aboutsummaryrefslogtreecommitdiffstats
path: root/doc/user-manual/user-manual-metadata.xml
blob: 66f49f63a92717676907841eab35a23b3e6e1e8c (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
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
    "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<chapter id="user-manual-metadata">

  <title>Metadata</title>
  <section>
    <title>Description</title>
    <para>The BitBake task executor, together with various types of
	    configuration files form the OpenEmbedded Core.
	    This section provides an overview of the BitBake task executor
	    and the configuration files by describing what they are used for
	    and how they interact.
    </para>
    <para>BitBake handles the parsing and execution of the data files.
	    The data itself is of various types: 
	    <itemizedlist>
		    <listitem>
			    <para>
				    <emphasis>Recipes:</emphasis>
				    Provides details about particular pieces
				    of software.
		    </para>
	    </listitem>
	    
	    <listitem>
		    <para>
		    			<emphasis>Class Data:</emphasis>
			An abstraction of common build information (e.g. how
			to build a Linux kernel).
		</para>
	</listitem>
      <listitem>
        <para>
		<emphasis>Configuration Data:</emphasis>Defines
		machine-specific settings, policy decisions, etc.
		Configuration data acts as the glue to bind everything
		together.		
			</para>
      </listitem>
    </itemizedlist>
    <para>What follows is a large number of examples of BitBake metadata.
	    Any syntax which is not supported in any of the aforementioned areas
	    will be documented as such.
    </para>
    </para>
    </section>

<section>
<title>Basic Syntax</title>
    <section>
      <title>Basic Variable Setting</title>
      <para>
        <screen>
        <varname>VARIABLE</varname> = "value"</screen>
      </para>
      <para>In this example, 
      <varname>VARIABLE</varname> is 
      <literal>value</literal>.</para>
    </section>
    
    <section>
      <title>Variable Expansion</title>
      <para>BitBake supports variables referencing one another's contents using
	      a syntax which is similar to shell scripting:
      </para>
      <para>
        <screen>
        <varname>A</varname> = "aval" 
        <varname>B</varname> = "pre${A}post"</screen>
      </para>
      <para>This results in 
      <varname>A</varname> containing 
      <literal>aval</literal> and 
      <varname>B</varname> containing 
      <literal>preavalpost</literal>.</para>
    </section>
    
    <section>
      <title>Setting a Default Value (?=)</title>
      <para>
        <screen>
        <varname>A</varname>?= "aval"</screen>
      </para>
      <para>If 
      <varname>A</varname> is set before the above is called, it
      will retain its previous value. If 
      <varname>A</varname> is unset prior to the above call, 
      <varname>A</varname> will be set to 
      <literal>aval</literal>. Note that this assignment is
      immediate, so if there are multiple ?= assignments to a
      single variable, the first of those will be used.</para>
    </section>
    
    <section>
      <title>Setting a Weak Default Value (??=)</title>
      <para>
        <screen>
        <varname>A</varname>??= "somevalue" 
        <varname>A</varname>??= "someothervalue"</screen>
      </para>
      <para>If 
	      <varname>A</varname> 
	      is set before the above, it will retain that value.
	      If 
	      <varname>A</varname> 
	      is unset prior to the above, 
	      <varname>A</varname> 
	      will be set to <literal>someothervalue</literal>. 
	      This is a lazy/weak assignment in that the assignment does not
	      occur until the end of the parsing process, so that the last,
	      rather than the first, ??= assignment to a given variable will 
	      be used.
	      Any other setting of A using = or ?= will however override the
	      value set with ??=.
      </para>
    </section>
    
    <section>
      <title>Immediate Variable Expansion (:=)</title>
      <para>:= results in a variable's contents being expanded immediately,
	      rather than when the variable is actually used.
      </para>
      
      <para>
        <screen>
        <varname>T</varname> = "123" 
        <varname>A</varname>:= "${B} ${A} test ${T}" 
        <varname>T</varname> = "456" 
        <varname>B</varname> = "${T} bval" 
        <varname>C</varname> = "cval" 
        <varname>C</varname>:= "${C}append"</screen>
      </para>
      <para>In this example, 
	      <varname>A</varname> 
	      would contain
	      <literal>test 123</literal>,
	      <varname>B</varname> 
	      would contain
	      <literal>456 bval</literal>, and
	      <varname>C</varname> 
	      would be
	      <literal>cvalappend</literal>.
      </para>
    </section>

