <note>
                    You can override any values and functions of the
                    inherited class within your recipe by doing so
                    after the "inherit" statement.
                </note>
            </para>
        </section>

        <section id='include-directive'>
            <title><filename>include</filename> Directive</title>

            <para>
                BitBake understands the <filename>include</filename>
                directive.
                This directive causes BitBake to parse whatever file you specify,
                and to insert that file at that location.
                The directive is much like its equivalent in Make except
                that if the path specified on the include line is a relative
                path, BitBake locates the first file it can find
                within <filename>BBPATH</filename>.
            </para>

            <para>
                As an example, suppose you needed a recipe to include some
                self-test definitions:
                <literallayout class='monospaced'>
     include test_defs.inc
                </literallayout>
                <note>
                    The <filename>include</filename> directive does not
                    produce an error when the file cannot be found.
                    Consequently, it is recommended that if the file you
                    are including is expected to exist, you should use
                    <link linkend='require-inclusion'><filename>require</filename></link>
                    instead of <filename>include</filename>.
                    Doing so makes sure that an error is produced if the
                    file cannot be found.
                </note>
            </para>
        </section>

        <section id='require-inclusion'>
            <title><filename>require</filename> Directive</title>

            <para>
                BitBake understands the <filename>require</filename>
                directive.
                This directive behaves just like the
                <filename>include</filename> directive with the exception that
                BitBake raises a parsing error if the file to be included cannot
                be found.
                Thus, any file you require is inserted into the file that is
                being parsed at the location of the directive.
            </para>

            <para>
                Similar to how BitBake handles
                <link linkend='include-directive'><filename>include</filename></link>,
                if the path specified
                on the require line is a relative path, BitBake locates
                the first file it can find within <filename>BBPATH</filename>.
            </para>

            <para>
                As an example, suppose you have two versions of a recipe
                (e.g. <filename>foo_1.2.2.bb</filename> and
                <filename>foo_2.0.0.bb</filename>) where
                each version contains some identical functionality that could be
                shared.
                You could create an include file named <filename>foo.inc</filename>
                that contains the common definitions needed to build "foo".
                You need to be sure <filename>foo.inc</filename> is located in the
                same directory as your two recipe files as well.
                Once these conditions are set up, you can share the functionality
                using a <filename>require</filename> directive from within each
                recipe:
                <literallayout class='monospaced'>
     require foo.inc
                </literallayout>
            </para>
        </section>

        <section id='inherit-configuration-directive'>
            <title><filename>INHERIT</filename> Configuration Directive</title>

            <para>
                When creating a configuration file (<filename>.conf</filename>),
                you can use the <filename>INHERIT</filename> directive to
                inherit a class.
                BitBake only supports this directive when used within
                a configuration file.
            </para>

            <para>
                As an example, suppose you needed to inherit a class
                file called <filename>abc.bbclass</filename> from a
                configuration file as follows:
                <literallayout class='monospaced'>
     INHERIT += "abc"
                </literallayout>
                This configuration directive causes the named
                class to be inherited at the point of the directive
                during parsing.
                As with the <filename>inherit</filename> directive, the
                <filename>.bbclass</filename> file must be located in a
                "classes" subdirectory in one of the directories specified
                in <filename>BBPATH</filename>.
                <note>
                    Because <filename>.conf</filename> files are parsed
                    first during BitBake's execution, using
                    <filename>INHERIT</filename> to inherit a class effectively
                    inherits the class globally (i.e. for all recipes).
                </note>
            </para>
        </section>
    </section>

    <section id='functions'>
        <title>Functions</title>

        <para>
            As with most languages, functions are the building blocks that
            are used to build up operations into tasks.
            BitBake supports these types of functions:
            <itemizedlist>
                <listitem><para><emphasis>Shell Functions:</emphasis>
                    Functions written in shell script and executed either
                    directly as functions, tasks, or both.
                    They can also be called by other shell functions.
                    </para></listitem>
                <listitem><para><emphasis>BitBake Style Python Functions:</emphasis>
                    Functions written in Python and executed by BitBake or other
                    Python functions using <filename>bb.build.exec_func()</filename>.
                    </para></listitem>
                <listitem><para><emphasis>Python Functions:</emphasis>
                    Functions written in Python and executed by Python.
                    </para></listitem>
                <listitem><para><emphasis>Anonymous Python Functions:</emphasis>
                    Python functions executed automatically during
                    parsing.
                    </para></listitem>
            </itemizedlist>
            Regardless of the type of function, you can only
            define them in class (<filename>.bbclass</filename>)
            and recipe (<filename>.bb</filename> or <filename>.inc</filename>)
            files.
        </para>

