aboutsummaryrefslogtreecommitdiffstats
path: root/lib/asn1/doc/src/asn1_ug.xml
blob: 1b399fb641d5e94220835a8ad7574c1181ff5754 (plain) (blame)
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
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
<?xml version="1.0" encoding="iso-8859-1" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>1997</year><year>2011</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>Asn1</title>
    <prepared>Kenneth Lundin</prepared>
    <docno></docno>
    <date>1999-03-25</date>
    <rev>D</rev>
    <file>asn1_ug.xml</file>
  </header>

  <section>
    <title>Introduction</title>

    <section>
      <title>Features</title>
      <p>The Asn1 application provides:
        </p>
      <list type="bulleted">
        <item>An ASN.1 compiler for Erlang, which generates encode and
         decode functions to be used by Erlang programs sending and
         receiving ASN.1 specified data.</item>
        <item>Run-time functions used by the generated code.</item>
        <item>The supported encoding rules are: 
	<list>
	  <item>
	    Basic Encoding Rules (<em>BER</em>)
	  </item>
	  <item>
	    Distinguished Encoding Rules (<em>DER</em>), a specialized form of BER that is used in security-conscious applications.
	  </item>
	  <item>
	    Packed Encoding Rules (<em>PER</em>) both the aligned and unaligned variant.
	  </item>
	  </list>
	</item>
      </list>
    </section>

    <section>
      <title>Overview</title>
      <p>ASN.1 (Abstract Syntax Notation 1) is a formal language for describing data structures to be exchanged between distributed computer systems.
        The purpose of ASN.1 is to have
        a platform and programming language independent notation to express 
	types   using a
        standardized set of rules  for the transformation of values of
        a defined type, into a stream of bytes. This stream of bytes
        can then be sent on a communication channel set  up by the
        lower layers in the stack of communication protocols e.g.
        TCP/IP or encapsulated within UDP packets. This way, two
        different applications written in two completely different
        programming languages running on different computers with
        different internal representation of data can exchange
        instances of structured data types  (instead of exchanging
        bytes or bits). This makes programming faster and easier since no code
        has to be written to process  the transport format of the
        data. 
        </p>
      <p>To write a network application which processes ASN.1 encoded
        messages, it is prudent and sometimes essential to have a set
        of off-line development tools such as an ASN.1 compiler which
        can generate the encode and decode logic for the specific ASN.1
        data types. It is also necessary to combine this with some
        general language-specific runtime support for ASN.1 encoding and
        decoding. 
        </p>
      <p>The ASN.1 compiler must be directed towards a target language
        or a set  of closely related languages. This manual describes a
        compiler which is directed towards the functional language
        Erlang. In order to use this  compiler, familiarity with the
        language Erlang is essential. Therefore, the runtime support for ASN.1 is
        also closely related to the language Erlang and
        consist of a number of functions, which the
        compiler uses. The types in ASN.1 and how to represent
        values of those types in Erlang are described in this manual.
        </p>
      <p>The following document is structured so that the first part describes 
        how to use ASN.1 compiler, and then there are descriptions of all
        the primitive and constructed ASN.1 types and their representation
        in Erlang,
        </p>
    </section>

    <section>
      <title>Prerequisites</title>
      <p>It is assumed that the reader is familiar with the ASN.1 notation
        as documented in the standard definition [<cite id="X.680"></cite>] which is
        the primary text. It may also be helpful, but not necessary,
        to read the standard definitions 
        [<cite id="X.681"></cite>] [<cite id="X.682"></cite>]  [<cite id="X.683"></cite>]
        [<cite id="X.690"></cite>] [<cite id="X.691"></cite>]. </p>
      <p>A very good book explaining those reference texts is 
        [<cite id="DUBUISSON"></cite>], free to download at 
        <url href="http://www.oss.com/asn1/dubuisson.html">http://www.oss.com/asn1/dubuisson.html </url>.
        </p>
    </section>

    <section>
      <title>Capability</title>
      <p>This application covers all features of ASN.1 up to the 1997
      edition of the specification. In the 2002 edition of ASN.1 a number of
      new features where introduced of which some are supported while
      others are not. For example the
      ECN (Encoding Control Notation) and XML notation are still
      unsupported. Though, the other features of the 2002 edition are
      fully or partly supported as shown below:</p>
      <list type="bulleted">
	<item>
	  <p>Decimal notation (e.g., "1.5e3") for REAL values. The
	  NR1, NR2 and NR3 formats as explained in ISO6093 are
	  supported.</p>
	</item>
	<item>
	  <p>The RELATIVE-OID type for relative object identifiers are
	  fully supported.</p>
	</item>
	<item>
	  <p>The subtype constraint (CONTAINING/ENCODED BY) to
	  constrain the content of an octet string or a bit string is
	  parsed when compiling, but no further action is taken. This
	  constraint is not a PER-visible constraint.</p>
	</item>
	<item>
	  <p>The subtype constraint by regular expressions (PATTERN) for character string types is parsed when compiling, but no further action is taken. This constraint is not a PER-visible constraint.</p>
	</item>
	<item>
	  <p>Multiple-line comments as in C, <c>/* ... */</c>, are
	  supported.</p>
	</item>
      </list>
      <p>It should also be added here that the encoding formats
      supported are <em>BER</em>, <em>DER</em>, <em>PER aligned
      basic</em> variant and <em>PER unaligned basic</em> variant.</p>
    </section>

  </section>

  <section>
    <title>Getting Started with Asn1</title>

    <section>
      <title>A First Example</title>
      <p>The following example demonstrates the basic functionality used to run 
        the Erlang ASN.1 compiler.</p>
      <p>First, create a file called <c>People.asn</c> containing the following:</p>
      <pre>
People DEFINITIONS IMPLICIT TAGS ::=

BEGIN
EXPORTS Person;