    <section>
      <title>Appending (+=) and Prepending (=+)</title>
      <para>
        <screen>
        <varname>B</varname> = "bval" 
        <varname>B</varname>+= "additionaldata" 
        <varname>C</varname> = "cval" 
        <varname>C</varname>=+ "test"</screen>
      </para>
      <para>In this example, 
	      <varname>B</varname> 
	      is now
	      <literal>bval additionaldata</literal> 
	      and
	      <varname>C</varname> 
	      is 
	      <literal>test cval</literal>.
      </para>
    </section>
    <section>
	    <title>Appending (.=) and Prepending (=.) without spaces</title>
	    <para>
		    <screen>
			    <varname>B</varname> = "bval" 
			    <varname>B</varname>.= "additionaldata" 
			    <varname>C</varname> = "cval" 
			    <varname>C</varname>=. "test"
		    </screen>
	    </para>
	    <para>In this example,
	    <varname>B</varname>		is now
		   <literal>bvaladditionaldata</literal> 
		and 
		<varname>C</varname> 
		is 
		<literal>testcval</literal>.
		In contrast to the above appending and prepending operators,
		no additional space will be introduced.
	</para>
    </section>

    <section>
                <title>Appending and Prepending (override style syntax)</title>
                    <para><screen><varname>B</varname> = "bval"
<varname>B_append</varname> = " additional data"
<varname>C</varname> = "cval"
<varname>C_prepend</varname> = "additional data "</screen></para>
                 <para>This example results in <varname>B</varname> becoming <literal>bval additional data</literal>
and <varname>C</varname> becoming <literal>additional data cval</literal>. Note the spaces in the append.
Unlike the += operator, additional space is not automatically added.  You must take steps to add space
yourself.</para>
            </section>
            <section>
                <title>Removing (override style syntax)</title>
                <para><screen><varname>FOO</varname> = "123 456 789 123456 123 456 123 456"
<varname>FOO_remove</varname> = "123"
<varname>FOO_remove</varname> = "456"</screen></para>
                <para>In this example, <varname>FOO</varname> is now <literal>789 123456</literal>.</para>
    </section>

<section>
	<title>Variable Flags</title>
	<para>Variables can have associated flags which provide a way of 
		tagging extra information onto a variable.
		Several flags are used internally by BitBake but they can be 
		used externally too if needed.
		The standard operations mentioned above also work on flags.
	</para>
	<para>
		<screen>
			<varname>VARIABLE</varname>
			[
			<varname>SOMEFLAG</varname>
			] = "value"
		</screen>
	</para>
	<para>In this example,
		<varname>VARIABLE</varname> 
		has a flag, 
		<varname>SOMEFLAG</varname> 
		that is set to 
		<literal>value</literal>.
	</para>
	</section>
	
<section>
	<title>Inline Python variable expansion</title>
	<para>
		<screen>
			<varname>DATE</varname> =
			 "${@time.strftime('%Y%m%d',time.gmtime())}"
		 </screen>
	</para>
	<para>This would result in the
		<varname>DATE</varname> 
		variable containing today's date.
	</para>
	</section>
</section>

<section>
     <title>Conditional Syntax (Overrides)</title>
     
    <section>
	    <title>Conditional Metadata</title>
	    <para>OVERRIDES is a 
		    <quote>:</quote> 
		    separated variable containing each item you want to satisfy
		    conditions.
		    So, if you have a variable that is conditional on
		    <quote>arm</quote>, and
		    <quote>arm</quote> 
		    is in OVERRIDES, then the 
		    <quote>arm</quote> 
		    specific version of the variable is used rather than the
		    non-conditional version.
		    Example:
	    </para>
	    <para>
		    <screen>
			    <varname>OVERRIDES</varname> 
			    = "architecture:os:machine" 
			    <varname>TEST</varname> 
			    = "defaultvalue" 
			    <varname>TEST_os</varname> 
			    = "osspecificvalue" 
			    <varname>TEST_condnotinoverrides</varname> 
			    = "othercondvalue"
		    </screen>
      </para>
      <para>In this example, 
	      <varname>TEST</varname> 
	      would be <literal>osspecificvalue</literal>, 
	      due to the condition
	      <quote>os</quote> being in 
	      <varname>OVERRIDES</varname>.
      </para>
    </section>
    