        <section id='shell-functions'>
            <title>Shell Functions</title>

            <para>
                Functions written in shell script and executed either
                directly as functions, tasks, or both.
                They can also be called by other shell functions.
                Here is an example shell function definition:
                <literallayout class='monospaced'>
     some_function () {
         echo "Hello World"
     }
                </literallayout>
                When you create these types of functions in your recipe
                or class files, you need to follow the shell programming
                rules.
                The scripts are executed by <filename>/bin/sh</filename>,
                which may not be a bash shell but might be something
                such as <filename>dash</filename>.
                You should not use Bash-specific script (bashisms).
            </para>
        </section>

        <section id='bitbake-style-python-functions'>
            <title>BitBake Style Python Functions</title>

            <para>
                These functions are written in Python and executed by
                BitBake or other Python functions using
                <filename>bb.build.exec_func()</filename>.
            </para>

            <para>
                An example BitBake function is:
                <literallayout class='monospaced'>
     python some_python_function () {
         d.setVar("TEXT", "Hello World")
         print d.getVar("TEXT", True)
     }
                </literallayout>
                Because the Python "bb" and "os" modules are already
                imported, you do not need to import these modules.
                Also in these types of functions, the datastore ("d")
                is a global variable and is always automatically
                available.
           </para>
        </section>

        <section id='python-functions'>
            <title>Python Functions</title>

            <para>
                These functions are written in Python and are executed by
                other Python code.
                Examples of Python functions are utility functions
                that you intend to call from in-line Python or
                from within other Python functions.
                Here is an example:
                <literallayout class='monospaced'>
     def get_depends(d):
         if d.getVar('SOMECONDITION', True):
             return "dependencywithcond"
         else:
             return "dependency"
     SOMECONDITION = "1"
     DEPENDS = "${@get_depends(d)}"
                </literallayout>
                This would result in <filename>DEPENDS</filename>
                containing <filename>dependencywithcond</filename>.
            </para>

            <para>
                Here are some things to know about Python functions:
                <itemizedlist>
                    <listitem><para>Python functions can take parameters.
                        </para></listitem>
                    <listitem><para>The BitBake datastore is not
                        automatically available.
                        Consequently, you must pass it in as a
                        parameter to the function.
                        </para></listitem>
                    <listitem><para>The "bb" and "os" Python modules are
                        automatically available.
                        You do not need to import them.
                        </para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='anonymous-python-functions'>
            <title>Anonymous Python Functions</title>

            <para>
                Sometimes it is useful to run some code during
                parsing to set variables or to perform other operations
                programmatically.
                To do this, you can define an anonymous Python function.
                Here is an example that conditionally sets a
                variable based on the value of another variable:
                <literallayout class='monospaced'>
     python __anonymous () {
         if d.getVar('SOMEVAR', True) == 'value':
             d.setVar('ANOTHERVAR', 'value2')
     }
                </literallayout>
                The "__anonymous" function name is optional, so the
                following example is functionally equivalent to the above:
                <literallayout class='monospaced'>
     python () {
         if d.getVar('SOMEVAR', True) == 'value':
             d.setVar('ANOTHERVAR', 'value2')
     }
                </literallayout>
                Because unlike other Python functions anonymous
                Python functions are executed during parsing, the
                "d" variable within an anonymous Python function represents
                the datastore for the entire recipe.
                Consequently, you can set variable values here and
                those values can be picked up by other functions.
            </para>
        </section>

        <section id='flexible-inheritance-for-class-functions'>
            <title>Flexible Inheritance for Class Functions</title>

            <para>
                Through coding techniques and the use of
                <filename>EXPORT_FUNCTIONS</filename>, BitBake supports
                exporting a function from a class such that the
                class function appears as the default implementation
                of the function, but can still be called if a recipe
                inheriting the class needs to define its own version of
                the function.
            </para>

            <para>
                To understand the benefits of this feature, consider
                the basic scenario where a class defines a task function
                and your recipe inherits the class.
                In this basic scenario, your recipe inherits the task
                function as defined in the class.
                If desired, your recipe can add to the start and end of the
                function by using the "_prepend" or "_append" operations
                respectively, or it can redefine the function completely.
                However, if it redefines the function, there is
                no means  for it to call the class version of the function.
                <filename>EXPORT_FUNCTIONS</filename> provides a mechanism
                that enables the recipe's version of the function to call
                the original version of the function.
            </para>