Person ::= [PRIVATE 19] SEQUENCE {
        name PrintableString,
        location INTEGER {home(0),field(1),roving(2)},
        age INTEGER OPTIONAL }
END      </pre>
      <p>This file (<c>people.asn</c>) must be compiled before it can  be 
        used. 
        The ASN.1 compiler checks that the syntax is correct and that the 
        text represents proper ASN.1 code before generating an abstract
        syntax tree. The code-generator then uses the abstract syntax 
        tree in order to generate code.
        </p>
      <p>The generated Erlang  files will be placed in the current directory or
        in the directory specified with the <c>{outdir,Dir}</c> option.
        The following shows how the compiler
        can be called from the  Erlang shell:</p>
      <pre>
1><input>asn1ct:compile("People",[ber_bin]).</input>
Erlang ASN.1 compiling "People.asn" 
--{generated,"People.asn1db"}--
--{generated,"People.hrl"}--
--{generated,"People.erl"}--
ok
2>      </pre>
      <p>The ASN.1 module People is now accepted and the abstract syntax tree
        is saved in the <c>People.asn1db</c> file, the
        generated Erlang code is compiled using the Erlang compiler and
        loaded into  the Erlang runtime system. Now there is a user interface
        of encode/2 and decode/2 in the module People, which is invoked by:
                <br></br>
<c><![CDATA['People':encode(<Type name>,<Value>),]]></c>        <br></br>

        or        <br></br>
<c><![CDATA['People':decode(<Type name>,<Value>),]]></c>        <br></br>

        Alternatively one can use the <c><![CDATA[asn1rt:encode(<Module name> ,<Type name>,<Value>)]]></c> and <c><![CDATA[asn1rt:decode(< Module name>,<Type name>,<Value>)]]></c> calls.
        However, they are not as efficient as the previous methods since they
        result in an additional <c>apply/3</c> call.</p>
      <p>Assume there is a network
        application which receives instances of the  ASN.1 defined
        type Person,  modifies and sends them back again:</p>
      <code type="none">
receive
   {Port,{data,Bytes}} ->
       case 'People':decode('Person',Bytes) of
           {ok,P} ->
               {ok,Answer} = 'People':encode('Person',mk_answer(P)),
               Port ! {self(),{command,Answer}};
           {error,Reason} ->
               exit({error,Reason})
       end
    end,      </code>
      <p>In the example above, a  series of bytes is received from an
        external source and the bytes are then decoded into a valid
        Erlang term. This was achieved with the call
        <c>'People':decode('Person',Bytes)</c> which   returned
        an Erlang value of the ASN.1 type <c>Person</c>.  Then an answer was
        constructed  and encoded using
        <c>'People':encode('Person',Answer)</c> which takes an
        instance of a defined ASN.1 type and transforms it to a
        (possibly) nested list of bytes  according to the BER or PER
        encoding-rules. 
                <br></br>
The encoder and the decoder can also be run from
        the shell. The following  dialogue with the shell illustrates
        how the functions   
        <c>asn1rt:encode/3</c> and <c>asn1rt:decode/3</c> are used.</p>
      <pre>
2> <input>Rockstar = {'Person',"Some Name",roving,50}.</input>
{'Person',"Some Name",roving,50}
3> <input>{ok,Bytes} = asn1rt:encode('People','Person',Rockstar).</input>
{ok,[&lt;&lt;243&gt;&gt;,
     [17],
     [19,9,"Some Name"],
     [2,1,[2]],
     [2,1,"2"]]}
4> <input>Bin = list_to_binary(Bytes).</input>
&lt;&lt;243,17,19,9,83,111,109,101,32,78,97,109,101,2,1,2,2,1,50&gt;&gt;
5> <input>{ok,Person} = asn1rt:decode('People','Person',Bin).</input>
{ok,{'Person',"Some Name",roving,50}}
6>      </pre>
      <p>Notice that the result from <c>encode</c> is a nested list which
        must be turned into a binary before the call to <c>decode</c>. A
        binary is necessary as input to decode since the module was compiled
        with the <c>ber_bin</c> option
        The reason for returning a nested list is that it is faster to produce 
        and  the <c>list_to_binary</c> operation is
        performed automatically when the list is sent via the Erlang port mechanism.</p>
    </section>

    <section>
      <title>Module dependencies</title>
      <p>It is common that asn1 modules import defined types, values and
        other entities from another asn1 module.</p>
      <p>Earlier versions of the asn1 compiler required that modules that
        were imported from had to be compiled before the module that
        imported. This caused problems when asn1 modules had circular
        dependencies.</p>
      <p>Now are referenced modules parsed when the compiler finds an
        entity that is imported. There will not be any code generated for
        the referenced module. However, the compiled module rely on
        that the referenced modules also will be compiled.</p>
    </section>
  </section>

  <section>
    <title>The Asn1 Application User Interface</title>
    <p>The Asn1 application provides two separate user interfaces:</p>
    <list type="bulleted">
      <item>
        <p>The module <c>asn1ct</c> which provides the compile-time functions 
          (including the compiler).</p>
      </item>
      <item>
        <p>The module <c>asn1rt</c> which provides the run-time functions. 
          However, it is preferable to use the generated <c>encode/2</c> and 
          <c>decode/2</c> functions in each module, ie. 
          Module:encode(Type,Value), in favor of the <c>asn1rt</c> 
          interface.</p>
      </item>
    </list>
    <p>The reason for the division  of the interface into  compile-time 
      and  run-time 
      is that only run-time modules (<c>asn1rt*</c>) need to be loaded in
      an embedded system. 
      </p>