    <section>
	    <title>Conditional Appending</title>
	    <para>BitBake also supports appending and prepending to variables 
		    based on whether something is in OVERRIDES.
		    Example:
	    </para>
	    <para>
		    <screen>
			    <varname>DEPENDS</varname> 
			    = "glibc ncurses" 
			    <varname>OVERRIDES</varname> 
			    = "machine:local" 
			    <varname>DEPENDS_append_machine</varname> 
			    = "libmad"
		    </screen>
		    </para>
		    <para>In this example,
		    <varname>DEPENDS</varname> is set to
		<literal>glibc ncurses libmad</literal>.
	</para>
	</section>

<section>
	<title>Variable Interaction: Worked Examples</title>
	<para>Despite the documentation of the different forms of variable 
		definition above, it can be hard to work out what happens when 
		variable operators are combined.
		This section documents some common questions people have 
		regarding the way variables interact.
	</para>
	</section>
	
	<section>
	<title>Override and Append Ordering</title>
	<para>There is often confusion about which order overrides and the 
		various append operators take effect.
	</para>
	<para>
		<screen>
			<varname>OVERRIDES</varname> 
			= "foo" 
			<varname>A_foo_append</varname> 
			= "X"</screen>
	</para>
	<para>In this case, X is unconditionally appended to the variable 
		<varname>A_foo</varname>. 
		Since foo is an override, A_foo would then replace 
		<varname>A</varname>.</para>
	<para>
		<screen>
			<varname>OVERRIDES</varname> 
			= "foo" 
			<varname>A</varname> 
			= "X" 
			<varname>A_append_foo</varname> 
			= "Y"
		</screen>
	</para>
	<para>In this case, only when foo is in OVERRIDES, Y is appended to the
		variable 
		<varname>A</varname> 
		so the value of 
		<varname>A</varname> 
		would become XY (NB: no spaces are appended).
	</para>
	<para>
		<screen>
			<varname>OVERRIDES</varname> 
			= "foo" 
			<varname>A_foo_append</varname> 
			= "X" 
			<varname>A_foo_append</varname>
			+= "Y"
		</screen>
        </para>
	<para>This behaves as per the first case above, but the value of
		<varname>A</varname> 
		would be "X Y" instead of just "X".
	</para>
	<para>
		<screen>
			<varname>A</varname> 
			= "1" 
			<varname>A_append</varname> 
			= "2" 
			<varname>A_append</varname> 
			= "3" 
			<varname>A</varname>
			+= "4" 
			<varname>A</varname>
			.= "5"
		</screen>
        </para>
	<para>Would ultimately result in 
		<varname>A</varname> 
		taking the value "1 4523" since the _append operator executes 
		at the same time as the expansion of other overrides.
	</para>
	</section>
	
	<section>
	<title>Key Expansion</title>
	<para>Key expansion happens at the data store finalisation time just
		before overrides are expanded.
	</para>
	<para>
		<screen>
			<varname>A${B}</varname> 
			= "X"
			<varname>B</varname> 
			= "2" 
			<varname>A2</varname> 
			= "Y"
		</screen>
	</para>
	<para>So in this case 
		<varname>A2</varname> 
		would take the value of "X".
	</para>
	</section>
	</section>
	
<section>
<title>Inheritance</title>
<section>
	<title>Inheritance</title>
	<para>
		<emphasis>NOTE:</emphasis> This is only supported in .bb 
		and .bbclass files.
	</para>
	<para>The <literal>inherit</literal> directive is a means of specifying
		what classes of functionality your .bb requires.
		It is a rudimentary form of inheritance.
		For example, you can easily abstract out the tasks involved in
		building a package that uses autoconf and automake, and put 
		that into a bbclass for your packages to make use of. 
		A given bbclass is located by searching for 
		classes/filename.bbclass in 
		<envar>BBPATH</envar>, 
		where filename is what you inherited.
	</para>
	</section>

<section>
	<title>Inclusion</title>
	<para>Next, there is the 
		<literal>include</literal> directive, that causes BitBake to
		parse whatever file you specify, and insert it at that
		location, which is not unlike
		<command>Make</command>. However, if the path specified on the
		<literal>include</literal> 
		line is a relative path, BitBake will locate the first one it
		can find within 
		<envar>BBPATH</envar>.
   FIX ME: This section requires improvement.
	</para>
	</section>
	
	<section>
	<title>Requiring Inclusion</title>
	<para>In contrast to the 
		<literal>include</literal> 
		directive, 
		<literal>require</literal> 
		will raise a ParseError if the file to be included cannot be
		found.
		Otherwise it will behave just like the 
		<literal>include</literal> 
		directive.
		FIX ME: This section requires improvement.
		