            <para>
                To make use of this technique, you need the following
                things in place:
                <itemizedlist>
                    <listitem><para>
                        The class needs to define the function as follows:
                        <literallayout class='monospaced'>
     &lt;classname&gt;_&lt;functionname&gt;
                        </literallayout>
                        For example, if you have a class file
                        <filename>bar.bbclass</filename> and a function named
                        <filename>do_foo</filename>, the class must define the function
                        as follows:
                        <literallayout class='monospaced'>
     bar_do_foo
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        The class needs to contain the <filename>EXPORT_FUNCTIONS</filename>
                        statement as follows:
                        <literallayout class='monospaced'>
     EXPORT_FUNCTIONS &lt;functionname&gt;
                        </literallayout>
                        For example, continuing with the same example, the
                        statement in the <filename>bar.bbclass</filename> would be
                        as follows:
                        <literallayout class='monospaced'>
     EXPORT_FUNCTIONS do_foo
                        </literallayout>
                        </para></listitem>
                    <listitem><para>
                        You need to call the function appropriately from within your
                        recipe.
                        Continuing with the same example, if your recipe
                        needs to call the class version of the function,
                        it should call <filename>bar_do_foo</filename>.
                        Assuming <filename>do_foo</filename> was a shell function
                        and <filename>EXPORT_FUNCTIONS</filename> was used as above,
                        the recipe's function could conditionally call the
                        class version of the function as follows:
                        <literallayout class='monospaced'>
     do_foo() {
             if [ somecondition ] ; then
                     bar_do_foo
             else
                     # Do something else
             fi
     }
                        </literallayout>
                        To call your modified version of the function as defined
                        in your recipe, call it as <filename>do_foo</filename>.
                        </para></listitem>
                </itemizedlist>
                With these conditions met, your single recipe
                can freely choose between the original function
                as defined in the class file and the modified function in your recipe.
                If you do not set up these conditions, you are limited to using one function
                or the other.
            </para>
        </section>
    </section>

    <section id='tasks'>
        <title>Tasks</title>

        <para>
            Tasks are BitBake execution units that originate as
            functions and make up the steps that BitBake needs to run
            for given recipe.
            Tasks are only supported in recipe (<filename>.bb</filename>
            or <filename>.inc</filename>) and class
            (<filename>.bbclass</filename>) files.
            By convention, task names begin with the string "do_".
        </para>

        <para>
            Here is an example of a task that prints out the date:
            <literallayout class='monospaced'>
     python do_printdate () {
         import time
         print time.strftime('%Y%m%d', time.gmtime())
     }
     addtask printdate after do_fetch before do_build
            </literallayout>
        </para>

        <section id='promoting-a-function-to-a-task'>
            <title>Promoting a Function to a Task</title>

            <para>
                Any function can be promoted to a task by applying the
                <filename>addtask</filename> command.
                The <filename>addtask</filename> command also describes
                inter-task dependencies.
                Here is the function from the previous section but with the
                <filename>addtask</filename> command promoting it to a task
                and defining some dependencies:
                <literallayout class='monospaced'>
     python do_printdate () {
         import time
         print time.strftime('%Y%m%d', time.gmtime())
     }
     addtask printdate after do_fetch before do_build
                </literallayout>
                In the example, the function is defined and then promoted
                as a task.
                The <filename>do_printdate</filename> task becomes a dependency of
                the <filename>do_build</filename> task, which is the default
                task.
                And, the <filename>do_printdate</filename> task is dependent upon
                the <filename>do_fetch</filename> task.
                Execution of the <filename>do_build</filename> task results
                in the <filename>do_printdate</filename> task running first.
            </para>
        </section>

        <section id='deleting-a-task'>
            <title>Deleting a Task</title>

            <para>
                As well as being able to add tasks, tasks can also be deleted.
                This is done simply with <filename>deltask</filename> command.
                For example, to delete the example task used in the previous
                sections, you would use:
                <literallayout class='monospaced'>
     deltask printdate
                </literallayout>
            </para>
        </section>

        <section id='passing-information-into-the-build-task-environment'>
            <title>Passing Information Into the Build Task Environment</title>