    <section>
      <title>Compile-time Functions</title>
      <p>The ASN.1 compiler can be invoked directly from the command-line
        by means of the <c>erlc</c> program. This is convenient when compiling
        many ASN.1 files from the command-line or when using Makefiles.
        Here are some examples of how the <c>erlc</c> command can be used to invoke the
        ASN.1 compiler:</p>
      <pre>
erlc Person.asn
erlc -bper_bin Person.asn
erlc -bber_bin +optimize ../Example.asn
erlc -o ../asnfiles -I ../asnfiles -I /usr/local/standards/asn1 Person.asn      </pre>
      <p>The useful options for the ASN.1 compiler are:</p>
      <taglist>
        <tag><c>-b[ber | per | ber_bin | per_bin | uper_bin]</c></tag>
        <item>
          <p>Choice of encoding rules, if omitted <c>ber</c> is the
          default. The <c>ber_bin</c> and <c>per_bin</c> options
          allows for optimizations and are therefore recommended
          instead of the <c>ber</c> and <c>per</c> options.</p>
        </item>
        <tag><c>-o OutDirectory</c></tag>
        <item>
          <p>Where to put the generated files, default is the current
            directory.</p>
        </item>
        <tag><c>-I IncludeDir</c></tag>
        <item>
          <p>Where to search for <c>.asn1db</c> files and asn1
            source specs in order to resolve references to other
            modules. This option can be repeated many times if there
            are several places to search in.  The compiler will always
            search the current directory first.</p>
        </item>
        <tag><c>+compact_bit_string</c></tag>
        <item>
          <p>Gives the user the option to use a compact format of the BIT
            STRING type to save memory space, typing space and
            increase encode/decode performance, for details see 
            <seealso marker="#BIT STRING">BIT STRING </seealso>type section.</p>
        </item>
        <tag><c>+der</c></tag>
        <item>
          <p>DER encoding rule. Only when using <c>-ber</c> or 
            <c>-ber_bin</c> option.</p>
        </item>
        <tag><c>+optimize</c></tag>
        <item>
          <p>This flag has effect only when used together with one of
            <c>per_bin</c> or <c>ber_bin</c> flags. It gives time optimized
            code in the generated modules and it uses another runtime module.
            In the <c>per_bin</c> case a nif is used. The
            result from an encode is a binary.</p>
          <p><em>When this flag is used you cannot use the old format</em><c>{TypeName,Value}</c> when you encode values. Since it is
            an unnecessary construct it has been removed in favor of
            performance. It
            is neither admitted to construct SEQUENCE or SET component values
            with the format <c>{ComponentName,Value}</c> since it also is
            unnecessary. The only case were it is necessary is in a CHOICE,
            were you have to pass values to the right component by specifying
            <c>{ComponentName,Value}</c>. See also about
            <seealso marker="#TypeNameValue">{Typename,Value}</seealso> below 
            and in the sections for each type.</p>
        </item>
        <tag><c>+driver</c></tag>
        <item>
           <p>As of R15B this means the same as the <c>nif</c> option. Kept for
	    backwards compatability reasons.</p>
        </item>
	<tag><c>+nif</c></tag>
        <item>
          <p>Together with the flags <c>ber_bin</c>
	  and <c>optimize</c> you choose to use a nif for considerable
	  faster encode and decode. </p>
        </item>
        <tag><c>+asn1config</c></tag>
        <item>
          <p>This functionality works together with the flags
            <c>ber_bin</c> and <c>optimize</c>. You enables the
            specialized decodes, see the <seealso marker="asn1_spec">Specialized Decode</seealso> chapter.
            </p>
        </item>
        <tag><c>+undec_rest</c></tag>
        <item>
          <p>A buffer that holds a message, being decoded may
            also have some following bytes. Now it is possible to get
            those following bytes returned together with the decoded
            value. If an asn1 spec is compiled with this option a tuple
            <c>{ok,Value,Rest}</c> is returned. <c>Rest</c> may be a
            list or a binary. Earlier versions of the compiler ignored
            those following bytes.</p>
        </item>
        <tag><c>{inline,OutputName}</c></tag>
        <item>
          <p>Compiling with this option gives one output module
            containing all asn1 run-time functionality. The asn1 specs
            are provided in a target module <c>Module.set.asn</c> as
            described in the <seealso marker="asn1ct#asn1set">reference manual</seealso>. The name of the resulting module
            containing generated encode/decode functions and inlined
            run-time functions will be <c>OutputName.erl</c>. The
            merging/inlining of code is done by the <c>igor</c> module
            of <c>syntax_tools</c>. By default the functions generated
            from the first asn1 spec in the <c>.set.asn</c> are
            exported, unless a <c>{export,[atom()]}</c> or
            <c>{export_all,true}</c> option are provided. The list of
            atoms are names of choosen asn1 specs from the
            <c>.set.asn</c> file. See further examples of usage <seealso marker="#inlineExamples">below</seealso></p>
        </item>
        <tag><c>+'Any Erlc Option'</c></tag>
        <item>
          <p>You may add any option to the Erlang compiler when
            compiling the generated Erlang files. Any option
            unrecognised by the asn1 compiler will be passed to the
            Erlang compiler.</p>
        </item>
      </taglist>
      <p>For a complete description of <c>erlc</c> see Erts Reference Manual.</p>
      <p>For preferred option use see <seealso marker="#preferred option use">Preferred Option Use</seealso> section.</p>
      <p>The compiler and other compile-time functions can also be invoked from 
        the Erlang shell. Below follows a brief
        description of the primary functions, for a
        complete description of each function see 
        <seealso marker="asn1ct">the Asn1 Reference Manual</seealso>, the
        <c>asn1ct</c> module.</p>
      <p>The compiler is invoked by using <c>asn1ct:compile/1</c> with
        default options, or <c>asn1ct:compile/2</c> if explicit options
        are given.
        Example:</p>
      <pre>
asn1ct:compile("H323-MESSAGES.asn1").      </pre>
      <p>which equals:</p>
      <pre>
asn1ct:compile("H323-MESSAGES.asn1",[ber]).      </pre>
      <p>If one wants PER encoding with optimizations:</p>
      <pre>
asn1ct:compile("H323-MESSAGES.asn1",[per_bin,optimize]).      </pre>
      <p>The generic encode and decode functions can be invoked like this:</p>
      <pre>
asn1ct:encode('H323-MESSAGES','SomeChoiceType',{call,"octetstring"}).
asn1ct:decode('H323-MESSAGES','SomeChoiceType',Bytes).      </pre>
      <p>Or, preferable like:</p>
      <pre>
'H323-MESSAGES':encode('SomeChoiceType',{call,"octetstring"}).
'H323-MESSAGES':decode('SomeChoiceType',Bytes).      </pre>
    </section>