	</para>
	</section>
	
	<section>
	<title>INHERIT Configuration Directive</title>
	<para>
	     This configuration directive causes the named class to be inherited at 
	     this point during parsing.
	     This variable is only valid in configuration files.
	     </para>
	     
	     <para>
	     </para>
	     
</section>
</section>

	<section>
	<title>Defining Python Functions into the Global Python Namespace
	</title>
	<para>
		<emphasis>NOTE:</emphasis> 
		This is only supported in .bb and .bbclass files.
	</para>
	<para>
		<emphasis>NOTE:</emphasis>
		Python functions are in the global namespace so should use
		unique names.
	</para>
	<para>
		<screen>
		def get_depends(d):
		   if d.getVar('SOMECONDITION', True):
		       return	"dependencywithcond"
		   else:
		       return "dependency" 
		SOMECONDITION = "1" 
		DEPENDS = "${@get_depends(d)}"
		</screen>
	</para>
	<para>This would result in 
		<varname>DEPENDS</varname> 
		containing 
		<literal>dependencywithcond</literal>.
	</para>
	</section>
	
	<section>
	<title>Functions</title>
   	   <para>NOTE: This is only supported in .bb and .bbclass files.</para>
      <para>As with most languages, functions are the building blocks that
       define operations. 
       Bitbake supports shell and python functions.
       An example shell function definition is:
       <screen>
some_function () {
        echo "Hello World"
}
</screen>

and an example python function definition is:
<screen>
python some_python_function () {
    d.setVar("TEXT", "Hello World")
    print d.getVar("TEXT", True)
}
</screen>
<para>In python functions, the "bb" and "os" modules are already imported,
there is no need to import those module.
The datastore, "d" is also a global variable and always available to these
 functions automatically.</para>
 
 <para>Bitbake will execute functions of this form using bb.build.exec_func()
which can also be called from python functions to execute other functions,
 either shell or python based.
  Shell functions can only execute other shell functions.</para>

<para>There is also a second way to declare python functions with parameters
which takes the form:
<screen>
def some_python_function(arg1, arg2):
    print arg1 + " " + arg2
</screen>

The difference is that the second form takes parameters, the datastore
is not available automatically and must be passed as a parameter and
these functions are not called with exec_func() but are executed with
direct python function calls. 
The "bb" and "os" modules are still automatically available and there is no 
need to import them.</para>
      </para>
</section>

<section>
	<title>Tasks</title>
	<para>
		<emphasis>NOTE:</emphasis> 
		This is only supported in .bb and .bbclass files.
	</para>
	<para>A shell or python function executable through exec_func can be promoted
	to become a task.
	Tasks are the execution unit Bitbake uses and each step that needs to be run 
	for a given .bb is known as a task. 
	There is a addtask command to add new tasks and promote functions which by 
	convention must start with “do_”.
	The addtask command is also used to describe intertask dependencies.
	</para>
	<para>
		<screen>
		python do_printdate () {
		   import time print
		   time.strftime('%Y%m%d', time.gmtime())
	 }
	 addtask printdate after do_fetch before do_build
		</screen>
	</para>
	<para>The above example defined a python function, then adds it as a task
	which is now a dependency of do_build, the default task and states it
	has to happen after do_fetch.
	If anyone executes the do_build task, that will result in do_printdate
	being run first.
	</para>
	</section>
	
	<section>
	<title>Task Flags</title>
	<para>Tasks support a number of flags which control various 
		functionality of the task.
		These are as follows:
	</para>
	<para>'dirs' - directories which should be created before the task 
		runs
	</para>
	<para>'cleandirs' - directories which should created before the task 
		runs but should be empty
	</para>
	<para>'noexec' - marks the tasks as being empty and no execution 
		required.
		These are used as dependency placeholders or used when added
		tasks need to be subsequently disabled.
	</para>
	<para>'nostamp' - don't generate a stamp file for a task.  
		This means the task is always executed.
	</para>
	<para>'fakeroot' - this task needs to be run in a fakeroot
		environment, obtained by adding the variables in FAKEROOTENV 
		to the environment.
	</para>
	<para>'umask' - the umask to run the task under.
	</para>
	<para>For the 'deptask', 'rdeptask', 'depends', 'rdepends'and 
		'recrdeptask' flags please see the dependencies section.
	</para>
	</section>