            <para>
                When running a task, BitBake tightly controls the execution
                environment of the build tasks to make
                sure unwanted contamination from the build machine cannot
                influence the build.
                Consequently, if you do want something to get passed into the
                build task environment, you must take these two steps:
                <orderedlist>
                    <listitem><para>
                        Tell BitBake to load what you want from the environment
                        into the datastore.
                        You can do so through the
                        <link linkend='var-BB_ENV_EXTRAWHITE'><filename>BB_ENV_EXTRAWHITE</filename></link>
                        variable.
                        For example, assume you want to prevent the build system from
                        accessing your <filename>$HOME/.ccache</filename>
                        directory.
                        The following command tells BitBake to load
                        <filename>CCACHE_DIR</filename> from the environment into
                        the datastore:
                        <literallayout class='monospaced'>
     export BB_ENV_EXTRAWHITE="$BB_ENV_EXTRAWHITE CCACHE_DIR"
                        </literallayout></para></listitem>
                    <listitem><para>
                        Tell BitBake to export what you have loaded into the
                        datastore to the task environment of every running task.
                        Loading something from the environment into the datastore
                        (previous step) only makes it available in the datastore.
                        To export it to the task environment of every running task,
                        use a command similar to the following in your local configuration
                        file <filename>local.conf</filename> or your
                        distribution configuration file:
                        <literallayout class='monospaced'>
     export CCACHE_DIR
                        </literallayout>
                        <note>
                            A side effect of the previous steps is that BitBake
                            records the variable as a dependency of the build process
                            in things like the setscene checksums.
                            If doing so results in unnecessary rebuilds of tasks, you can
                            whitelist the variable so that the setscene code
                            ignores the dependency when it creates checksums.
                        </note></para></listitem>
                </orderedlist>
            </para>

            <para>
                Sometimes, it is useful to be able to obtain information
                from the original execution environment.
                Bitbake saves a copy of the original environment into
                a special variable named
                <link linkend='var-BB_ORIGENV'><filename>BB_ORIGENV</filename></link>.
            </para>

            <para>
                The <filename>BB_ORIGENV</filename> variable returns a datastore
                object that can be queried using the standard datastore operators
                such as <filename>getVar()</filename>.
                The datastore object is useful, for example, to find the original
                <filename>DISPLAY</filename> variable.
                Here is an example:
                <literallayout class='monospaced'>
     BB_ORIGENV - add example?

     origenv = d.getVar("BB_ORIGENV", False)
     bar = origenv.getVar("BAR", False)
                </literallayout>
                The previous example returns <filename>BAR</filename> from the original
                execution environment.
            </para>

            <para>
                By default, BitBake cleans the environment to include only those
                things exported or listed in its whitelist to ensure that the build
                environment is reproducible and consistent.
            </para>
        </section>
    </section>

    <section id='variable-flags'>
        <title>Variable Flags</title>

        <para>
            Variable flags (varflags) help control a task's functionality
            and dependencies.
            BitBake reads and writes varflags to the datastore using the following
            command forms:
            <literallayout class='monospaced'>
     &lt;variable&gt; = d.getVarFlags("&lt;variable&gt;")
     self.d.setVarFlags("FOO", {"func": True})
            </literallayout>
        </para>

        <para>
            When working with varflags, the same syntax, with the exception of
            overrides, applies.
            In other words, you can set, append, and prepend varflags just like
            variables.
            See the
            "<link linkend='variable-flag-syntax'>Variable Flag Syntax</link>"
            section for details.
        </para>