    <section>
      <marker id="preferred option use"></marker>
      <title>Preferred Option Use</title>
      <p>
	It may not be obvious which compile options best fit a
	situation. This section describes the format of the result of
	encode and decode. It also gives some performance statistics
	when using certain options. Finally there is a recommendation
	which option combinations should be used.
      </p>
      <p>
	The default option is <c>ber</c>. It is the same backend as
	<c>ber_bin</c> except that the result of encode is transformed
	to a flat list. Below is a table that gives the different
	formats of input and output of encode and decode using the
	<em>allowed combinations</em> of coding and optimization
	options: (EAVF stands for how ASN1 values are represented in
	Erlang which is described in the <seealso
	marker="#ASN1Types">ASN1 Types chapter</seealso>)
      </p>
      <table>
      <row>
        <cell align="left" valign="middle"><em>Encoding Rule</em></cell>
        <cell align="left" valign="middle"><em>Compile options, allowed combinations</em></cell>
        <cell align="left" valign="middle"><em>encode input</em></cell>
        <cell align="left" valign="middle"><em>encode output</em></cell>
        <cell align="left" valign="middle"><em>decode input</em></cell>
        <cell align="left" valign="middle"><em>decode output</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle">BER</cell>
        <cell align="left" valign="middle">[ber] (default)</cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">flat list</cell>
        <cell align="left" valign="middle">flat list / binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">BER</cell>
        <cell align="left" valign="middle">[ber_bin]</cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">iolist</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">BER</cell>
        <cell align="left" valign="middle"><em>[ber_bin, optimize]</em></cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">iolist</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">BER</cell>
        <cell align="left" valign="middle"><em>[ber_bin, optimize, nif]</em></cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">iolist</cell>
        <cell align="left" valign="middle">iolist / binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">PER aligned variant</cell>
        <cell align="left" valign="middle">[per]</cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">flat list</cell>
        <cell align="left" valign="middle">flat list</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">PER aligned variant</cell>
        <cell align="left" valign="middle"><em>[per_bin]</em></cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">iolist / binary</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">PER aligned variant</cell>
        <cell align="left" valign="middle"><em>[per_bin, optimize]</em></cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">PER unaligned variant</cell>
        <cell align="left" valign="middle"><em>[uper_bin]</em></cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>

      <row>
        <cell align="left" valign="middle">DER</cell>
        <cell align="left" valign="middle">[(ber), der]</cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">flat list</cell>
        <cell align="left" valign="middle">flat list / binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">DER</cell>
        <cell align="left" valign="middle">[ber_bin, der]</cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">iolist</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">DER</cell>
        <cell align="left" valign="middle"><em>[ber_bin, optimize, der]</em></cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">iolist</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>
      <row>
        <cell align="left" valign="middle">DER</cell>
        <cell align="left" valign="middle"><em>[ber_bin, optimize, nif, der]</em></cell>
        <cell align="left" valign="middle">EAVF</cell>
        <cell align="left" valign="middle">iolist</cell>
        <cell align="left" valign="middle">binary</cell>
        <cell align="left" valign="middle">EAVF</cell>
      </row>


      <tcaption>The output / input formats for different combinations of compile options.</tcaption>
    </table>
    <p>
      Encode / decode speed comparison in one user case for the above
      alternatives (except <c>DER</c>) is showed in the table below. The
      <c>DER</c> alternatives are slower than their corresponding
      <c>BER</c> alternative.
    </p>

    <table>
      <row>
	<cell align="left" valign="middle"><em>compile options</em></cell>
	<cell align="left" valign="middle"><em>encode time</em></cell>
	<cell align="left" valign="middle"><em>decode time</em></cell>
      </row>
      <row>
	<cell align="left" valign="middle">[ber]</cell>
	<cell align="left" valign="middle">120</cell>
	<cell align="left" valign="middle">162</cell>
      </row>
      <row>
	<cell align="left" valign="middle">[ber_bin]</cell>
	<cell align="left" valign="middle">124</cell>
	<cell align="left" valign="middle">154</cell>
      </row>
      <row>
	<cell align="left" valign="middle"><em>[ber_bin, optimize]</em></cell>
	<cell align="left" valign="middle">50</cell>
	<cell align="left" valign="middle">78</cell>
      </row>
      <row>
	<cell align="left" valign="middle"><em>[ber_bin, optimize, driver]</em></cell>
	<cell align="left" valign="middle">50</cell>
	<cell align="left" valign="middle">62</cell>
      </row>
      <row>
	<cell align="left" valign="middle">[per]</cell>
	<cell align="left" valign="middle">141</cell>
	<cell align="left" valign="middle">133</cell>
      </row>
      <row>
	<cell align="left" valign="middle"><em>[per_bin]</em></cell>
	<cell align="left" valign="middle">125</cell>
	<cell align="left" valign="middle">123</cell>
      </row>
      <row>
	<cell align="left" valign="middle"><em>[per_bin, optimize]</em></cell>
	<cell align="left" valign="middle">77</cell>
	<cell align="left" valign="middle">72</cell>
      </row>
      <row>
	<cell align="left" valign="middle"><em>[uper_bin]</em></cell>
	<cell align="left" valign="middle">97</cell>
	<cell align="left" valign="middle">104</cell>
      </row>
      <tcaption>
	One example of difference in speed for the compile option alternatives.
      </tcaption>
    </table>

    <p>
      The compile options <c>ber</c>, <c>per</c> and
      <c>driver</c> are kept for backwards compatibility and should not be 
      used in new code. The nif implementation which replaces the linked-in
      driver has been shown to be about 5-15% faster.
    </p>
    <p>
      You are strongly recommended to use the appropriate alternative
      of the bold typed options. The <c>optimize</c> and
      <c>nif</c> options does not affect the encode or decode
      result, just the time spent in run-time. When <c>ber_bin</c> and
      <c>nif</c> or <c>per_bin</c> and <c>optimize</c> is
      combined the C-code nif is used in chosen parts of encode /
      decode procedure.
    </p>
    <table>
      <row>
        <cell align="left" valign="middle"><em>Compile options, allowed combinations</em></cell>
        <cell align="left" valign="middle"><em>use of nif</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle">[ber]</cell>
        <cell align="left" valign="middle">no</cell>
      </row>
      <row>
        <cell align="left" valign="middle">[ber_bin]</cell>
        <cell align="left" valign="middle">no</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><em>[ber_bin, optimize]</em></cell>
        <cell align="left" valign="middle">no</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><em>[ber_bin, optimize, nif]</em></cell>
        <cell align="left" valign="middle">yes</cell>
      </row>
      <row>
        <cell align="left" valign="middle">[per]</cell>
        <cell align="left" valign="middle">no</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><em>[per_bin]</em></cell>
        <cell align="left" valign="middle">no</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><em>[per_bin, optimize]</em></cell>
        <cell align="left" valign="middle">yes</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><em>[uper_bin]</em></cell>
        <cell align="left" valign="middle">no</cell>
      </row>