	
<section>
	<title>Parsing and Execution</title>
<section>
<title>Parsing Overview</title>
<para>
            BitBake parses configuration files, classes, and 
            <filename>.bb</filename> files.
        </para>

        <para>
            The first thing BitBake does is look for the 
            <filename>bitbake.conf</filename> file.
            This file resides in the within the <filename>/conf/</filename>
             directory.
            BitBake finds it by examining its
            <link linkend='var-BBPATH'><filename>BBPATH</filename></link>
             environment variable and looking for the 
             <filename>/conf/</filename> directory.
        </para>

        <para>
            The <filename>bitbake.conf</filename> file lists other configuration
            files to include from a <filename>conf/</filename> directory below
            the directories listed in <filename>BBPATH</filename>.
            In general, the most important configuration file from a user's
            perspective is <filename>local.conf</filename>, which contains a
            user's customized settings for the build environment.
            Other notable configuration files are the distribution configuration
            file (set by the <filename>DISTRO</filename>
            variable) and the machine configuration file (set by the
            <filename>MACHINE</filename> variable).
            The <filename>DISTRO</filename> and <filename>MACHINE</filename>
            BitBake environment variables are both usually set in
            the <filename>local.conf</filename> file.
            Valid distribution configuration files are available in the 
            <filename>/conf/distro/</filename> directory and valid machine
            configuration files in the <filename>meta/conf/machine/</filename>
            directory.
            Within the <filename>/conf/machine/include/</filename> directory are
            various <filename>tune-*.inc</filename> configuration files that
            provide common "tuning" settings specific to and shared between
            particular architectures and machines.
        </para>

        <para>
            After parsing of the configuration files, some standard classes are 
            included.
            The <filename>base.bbclass</filename> file is always included.
            Other classes that are specified in the configuration using the
            <filename>INHERIT</filename> variable are also included.
            Class files are searched for in a <filename>classes</filename>
            subdirectory under the paths in <filename>BBPATH</filename> in the 
            same way as configuration files.
        </para>

        <para>
            After classes are included, the variable 
            <filename>BBFILES</filename> is set, usually in
            <filename>local.conf</filename>, and defines the list of places to 
            search for <filename>.bb</filename> files.
            Adding extra content to <filename>BBFILES</filename> is best 
            achieved through the use of BitBake layers as described in the
            Layers section below.
        </para>

        <para>
            BitBake parses each <filename>.bb</filename> file in <filename>BBFILES</filename> and
            stores the values of various variables.
            In summary, for each <filename>.bb</filename>
            file the configuration plus the base class of variables are set, followed
            by the data in the <filename>.bb</filename> file
            itself, followed by any inherit commands that
            <filename>.bb</filename> file might contain.
        </para>

        <para>
            Because parsing <filename>.bb</filename> files is a time
            consuming process, a cache is kept to speed up subsequent parsing.
            This cache is invalid if the timestamp of the <filename>.bb</filename>
            file itself changes, or if the timestamps of any of the include,
            configuration files or class files on which the
            <filename>.bb</filename> file depends change.
        </para>
</section>	
	<section>
			<title>Configuration files</title>
		<para>Prior to parsing configuration files, Bitbake looks at certain
		variables, including:
				
		<itemizedlist>
		<listitem>BB-ENV-WHITELIST</listitem>
		<listitem>BB_PRESERVE-ENV</listitem>
		<listitem>BB_ENV_EXTRAWHITE</listitem>
		<listitem>BB_ORIG_ENV</listitem>
		<listitem>PREFERRED_VERSIONS</listitem>
		<listitem>PREFERRED_PROVIDERS</listitem>
		</itemizedlist>
		</para>
		<para>The first kind of metadata in BitBake is configuration
			metadata.
			This metadata is global, and therefore affects
			<emphasis>all</emphasis> 
			packages and tasks that are executed.
		</para>
		<para>BitBake will first search the current working directory 
			for an optional "conf/bblayers.conf" configuration 
			file.
			This file is expected to contain a BBLAYERS variable 
			that is a space delimited list of 'layer' directories.
			For each directory in this list, a "conf/layer.conf" 
			file will be searched for and parsed with the LAYERDIR 
			variable being set to the directory where the layer was
			found.
			The idea is these files will setup BBPATH and other 
			variables correctly for a given build directory
			automatically for the user.
		</para>
		<para>BitBake will then expect to find 'conf/bitbake.conf'
			somewhere in the user specified 
			<envar>BBPATH</envar>. 
			That configuration file generally has include 
			directives to pull in any other metadata  (generally 
			files specific to architecture, machine,
			<emphasis>local</emphasis>and so on).
		</para>
		<para>Only variable definitions and include directives are
			allowed in .conf files.  The following variables include:
			