        <para>
            BitBake has a defined set of varflags available for recipes and
            classes.
            Tasks support a number of these flags which control various
            functionality of the task:
            <itemizedlist>
                <listitem><para><emphasis>dirs:</emphasis>
                    Directories that should be created before the task runs.
                    </para></listitem>
                <listitem><para><emphasis>cleandirs:</emphasis>
                    Empty directories that should created before the task runs.
                    </para></listitem>
                <listitem><para><emphasis>noexec:</emphasis>
                    Marks the tasks as being empty and no execution required.
                    The <filename>noexec</filename> flag can be used to set up
                    tasks as dependency placeholders, or to disable tasks defined
                    elsewhere that are not needed in a particular recipe.
                    </para></listitem>
                <listitem><para><emphasis>nostamp:</emphasis>
                    Tells BitBake to not generate a stamp file for a task,
                    which implies the task should always be executed.
                    </para></listitem>
                <listitem><para><emphasis>fakeroot:</emphasis>
                    Causes a task to be run in a fakeroot environment,
                    obtained by adding the variables in
                    <link linkend='var-FAKEROOTENV'><filename>FAKEROOTENV</filename></link>
                    to the environment.
                    </para></listitem>
                <listitem><para><emphasis>umask:</emphasis>
                    The umask to run the task under.
                    </para></listitem>
                <listitem><para><emphasis>deptask:</emphasis>
                    Controls task build-time dependencies.
                    See the
                    <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
                    variable and the
                    "<link linkend='build-dependencies'>Build Dependencies</link>"
                    section for more information.
                    </para></listitem>
                <listitem><para><emphasis>rdeptask:</emphasis>
                    Controls task runtime dependencies.
                    See the
                    <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>
                    variable, the
                    <link linkend='var-RRECOMMENDS'><filename>RRECOMMENDS</filename></link>
                    variable, and the
                    "<link linkend='runtime-dependencies'>Runtime Dependencies</link>"
                    section for more information.
                    </para></listitem>
                <listitem><para><emphasis>recrdeptask:</emphasis>
                    Controls task recursive runtime dependencies.
                    See the
                    <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>
                    variable, the
                    <link linkend='var-RRECOMMENDS'><filename>RRECOMMENDS</filename></link>
                    variable, and the
                    "<link linkend='recursive-dependencies'>Recursive Dependencies</link>"
                    section for more information.
                    </para></listitem>
                <listitem><para><emphasis>depends:</emphasis>
                    Controls inter-task dependencies.
                    See the
                    <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
                    variable and the
                    "<link linkend='inter-task-dependencies'>Inter-Task Dependencies</link>"
                    section for more information.
                    </para></listitem>
                <listitem><para><emphasis>rdepends:</emphasis>
                    Controls inter-task runtime dependencies.
                    See the
                    <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>
                    variable, the
                    <link linkend='var-RRECOMMENDS'><filename>RRECOMMENDS</filename></link>
                    variable, and the
                    "<link linkend='inter-task-dependencies'>Inter-Task Dependencies</link>"
                    section for more information.
                    </para></listitem>
                <listitem><para><emphasis>postfuncs:</emphasis>
                    List of functions to call after the completion of the task.
                    </para></listitem>
                <listitem><para><emphasis>prefuncs:</emphasis>
                    List of functions to call before the task executes.
                    </para></listitem>
                <listitem><para><emphasis>stamp-extra-info:</emphasis>
                    Extra stamp information to append to the task's stamp.
                    As an example, OpenEmbedded uses this flag to allow
                    machine-specific tasks.
                    </para></listitem>
            </itemizedlist>
        </para>

        <para>
            Several varflags are useful for controlling how signatures are
            calculated for variables.
            For more information on this process, see the
            "<link linkend='checksums'>Checksums (Signatures)</link>"
            section.
            <itemizedlist>
                <listitem><para><emphasis>vardeps:</emphasis>
                    Specifies a space-separated list of additional
                    variables to add to a variable's dependencies
                    for the purposes of calculating its signature.
                    Adding variables to this list is useful, for example, when
                    a function refers to a variable in a manner that
                    does not allow BitBake to automatically determine
                    that the variable is referred to.
                    </para></listitem>
                <listitem><para><emphasis>vardepvalue:</emphasis>
                    If set, instructs BitBake to ignore the actual
                    value of the variable and instead use the specified
                    value when calculating the variable's signature.
                    </para></listitem>
                <listitem><para><emphasis>vardepsexclude:</emphasis>
                    Specifies a space-separated list of variables
                    that should be excluded from a variable's dependencies
                    for the purposes of calculating its signature.
                    </para></listitem>
                <listitem><para><emphasis>vardepvalueexclude:</emphasis>
                    Specifies a pipe-separated list of strings to exclude
                    from the variable's value when calculating the
                    variable's signature.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='events'>
        <title>Events</title>

        <para>
            BitBake allows installation of event handlers within
            recipe and class files.
            Events are triggered at certain points during operation,
            such as the beginning of operation against a given
            <filename>.bb</filename>, the start of a given task,
            task failure, task success, and so forth.
            The intent is to make it easy to do things like email
            notification on build failure.
        </para>

        <para>
            Following is an example event handler that
            prints the name of the event and the content of
            the <filename>FILE</filename> variable:
            <literallayout class='monospaced'>
     addhandler myclass_eventhandler
     python myclass_eventhandler() {
         from bb.event import getName
         from bb import data
         print("The name of the Event is %s" % getName(e))
         print("The file we run for is %s" % data.getVar('FILE', e.data, True))
     }
            </literallayout>
            This event handler gets called every time an event is
            triggered.
            A global variable "<filename>e</filename>" is defined and
            "<filename>e.data</filename>" contains an instance of
            "<filename>bb.data</filename>".
            With the <filename>getName(e)</filename> method, one can get
            the name of the triggered event.
        </para>