      <row>
        <cell align="left" valign="middle">[(ber), der]</cell>
        <cell align="left" valign="middle">no</cell>
      </row>
      <row>
        <cell align="left" valign="middle">[ber_bin, der]</cell>
        <cell align="left" valign="middle">no</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><em>[ber_bin, optimize, der]</em></cell>
        <cell align="left" valign="middle">no</cell>
      </row>
      <row>
        <cell align="left" valign="middle"><em>[ber_bin, optimize, nif, der]</em></cell>
        <cell align="left" valign="middle">yes</cell>
      </row>


      <tcaption>When the ASN1 nif is used.</tcaption>
    </table>

    </section>
    <section>
      <title>Run-time Functions</title>
      <p>A brief description of the major functions is given here. For a
        complete description of each function see
        <seealso marker="asn1rt"> the Asn1 Reference Manual</seealso>, the <c>asn1rt</c> module.</p>
      <p>The generic run-time encode and decode functions can be invoked as below:</p>
      <pre>
asn1rt:encode('H323-MESSAGES','SomeChoiceType',{call,"octetstring"}).
asn1rt:decode('H323-MESSAGES','SomeChoiceType',Bytes).      </pre>
      <p>Or, preferable like:</p>
      <pre>
'H323-MESSAGES':encode('SomeChoiceType',{call,"octetstring"}).
'H323-MESSAGES':decode('SomeChoiceType',Bytes).      </pre>
      <p>The asn1 nif is enabled in two occasions: encoding of
        asn1 values when the asn1 spec is compiled with <c>per_bin</c> and
        <c>optimize</c> or decode of encoded asn1 values when the asn1 spec is
        compiled with <c>ber_bin</c>, <c>optimize</c> and <c>nif</c>. In
        those cases the nif will be loaded automatically at the first call
        to <c>encode</c>/<c>decode</c>. If one doesn't want the performance
        overhead of the nif being loaded at the first call it is possible
        to load the nif separately by loading the <c>asn1rt_nif</c> module.</p>
      <p>By invoking the function <c>info/0</c> in a generated module, one
        gets information about which compiler options were used.</p>
    </section>

    <section>
      <title>Errors</title>
      <p>Errors detected at
        compile time appear on the screen together with
        a line number indicating where in the source file the error 
        was detected. If no errors are found, an Erlang ASN.1 module will
        be created as default.</p>
      <p>The run-time encoders and decoders (in the <c>asn1rt</c> module) do
        execute within a catch and returns <c>{ok, Data}</c> or
        <c>{error, {asn1, Description}}</c> where
        <c>Description</c> is
        an Erlang term describing the error. </p>
    </section>
  </section>

  <section>
    <marker id="inlineExamples"></marker>
    <title>Multi File Compilation</title>
    <p>There are various reasons for using a multi file compilation:</p>
    <list type="bulleted">
      <item>You want to choose name for the generated module by
       any reason. Maybe you need to compile the same specs for
       different encoding/decoding standards.</item>
      <item>You want only one resulting module.</item>
      <item>If it is crucial to have a minimal system. Using
      <c>{inline,OutputModule}</c> includes all necessary run-time
       functions of the asn1 application, but skips those modules not
       used.</item>
      <item>Upgrading issues: Even if you upgrade your Erlang system
       you may want to continue running the old asn1 run-time
       functionality.</item>
      <item>Performance issues: If you have an asn1 system with a lot
       of cross references you may gain in performance. Measurements
       must be done for each case.</item>
    </list>
    <p>You may choose either the plain multi file compilation that just
      merges the chosen asn1 specs or the <c>{inline,OutputModule}</c>
      that also includes the used asn1 run-time functionality.</p>
    <p>For both cases you need to specify which asn1 specs you will
      compile in a module that must have the extension
      <c>.set.asn</c>. You chose name of the module and provide the
      names of the asn1 specs. For instance, if you have the specs
      <c>File1.asn</c>, <c>File2.asn</c> and <c>File3.asn</c> your
      module <c>MyModule.set.asn</c> will look like:</p>
    <pre>
File1.asn
File2.asn
File3.asn    </pre>
    <p>If you compile with:</p>
    <code type="none">
~> erlc MyModule.set.asn    </code>
    <p>the result will be one merged module <c>MyModule.erl</c> with
      the generated code from the three asn1 specs. But if you compile
      with:</p>
    <code type="none">
~> erlc +"{inline,'OutputModule'}" MyModule.set.asn    </code>
    <p>the result will be a module <c>OutputModule.erl</c> that
      contains all encode/decode functions for the three asn1 specs and
      all used functions from the asn1 run-time modules, in this case
      <c>asn1rt_ber_bin</c>. In the former case all encode/decode
      functions are exported but in the latter only the encode/decode
      functions of the first spec in the <c>.set.asn</c>, i.e. those
      from <c>File1.asn</c>.
      </p>
  </section>