			<itemizedlist>
			<listitem>BITBAKE_UI</listitem>
			<listitem>BBDEBUG</listitem>
			<listitem>MULTI_PROVIDER_WHITELIST</listitem>
			<listitem>BB_NUMBER_PARSE_THREADS</listitem>
			<listitem>BBPKGS</listitem>
			<listitem>BB_DEFAULT_TASK</listitem>
			<listitem>TOPDIR</listitem>
			<listitem>BB_VERBOSE_LOGS</listitem>
			<listitem>BB_NICE_LEVEL</listitem>
			<listitem>BBFILE_COLLECTIONS</listitem>
			<listitem>ASSUME_PROVIDED</listitem>
			<listitem>BB_DANGLINGAPPENDS_WARNONLY</listitem>
			<listitem>BBINCLUDED</listitem>
			<listitem>BBFILE_PRIORITY</listitem>
			<listitem>BUILDNAME</listitem>
			<listitem>BBMASK</listitem>
			</itemizedlist>
		</para>
		<section>
		<title>Layers</title>
		<para>Layers allow you to isolate different types of customizations from
            each other.
            You might find it tempting to keep everything in one layer when
            working on a single project.
            However, the more modular you organize your Metadata, the easier
            it is to cope with future changes.</para>
		
		<para>
            To illustrate how layers are used to keep things modular, consider
            machine customizations.
            These types of customizations typically reside in a special layer,
            rather than a general layer, called a Board Specific Package (BSP)
            Layer.
            Furthermore, the machine customizations should be isolated from
            recipes and Metadata that support a new GUI environment,
            for example.
            This situation gives you a couple of layers: one for the machine
            configurations, and one for the GUI environment.
            It is important to understand, however, that the BSP layer can
            still make machine-specific additions to recipes within the GUI
            environment layer without polluting the GUI layer itself
            with those machine-specific changes.
            You can accomplish this through a recipe that is a BitBake append
            (<filename>.bbappend</filename>) file, which is described later
            in this section.
        </para>
        <para>There are certain variable specific to layers, including:
		<itemizedlist>
		<listitem>LAYERDEPENDS</listitem>
		<listitem>LAYERVERSION</listitem>
		</itemizedlist>
		</para>
		</section>
		
		<section>
		<title>Schedulers</title>
		<para>
		There are variables specific to scheduling functionality including:
		<itemizedlist>
		<listitem>BB_SCHEDULER</listitem>
		<listitem>BB_SCHEDULERS</listitem>
		</itemizedlist>
		</para></section>
	</section>
	
<section>
	<title>Classes</title>
	<para>BitBake classes are our rudimentary inheritance mechanism. 
		As briefly mentioned in the metadata introduction, they're 
		parsed when an
		<literal>inherit</literal> 
		directive is encountered, and they are located in classes/ 
		relative to the directories in
		<envar>BBPATH</envar>.
	</para>
	</section>
	
<section>
        <title>.bb files</title>
	<para>A BitBake (.bb) file is a logical unit of tasks to be executed.
		Normally this is a package to be built.
		Inter-.bb dependencies are obeyed.
		The files themselves are located via the 
		<varname>BBFILES</varname> 
		variable, which is set to a space separated list of .bb files,
		and does handle wildcards.
	</para>
	</section>
</section>

	<section>
	<title>Events</title>
	<para>
		<emphasis>NOTE:</emphasis> 
		This is only supported in .bb and .bbclass files.
	</para>
	<para>BitBake allows installation of event handlers.
		Events are triggered at certain points during operation, such 
		as the beginning of operation against a given .bb, the start of
		a given task, task failure, task success, et cetera.
		The intent is to make it easy to do things like email 
		notification on build failure.
	</para>
	<para>
		<screen>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)) }
		</screen>
	</para>
	<para>This event handler gets called every time an event is triggered.
		A global variable 
		<varname>e</varname> 
		is defined. 
		<varname>e</varname>.data
		contains an instance of bb.data.
		With the getName(<varname>e</varname>) method one can get the 
		name of the triggered event.
	</para>
	<para>The above event handler prints the name of the event and the 
		content of the 
		<varname>FILE</varname> 
		variable.
	</para>
	