        <para>
            During a standard build, the following common events might occur:
            <itemizedlist>
                <listitem><para>
                    <filename>bb.event.ConfigParsed()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ParseStarted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ParseProgress()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ParseCompleted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.BuildStarted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskStarted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskInvalid()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskFailedSilent()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskFailed()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.build.TaskSucceeded()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.BuildCompleted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.cooker.CookerExit()</filename>
                    </para></listitem>
            </itemizedlist>
            Here is a list of other events that occur based on specific requests
            to the server:
            <itemizedlist>
                <listitem><para>
                    <filename>bb.event.TreeDataPreparationStarted()</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.TreeDataPreparationProgress</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.TreeDataPreparationCompleted</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.DepTreeGenerated</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.CoreBaseFilesFound</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ConfigFilePathFound</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.FilesMatchingFound</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.ConfigFilesFound</filename>
                    </para></listitem>
                <listitem><para>
                    <filename>bb.event.TargetsTreeGenerated</filename>
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='variants-class-extension-mechanism'>
        <title>Variants - Class Extension Mechanism</title>

        <para>
            BitBake supports two features that facilitate creating
            from a single recipe file multiple incarnations of that
            recipe file where all incarnations are buildable.
            These features are enabled through the
            <link linkend='var-BBCLASSEXTEND'><filename>BBCLASSEXTEND</filename></link>
            and
            <link linkend='var-BBVERSIONS'><filename>BBVERSIONS</filename></link>
            variables.
            <note>
                The mechanism for this class extension is extremely
                specific to the implementation.
                Usually, the recipe's
                <link linkend='var-PROVIDES'><filename>PROVIDES</filename></link>,
                <link linkend='var-PN'><filename>PN</filename></link>, and
                <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
                variables would need to be modified by the extension class.
                For specific examples, see the OE-Core
                <filename>native</filename>, <filename>nativesdk</filename>,
                and <filename>multilib</filename> classes.
            </note>
            <itemizedlist>
                <listitem><para><emphasis><filename>BBCLASSEXTEND</filename>:</emphasis>
                    This variable is a space separated list of classes used to "extend" the
                    recipe for each variant.
                    Here is an example that results in a second incarnation of the current
                    recipe being available.
                    This second incarnation will have the "native" class inherited.
                    <literallayout class='monospaced'>
     BBCLASSEXTEND = "native"
                    </literallayout></para></listitem>
                <listitem><para><emphasis><filename>BBVERSIONS</filename>:</emphasis>
                    This variable allows a single recipe to build multiple versions of a
                    project from a single recipe file.
                    You can also specify conditional metadata
                    (using the
                    <link linkend='var-OVERRIDES'><filename>OVERRIDES</filename></link>
                    mechanism) for a single version, or an optionally named range of versions.
                    Here is an example:
                    <literallayout class='monospaced'>
     BBVERSIONS = "1.0 2.0 git"
     SRC_URI_git = "git://someurl/somepath.git"

     BBVERSIONS = "1.0.[0-6]:1.0.0+ \ 1.0.[7-9]:1.0.7+"
     SRC_URI_append_1.0.7+ = "file://some_patch_which_the_new_versions_need.patch;patch=1"
                    </literallayout>
                    The name of the range defaults to the original version of the
                    recipe.
                    For example, in OpenEmbedded, the recipe file
                    <filename>foo_1.0.0+.bb</filename> creates a default name range
                    of <filename>1.0.0+</filename>.
                    This is useful because the range name is not only placed
                    into overrides, but it is also made available for the metadata to use
                    in the variable that defines the base recipe versions for use in
                    <filename>file://</filename> search paths
                    (<link linkend='var-FILESPATH'><filename>FILESPATH</filename></link>).
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id='dependencies'>
        <title>Dependencies</title>

        <para>
            To allow for efficient operation given multiple processes
            executing in parallel, BitBake handles dependencies at
            the task level.
            BitBake supports a robust method to handle these dependencies.
        </para>

        <para>
            This section describes several types of dependency mechanisms.
        </para>

        <section id='dependencies-internal-to-the-bb-file'>
            <title>Dependencies Internal to the <filename>.bb</filename> File</title>