  <section>
    <marker id="ASN1Types"></marker>
    <title>The ASN.1 Types</title>
    <p>This section describes the ASN.1 types including their
      functionality, purpose and how values are assigned in Erlang.
      </p>
    <p>ASN.1 has both primitive and constructed types:</p>
    <p></p>
    <table>
      <row>
        <cell align="left" valign="middle"><em>Primitive types</em></cell>
        <cell align="left" valign="middle"><em>Constructed types</em></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#BOOLEAN">BOOLEAN</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#SEQUENCE">SEQUENCE</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#INTEGER">INTEGER</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#SET">SET</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#REAL">REAL</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#CHOICE">CHOICE</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#NULL">NULL</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#SOF">SET OF and SEQUENCE OF</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#ENUMERATED">ENUMERATED</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#ANY">ANY</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#BIT STRING">BIT STRING</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#ANY">ANY DEFINED BY</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#OCTET STRING">OCTET STRING</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#NegotiationTypes">EXTERNAL</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#Character Strings">Character Strings</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#NegotiationTypes">EMBEDDED PDV</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#OBJECT IDENTIFIER">OBJECT IDENTIFIER</seealso></cell>
        <cell align="left" valign="middle"><seealso marker="#NegotiationTypes">CHARACTER STRING</seealso></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#Object Descriptor">Object Descriptor</seealso></cell>
        <cell align="left" valign="middle"></cell>
      </row>
      <row>
        <cell align="left" valign="middle"><seealso marker="#The TIME types">The TIME types</seealso></cell>
        <cell align="left" valign="middle"></cell>
      </row>
      <tcaption>The supported ASN.1 types</tcaption>
    </table>
    <marker id="TypeNameValue"></marker>
    <note>
      <p>Values of each ASN.1 type has its own representation in Erlang
        described in the following subsections. Users shall provide
        these values for encoding according to the representation, as
        in the example below.</p>
    </note>
    <pre>
Operational ::= BOOLEAN --ASN.1 definition    </pre>
    <p>In Erlang code it may look like:</p>
    <pre>
Val = true,
{ok,Bytes}=asn1rt:encode(MyModule,'Operational',Val),    </pre>
    <p>For historical reasons it is also possible to assign ASN.1 values 
      in Erlang using a tuple notation
      with type and value as this</p>
    <pre>
Val = {'Operational',true}    </pre>
    <warning>
      <marker id="warning"></marker>
      <p>The tuple notation, <c>{Typename, Value}</c> is only kept
        because of backward compatibility and may be withdrawn in a
        future release. If the notation is used the <c>Typename</c>
        element must be spelled correctly, otherwise a run-time error
        will occur. 
        </p>
      <p>If the ASN.1 module is compiled with the flags
        <c>per_bin</c> or <c>ber_bin</c> and <c>optimize</c> it is not
        allowed to use the tuple notation. That possibility has been
        removed due to performance reasons. Neither is it allowed to
        use the <c>{ComponentName,Value}</c> notation in case of a
        SEQUENCE or SET type.</p>
    </warning>
    <p>Below follows a description of how
      values of each type can be represented in Erlang. 
      </p>

    <section>
      <marker id="BOOLEAN"></marker>
      <title>BOOLEAN</title>
      <p>Booleans in ASN.1  express values that can be either 
        TRUE or FALSE.
        The meanings assigned to TRUE or FALSE is beyond the scope 
        of this text.          <br></br>

        In ASN.1 it is possible to have:</p>
      <pre>
Operational ::= BOOLEAN
      </pre>
      <p>Assigning a value to the type Operational in Erlang is possible by 
        using the following Erlang code:</p>
      <code type="erl">
Myvar1 = true,
      </code>
      <p>Thus, in Erlang the atoms <c>true</c> and <c>false</c> are used
        to encode a boolean value.</p>
    </section>

    <section>
      <marker id="INTEGER"></marker>
      <title>INTEGER</title>
      <p>ASN.1 itself specifies indefinitely large integers, and the Erlang 
        systems with versions 4.3 and higher, support very large 
        integers, in practice indefinitely large integers.</p>
      <p>The concept of sub-typing can be applied to integers as well
        as to  other ASN.1 types. The details of sub-typing are not
        explained here, for further info see  [<cite id="X.680"></cite>]. A variety 
        of syntaxes are allowed when defining a  type as an integer:</p>
      <pre>
T1 ::= INTEGER
T2 ::= INTEGER (-2..7)
T3 ::= INTEGER (0..MAX)
T4 ::= INTEGER (0&lt;..MAX)
T5 ::= INTEGER (MIN&lt;..-99)
T6 ::= INTEGER {red(0),blue(1),white(2)}
      </pre>
      <p>The Erlang representation of an ASN.1 INTEGER is an integer or
        an atom if a so called <c>Named Number List</c> (see T6 above)
        is specified.</p>
      <p>Below is an example of Erlang code which assigns values for the
        above types: </p>
      <pre>
T1value = 0,
T2value = 6,
T6value1 = blue,
T6value2 = 0,
T6value3 = white
      </pre>
      <p>The Erlang variables above are now bound to valid instances of
        ASN.1 defined types. This style of value can be passed directly 
        to the encoder for transformation into a series of bytes.</p>
      <p>The decoder will return an atom if the value corresponds to a
        symbol in the Named Number List.</p>
    </section>

    <section>
      <marker id="REAL"></marker>
      <title>REAL</title>
      <p>In this version reals are not implemented. When they are, 
        the following
        ASN.1 type is used:</p>
      <pre>
R1 ::= REAL
      </pre>
      <p>Can be assigned a value in Erlang as:</p>
      <pre>
R1value1 = 2.14,
R1value2 = {256,10,-2},
      </pre>
      <p>In the last line note that the tuple {256,10,-2} is the real number
        2.56 in a special notation, which will encode faster than simply 
        stating the number as 2.56. The arity three tuple is 
        <c>{Mantissa,Base,Exponent}</c> i.e. Mantissa * Base^Exponent.</p>
    </section>

    <section>
      <marker id="NULL"></marker>
      <title>NULL</title>
      <p>Null is suitable in cases where supply and recognition of a value 
        is important but the actual value is not.</p>
      <pre>
Notype ::= NULL
      </pre>
      <p>The NULL type can be assigned in Erlang:</p>
      <pre>
N1 = 'NULL',
      </pre>
      <p>The actual value is the quoted atom 'NULL'.</p>
    </section>