	<para>
	During a Build, the following common events occur:
<itemizedlist>	
<listitem>bb.event.ConfigParsed()</listitem>
<listitem>bb.event.ParseStarted()</listitem>
<listitem>bb.event.ParseProgress()</listitem>
<listitem>bb.event.ParseCompleted()</listitem>
<listitem>bb.event.BuildStarted()</listitem>
<listitem>bb.build.TaskStarted()</listitem>
<listitem>bb.build.TaskInvalid()</listitem>
<listitem>bb.build.TaskFailedSilent()</listitem>
<listitem>bb.build.TaskFailed()</listitem>
<listitem>bb.build.TaskSucceeded()</listitem>
<listitem>bb.event.BuildCompleted()</listitem>
<listitem>bb.cooker.CookerExit()</listitem>
</itemizedlist>

</para>

<para>
There are also other events that occur based on specific requests to the server:

<itemizedlist>
<listitem>bb.event.TreeDataPreparationStarted()</listitem>
<listitem>bb.event.TreeDataPreparationProgress</listitem>
<listitem>bb.event.TreeDataPreparationCompleted</listitem>
<listitem>bb.event.DepTreeGenerated</listitem>
<listitem>bb.event.CoreBaseFilesFound</listitem>
<listitem>bb.event.ConfigFilePathFound</listitem>
<listitem>bb.event.FilesMatchingFound</listitem>
<listitem>bb.event.ConfigFilesFound</listitem>
<listitem>bb.event.TargetsTreeGenerated</listitem>
</itemizedlist>
</para>
	</section>
	
	<section>
	<title>Variants - Class Extension Mechanism</title>
	<para>Two BitBake features exist to facilitate the creation of multiple
		buildable incarnations from a single recipe file.
	</para>
	<para>The first is 
		<varname>BBCLASSEXTEND</varname>. 
		This variable is a space separated list of classes used to 
		"extend" the recipe for each variant.
		As an example, setting
		<screen>BBCLASSEXTEND = "native"</screen> 
		results in a second incarnation of the current recipe being
		available.
		This second incarnation will have the "native" class 
		inherited.
	</para>
	<para>The second feature is
		<varname>BBVERSIONS</varname>. 
		This variable allows a single recipe to build multiple versions
		of a project from a single recipe file, and allows you to 
		specify conditional metadata (using the
		<varname>OVERRIDES</varname> mechanism) for a single version,
		or an optionally named range of versions:
	</para>
	<para>
		<screen>BBVERSIONS = "1.0 2.0 git" SRC_URI_git =
			"git://someurl/somepath.git"
		</screen>
	</para>
	<para>
		<screen>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"
		</screen>
	</para>
	<para>Note that the name of the range will default to the original 
		version of the recipe, so given OE, a recipe file of
		foo_1.0.0+.bb will default the name of its versions to 
		1.0.0+. 
		This is useful, as the range name is not only placed into 
		overrides; it's also made available for the metadata to use in 
		the form of the
		<varname>BPV</varname> 
		variable, for use in file:// search paths 
		(<varname>FILESPATH</varname>).
	</para>
	</section>
		
	<section>
	<title>Dependencies</title>
	<section>
	<title>Dependencies</title>
	<para>BitBake handles dependencies at the task level since to allow for
		efficient operation with multiple processes executing in 
		parallel, a robust method of specifying task dependencies is
		needed.
	</para>
	</section>
	
	<section>
	<title>Dependencies Internal to the .bb File</title>
	<para>Where the dependencies are internal to a given .bb file, the
		dependencies are handled by the previously detailed addtask
		directive.
	</para>
	</section>
	
	<section>
	<title>Build Dependencies</title>
	<para>DEPENDS lists build time dependencies.
		The 'deptask' flag for tasks is used to signify the task of 
		each item listed in DEPENDS which must have completed before 
		that task can be executed.
	</para>
	<para>
		<screen>do_configure[deptask] =
			"do_populate_staging"
		</screen>
	</para>

	<para>means the do_populate_staging task of each item in DEPENDS must 
		have completed before do_configure can execute.
	</para>
	</section>
	
	<section>
	<title>Runtime Dependencies</title>
	<para>The PACKAGES variable lists runtime packages and each of these 
		can have RDEPENDS and RRECOMMENDS 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.
	</para>
	<para>
		<screen>do_package_write[rdeptask] = 
			"do_package"
		</screen>
	</para>
	<para>means the do_package task of each item in RDEPENDS must have 
		completed before do_package_write can execute.
	</para>
	</section>
	