            <para>
                BitBake uses the <filename>addtask</filename> directive
                to manage dependencies that are internal to a given recipe
                file.
                You can use the <filename>addtask</filename> directive to
                indicate when a task is dependent on other tasks or when
                other tasks depend on that recipe.
                Here is an example:
                <literallayout class='monospaced'>
     addtask printdate after do_fetch before do_build
                </literallayout>
                In this example, the <filename>printdate</filename> task is
                depends on the completion of the <filename>do_fetch</filename>
                task.
                And, the <filename>do_build</filename> depends on the completion
                of the <filename>printdate</filename> task.
            </para>
        </section>

        <section id='build-dependencies'>
            <title>Build Dependencies</title>

            <para>
                BitBake uses the
                <link linkend='var-DEPENDS'><filename>DEPENDS</filename></link>
                variable to manage build time dependencies.
                The "deptask" varflag for tasks signifies the task of each
                item listed in <filename>DEPENDS</filename> that must
                complete before that task can be executed.
                Here is an example:
                <literallayout class='monospaced'>
     do_configure[deptask] = "do_populate_sysroot"
                </literallayout>
                In this example, the <filename>do_populate_sysroot</filename>
                task of each item in <filename>DEPENDS</filename> must complete before
                <filename>do_configure</filename> can execute.
            </para>
        </section>

        <section id='runtime-dependencies'>
            <title>Runtime Dependencies</title>

            <para>
                BitBake uses the
                <link linkend='var-PACKAGES'><filename>PACKAGES</filename></link>,
                <link linkend='var-RDEPENDS'><filename>RDEPENDS</filename></link>, and
                <link linkend='var-RRECOMMENDS'><filename>RRECOMMENDS</filename></link>
                variables to manage runtime dependencies.
            </para>

            <para>
                The <filename>PACKAGES</filename> variable lists runtime
                packages.
                Each of those packages can have <filename>RDEPENDS</filename> and
                <filename>RRECOMMENDS</filename> runtime dependencies.
                The "rdeptask" flag for tasks is used to signify the task of each
                item runtime dependency which must have completed before that
                task can be executed.
                <literallayout class='monospaced'>
     do_package_write[rdeptask] = "do_package"
                </literallayout>
                In the previous example, the <filename>do_package</filename>
                task of each item in <filename>RDEPENDS</filename> must have
                completed before <filename>do_package_write</filename> can execute.
            </para>
        </section>

        <section id='recursive-dependencies'>
            <title>Recursive Dependencies</title>

            <para>
                BitBake uses the "recrdeptask" flag to manage
                recursive task dependencies.
                BitBake looks through the build-time and runtime
                dependencies of the current recipe, looks through
                the task's inter-task
                dependencies, and then adds dependencies for the
                listed task.
                Once BitBake has accomplished this, it recursively works through
                the dependencies of those tasks.
                Iterative passes continue until all dependencies are discovered
                and added.
            </para>

            <para>
                You might want to not only have BitBake look for
                dependencies of those tasks, but also have BitBake look
                for build-time and runtime dependencies of the dependent
                tasks as well.
                If that is the case, you need to reference the task name
                itself in the task list:
                <literallayout class='monospaced'>
     do_a[recrdeptask] = "do_a do_b"
                </literallayout>
            </para>
        </section>

        <section id='inter-task-dependencies'>
            <title>Inter-Task Dependencies</title>

            <para>
                BitBake uses the "depends" flag in a more generic form
                to manage inter-task dependencies.
                This more generic form allows for inter-dependency
                checks for specific tasks rather than checks for
                the data in <filename>DEPENDS</filename>.
                Here is an example:
                <literallayout class='monospaced'>
     do_patch[depends] = "quilt-native:do_populate_sysroot"
                </literallayout>
                In this example, the <filename>do_populate_sysroot</filename>
                task of the target <filename>quilt-native</filename>
                must have completed before the
                <filename>do_patch</filename> task can execute.
            </para>

            <para>
                The "rdepends" flag works in a similar way but takes targets
                in the runtime namespace instead of the build-time dependency
                namespace.
            </para>
        </section>
    </section>

    <section id='accessing-datastore-variables-using-python'>
        <title>Accessing Datastore Variables Using Python</title>

        <para>
            It is often necessary to access variables in the
            BitBake datastore using Python functions.
            The Bitbake datastore has an API that allows you this
            access.
            Here is a list of available operations:
        </para>