    <section>
      <marker id="ENUMERATED"></marker>
      <title>ENUMERATED</title>
      <p>The enumerated type can be used, when the value we wish to 
        describe, may only take one of a set of predefined values.</p>
      <pre>
DaysOfTheWeek ::= ENUMERATED { 
    sunday(1),monday(2),tuesday(3),
    wednesday(4),thursday(5),friday(6),saturday(7) }
      </pre>
      <p>For example to assign a weekday value in Erlang use the same atom
        as in the <c>Enumerations</c> of the type definition:</p>
      <pre>
Day1 = saturday,
      </pre>
      <p>The enumerated type is very similar to an integer type, when
        defined with a set of predefined values. An enumerated type
        differs from an integer in that it may only have  specified
        values, whereas an integer can also have any other value.</p>
    </section>

    <section>
      <marker id="BIT STRING"></marker>
      <title>BIT STRING </title>
      <p>The BIT STRING type can be used to model information which
        is made up of  arbitrary length series of bits. It is intended
        to be used for a selection of flags, not for binary files.        <br></br>

        In ASN.1 BIT STRING definitions may look like:
        </p>
      <pre>
Bits1 ::= BIT STRING
Bits2 ::= BIT STRING {foo(0),bar(1),gnu(2),gnome(3),punk(14)}
      </pre>
      <p>There are four different notations available for representation of
        BIT STRING values in Erlang and as input to the encode functions.</p>
      <list type="ordered">
        <item>A list of binary digits (0 or 1).</item>
        <item>A hexadecimal number (or an integer). This format should be 
         avoided, since it is easy to misinterpret a <c>BIT STRING</c> 
         value in this format. This format may be withdrawn in a future 
         release.</item>
        <item>A list of atoms corresponding to atoms in the <c>NamedBitList</c> 
         in the BIT STRING definition.</item>
        <item>As <c>{Unused,Binary}</c> where <c>Unused</c> denotes how
         many trailing zero-bits 0 to 7 that are unused in the least
         significant byte in <c>Binary</c>. This notation is only
         available when the ASN.1 files have been compiled with the
        <em>+compact_bit_string</em> flag in the option list. In
         this case it is possible to use all kinds of notation when
         encoding. But the result when decoding is always in the
         compact form. The benefit from this notation is a more
         compact notation when one has large BIT STRINGs. The
         encode/decode performance is also much better in the case of
         large BIT STRINGs. </item>
      </list>
      <note>
        <p>Note that it is advised not to use the integer format of a
          BIT STRING, see the second point above.</p>
      </note>
      <pre>
Bits1Val1 = [0,1,0,1,1],
Bits1Val2 = 16#1A,
Bits1Val3 = {3,&lt;&lt;0:1,1:1,0:1,1:1,1:1,0:3&gt;&gt;}
      </pre>
      <p>Note that <c>Bits1Val1</c>, <c>Bits1Val2</c> and <c>Bits1Val3</c>
        denote the same value.</p>
      <pre>
Bits2Val1 = [gnu,punk],
Bits2Val2 = 2#1110,
Bits2Val3 = [bar,gnu,gnome],
Bits2Val4 = [0,1,1,1]
      </pre>
      <p>The above <c>Bits2Val2</c>, <c>Bits2Val3</c> and <c>Bits2Val4</c> 
        also all denote the same value.</p>
      <p><c>Bits2Val1</c> is assigned symbolic values. The assignment means 
        that the bits corresponding to <c>gnu</c> and <c>punk</c> i.e. bits
        2 and 14 are set to 1 and the rest set to 0. The symbolic values
        appear as a list of values. If a named value appears, which is not
        specified in the type definition, a run-time error will occur.</p>
      <p>The compact notation equivalent to the empty BIT STRING is
        <c><![CDATA[{0,<<>>}]]></c>, which in the other notations is <c>[]</c> or
        <c>0</c>.</p>
      <p>BIT STRINGS may also be sub-typed with for example a SIZE
        specification:</p>
      <pre>
Bits3 ::= BIT STRING (SIZE(0..31))      </pre>
      <p>This means that no bit higher than 31 can ever be set.</p>
    </section>

    <section>
      <marker id="OCTET STRING"></marker>
      <title>OCTET STRING</title>
      <p>The OCTET STRING is the simplest of all ASN.1 types The OCTET STRING
        only moves or transfers e.g. binary files or other unstructured
        information complying to two rules.
        Firstly, the bytes consist of octets and secondly, encoding is 
        not required.</p>
      <p>It is possible to have the following ASN.1 type definitions:</p>
      <pre>
O1 ::= OCTET STRING
O2 ::= OCTET STRING (SIZE(28))      </pre>
      <p>With the following example assignments in Erlang:</p>
      <pre>
O1Val = [17,13,19,20,0,0,255,254],
O2Val = "must be exactly 28 chars....",      </pre>
      <p>Observe that <c>O1Val</c> is assigned a series of numbers between 0 
        and 255 i.e. octets.
        <c>O2Val</c> is assigned using the string notation.
        </p>
    </section>