	<section>
	<title>Recursive Dependencies</title>
	<para>These are specified with the 'recrdeptask' flag which is used
		to signify the task(s) of dependencies which must have
		completed before that task can be executed.
		It works by looking though the build and runtime dependencies
		of the current recipe as well as any inter-task dependencies
		the task has, then adding a dependency on the listed task.
		It will then recurse through the dependencies of those tasks
		and so on.
	</para>
	<para>It may be desireable to recurse not just through the dependencies
		of those tasks but through the build and runtime dependencies 
		of dependent tasks too.
		If that is the case, the taskname itself should be referenced 
		in the task list, e.g. do_a[recrdeptask] = "do_a do_b".
	</para>
	</section>
	
	<section>
	<title>Inter Task Dependencies</title>
	<para>The 'depends' flag for tasks is a more generic form which allows 
		an interdependency on specific tasks rather than specifying 
		the data in DEPENDS.
	</para>
	<para>
		<screen>do_patch[depends] = 
			"quilt-native:do_populate_staging"
		</screen>
        </para>
	<para>means the do_populate_staging task of the target 
		quilt-native must have completed before the do_patch 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>
	   <title>Accessing Variable and the Data Store from Python</title>
	
	   <para>
	   <emphasis>NOTE: This section is in draft.</emphasis>
	   
	   It is often necessary to manipulate variables within python functions
	   and the Bitbake data store has an API which allows this.
	   The operations available are:
	   
	   <screen>
	   d.getVar("X", expand=False)
	   </screen>
	   
	   returns the value of variable "X", expanding the value if specified
	   
	   <screen>
	   d.setVar("X", value)
	   </screen>
	   
	   sets the value of "X" to value
	   
	   <screen>
	   d.appendVar("X", value)
	   </screen>
	   
	   adds value to the end of variable X
	   
	   <screen>
	   d.prependVar("X", value)
	   </screen>
	   
	   adds value to the start of variable X
	   
	   <screen>
	   d.delVar("X")
	   </screen>
	   
	   deletes the variable X from the data store
	   
	   <screen>
	   d.renameVar("X", "Y")
	   </screen>
	   
	   renames variable X to Y
	   
	   <screen>	   
	   d.getVarFlag("X", flag, expand=False)
	   </screen>
	   
	   gets given flag from variable X but does not expand it.
	   
	   <screen>
	   d.setVarFlag("X", flag, value)
	   </screen>
	   
	   sets given flag for variable X to value.
	   Note, setVarFlags will not clear previous flags.
	   Think of this method as addVarFlags.
	   
	   <screen>
	   d.appendVarFlag("X", flag, value)
	   </screen>
	   
	   <screen>
	   d.prependVarFlag("X", flag, value)
	   </screen>
	   
	   <screen>
	   d.delVarFlag("X", flag)
	   </screen>
	   
	   <screen>
	   d.setVarFlags("X", flagsdict)
	   </screen>
	   
	   sets the flags specified in the dict() parameter
	   
	   <screen>
	   d.getVarFlags("X")
	   </screen>
	   
	   returns a dict of the flags for X
	   
	   <screen>
	   d.delVarFlags
	   </screen>
	   
	   deletes all the flags for a variable
	   </para>
	
</section>
<!--
<section>
	   <title>Task Checksums and Setscene</title>
	
	   <para>
	   <emphasis>NOTE:  This section is a work in progress.</emphasis>
	   	   
	   This list is a place holder of content that needs explanation here. Items
	   should be moved to appropriate sections below as completed.
	   
	   <itemizedlist>
	   <listitem>STAMP</listitem>
	   <listitem>STAMPCLEAN</listitem>
	   <listitem>BB_STAMP_WHITELIST</listitem>
	   <listitem>BB_STAMP_POLICY</listitem>
	   <listitem>BB_HASHCHECK_FUNCTION</listitem>
	   <listitem>BB_SETSCENE_VERIFY_FUNCTION</listitem>
	   <listitem>BB_SETSCENE_DEPVALID</listitem>
	   <listitem>BB_TASKHASH</listitem>
	   </itemizedlist>
	   </para>
	   
	   <section>
	   <title>Task Signatures</title>
	   <para></para>
	   </section>
	   
	   <section>
	   <title>Hash Generation</title>
	   <para></para>
	   </section>

</section>
-->
</chapter>