        <para>
            <informaltable frame='none'>
                <tgroup cols='2' align='left' colsep='1' rowsep='1'>
                    <colspec colname='c1' colwidth='1*'/>
                    <colspec colname='c2' colwidth='1*'/>
                    <thead>
                        <row>
                            <entry align="left"><emphasis>Operation</emphasis></entry>
                            <entry align="left"><emphasis>Description</emphasis></entry>
                        </row>
                    </thead>
                    <tbody>
                        <row>
                            <entry align="left"><filename>d.getVar("X", expand=False)</filename></entry>
                            <entry align="left">Returns the value of variable "X".
                                Using "expand=True" expands the value.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.setVar("X", "value")</filename></entry>
                            <entry align="left">Sets the variable "X" to "value".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.appendVar("X", "value")</filename></entry>
                            <entry align="left">Adds "value" to the end of the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.prependVar("X", "value")</filename></entry>
                            <entry align="left">Adds "value" to the start of the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.delVar("X")</filename></entry>
                            <entry align="left">Deletes the variable "X" from the datastore.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.renameVar("X", "Y")</filename></entry>
                            <entry align="left">Renames the variable "X" to "Y".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.getVarFlag("X", flag, expand=False)</filename></entry>
                            <entry align="left">Gets then named flag from the variable "X".
                                Using "expand=True" expands the named flag.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.setVarFlag("X", flag, "value")</filename></entry>
                            <entry align="left">Sets the named flag for variable "X" to "value".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.appendVarFlag("X", flag, "value")</filename></entry>
                            <entry align="left">Appends "value" to the named flag on the
                            variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.prependVarFlag("X", flag, "value")</filename></entry>
                            <entry align="left">Prepends "value" to the named flag on
                               the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.delVarFlag("X", flag)</filename></entry>
                            <entry align="left">Deletes the named flag on the variable
                                "X" from the datastore.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.setVarFlags("X", flagsdict)</filename></entry>
                            <entry align="left">Sets the flags specified in
                                the <filename>flagsdict()</filename> parameter.
                                <filename>setVarFlags</filename> does not clear previous flags.
                                Think of this operation as <filename>addVarFlags</filename>.</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.getVarFlags("X")</filename></entry>
                            <entry align="left">Returns a <filename>flagsdict</filename> of the flags for
                                the variable "X".</entry>
                        </row>
                        <row>
                            <entry align="left"><filename>d.delVarFlags("X")</filename></entry>
                            <entry align="left">Deletes all the flags for the variable "X".</entry>
                        </row>
                    </tbody>
                </tgroup>
            </informaltable>
        </para>
    </section>

    <section id='task-checksums-and-setscene'>
        <title>Task Checksums and Setscene</title>

        <para>
            BitBake uses checksums (or signatures) along with the setscene
            to determine if a task needs to be run.
            This section describes the process.
            To help understand how BitBake does this, the section assumes an
            OpenEmbedded metadata-based example.
        </para>

        <para>
            This list is a place holder of content existed from previous work
            on the manual.
            Some or all of it probably needs integrated into the subsections
            that make up this section.
            For now, I have just provided a short glossary-like description
            for each variable.
            Ultimately, this list goes away.
            <itemizedlist>
                <listitem><para><filename>STAMP</filename>:
                    The base path to create stamp files.</para></listitem>
                <listitem><para><filename>STAMPCLEAN</filename>
                    Again, the base path to create stamp files but can use wildcards
                    for matching a range of files for clean operations.
                    </para></listitem>
                <listitem><para><filename>BB_STAMP_WHITELIST</filename>
                    Lists stamp files that are looked at when the stamp policy
                    is "whitelist".
                    </para></listitem>
                <listitem><para><filename>BB_STAMP_POLICY</filename>
                    Defines the mode for comparing timestamps of stamp files.
                    </para></listitem>
                <listitem><para><filename>BB_HASHCHECK_FUNCTION</filename>
                    Specifies the name of the function to call during
                    the "setscene" part of the task's execution in order
                    to validate the list of task hashes.
                    </para></listitem>
                <listitem><para><filename>BB_SETSCENE_VERIFY_FUNCTION</filename>
                    Specifies a function to call that verifies the list of
                    planned task execution before the main task execution
                    happens.
                    </para></listitem>
                <listitem><para><filename>BB_SETSCENE_DEPVALID</filename>
                    Specifies a function BitBake calls that determines
                    whether BitBake requires a setscene dependency to
                    be met.
                    </para></listitem>
                <listitem><para><filename>BB_TASKHASH</filename>
                    Within an executing task, this variable holds the hash
                    of the task as returned by the currently enabled
                    signature generator.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>
</chapter>