    <section>
      <marker id="Character Strings"></marker>
      <title>Character Strings</title>
      <p>ASN.1 supports a wide variety of character sets. The main difference
        between OCTET STRINGS and the Character strings is that OCTET
        STRINGS have no imposed semantics on the bytes delivered.</p>
      <p>However, when using for instance the IA5String (which closely
        resembles ASCII)  the byte 65 (in decimal
        notation)  <em>means</em> the character 'A'.
        </p>
      <p>For example, if a defined type is to be a VideotexString and
        an octet is received with the unsigned integer value X, then
        the octet should be interpreted as specified in the standard
        ITU-T T.100,T.101. 
        </p>
      <p>The  ASN.1 to Erlang compiler
        will not determine the correct interpretation  of each BER  
        (Basic Encoding Rules) string octet value with different
        Character strings. Interpretation of octets is the
        responsibility of the application. Therefore, from the BER
        string point of view, octets appear to be very similar to
        character strings and are compiled in the same way.
        </p>
      <p>It should be noted that when PER (Packed Encoding Rules) is
        used, there is a significant difference in the encoding scheme 
        between OCTET STRINGS and other strings. The constraints 
        specified for a type are especially important for PER, where
        they affect the encoding.
        </p>
      <p>Please note that <em>all</em> the Character strings are 
        supported and it is possible to use the following ASN.1 type 
        definitions:</p>
      <pre>
Digs ::= NumericString (SIZE(1..3))
TextFile ::= IA5String (SIZE(0..64000))      </pre>
      <p>and the following Erlang assignments:</p>
      <pre>
DigsVal1 = "456",
DigsVal2 = "123",
TextFileVal1 = "abc...xyz...",
TextFileVal2 = [88,76,55,44,99,121 .......... a lot of characters here ....]        </pre>
      <p>The Erlang representation for "BMPString" and
        "UniversalString" is either a list of ASCII values or a list
        of quadruples. The quadruple representation associates to the
        Unicode standard representation of characters. The ASCII
        characters are all represented by quadruples beginning with
        three zeros like {0,0,0,65} for the 'A' character. When
        decoding a value for these strings the result is a list of
        quadruples, or integers when the value is an ASCII character.
        The following example shows how it works:</p>
      <p>In a file <c>PrimStrings.asn1</c> the type <c>BMP</c> is defined as
                <br></br>
<c>BMP ::= BMPString</c> then using BER encoding (<c>ber_bin</c> 
        option)the input/output format will be:</p>
      <pre>
1> <input>{ok,Bytes1} = asn1rt:encode('PrimStrings','BMP',[{0,0,53,53},{0,0,45,56}]).</input>
{ok,[30,4,"55-8"]}
2> <input>asn1rt:decode('PrimStrings','BMP',list_to_binary(Bytes1)).</input>
{ok,[{0,0,53,53},{0,0,45,56}]}
3> <input>{ok,Bytes2} = asn1rt:encode('PrimStrings','BMP',[{0,0,53,53},{0,0,0,65}]).</input>
{ok,[30,4,[53,53,0,65]]}
4> <input>asn1rt:decode('PrimStrings','BMP',list_to_binary(Bytes2)).</input>
{ok,[{0,0,53,53},65]}
5> <input>{ok,Bytes3} = asn1rt:encode('PrimStrings','BMP',"BMP string").</input>
{ok,[30,20,[0,66,0,77,0,80,0,32,0,115,0,116,0,114,0,105,0,110,0,103]]}
6> <input>asn1rt:decode('PrimStrings','BMP',list_to_binary(Bytes3)).</input>
{ok,"BMP string"}      </pre>
      <p>The UTF8String is represented in Erlang as a list of integers,
        where each integer represents the unicode value of one
        character. When a value shall be encoded one first has to
        transform it to a UTF8 encoded binary, then it can be encoded by
        asn1. When decoding the result is a UTF8 encoded binary, which
        may be transformed to an integer list. The transformation
        functions, <c>utf8_binary_to_list</c> and
        <c>utf8_list_to_binary</c>, are in the <c>asn1rt</c> module. In
        the example below we assume an asn1 definition <c>UTF ::= UTF8String</c> in a module <c>UTF.asn</c>:</p>
      <pre>
1> <input>asn1ct:compile('UTF',[ber_bin]).</input>
Erlang ASN.1 version "1.4.3.3" compiling "UTF.asn" 
Compiler Options: [ber_bin]
--{generated,"UTF.asn1db"}--
--{generated,"UTF.erl"}--
ok
2> <input>UTF8Val1 = "hello".</input>
"hello"
3> <input>{ok,UTF8bin1} = asn1rt:utf8_list_to_binary(UTF8Val1).</input>
{ok,&lt;&lt;104,101,108,108,111&gt;&gt;}
4> <input>{ok,B}='UTF':encode('UTF',UTF8bin1).</input>
{ok,[12,
     5,
     &lt;&lt;104,101,108,108,111&gt;&gt;]}
5> <input>Bin = list_to_binary(B).</input>
&lt;&lt;12,5,104,101,108,108,111&gt;&gt;
6> <input>{ok,UTF8bin1}='UTF':decode('UTF',Bin).</input>
{ok,&lt;&lt;104,101,108,108,111&gt;&gt;}
7> <input>asn1rt:utf8_binary_to_list(UTF8bin1).</input>
{ok,"hello"}           
8> <input>UTF8Val2 = [16#00,16#100,16#ffff,16#ffffff].</input>
[0,256,65535,16777215]
9> <input>{ok,UTF8bin2} = asn1rt:utf8_list_to_binary(UTF8Val2).</input>
{ok,&lt;&lt;0,196,128,239,191,191,248,191,191,191,191&gt;&gt;}
10> <input>{ok,B2} = 'UTF':encode('UTF',UTF8bin2).</input>
{ok,[12,
     11,
     &lt;&lt;0,196,128,239,191,191,248,191,191,191,191&gt;&gt;]}
11> <input>Bin2 = list_to_binary(B2).</input>
&lt;&lt;12,11,0,196,128,239,191,191,248,191,191,191,191&gt;&gt;
12> <input>{ok,UTF8bin2} = 'UTF':decode('UTF',Bin2).</input>
{ok,&lt;&lt;0,196,128,239,191,191,248,191,191,191,191&gt;&gt;}
13> <input>asn1rt:utf8_binary_to_list(UTF8bin2).</input>
{ok,[0,256,65535,16777215]}
14>       </pre>
    </section>

    <section>
      <marker id="OBJECT IDENTIFIER"></marker>
      <title>OBJECT IDENTIFIER</title>
      <p>The OBJECT IDENTIFIER is used whenever a unique identity is required.
        An ASN.1 module, a transfer syntax, etc. is identified with an
        OBJECT IDENTIFIER. Assume the example below:</p>
      <pre>
Oid ::= OBJECT IDENTIFIER
      </pre>
      <p>Therefore, the example below is a valid Erlang instance of the 
        type 'Oid'.</p>
      <pre>
OidVal1 = {1,2,55},
      </pre>
      <p>The OBJECT IDENTIFIER value is simply a tuple with the
        consecutive values which must be integers. 
        </p>
      <p>The first value is limited to the values 0, 1 or 2 and the
        second value must be in the range 0..39 when the first value
        is 0 or 1.
        </p>
      <p>The OBJECT IDENTIFIER is a very important type and it is
        widely used within different standards to uniquely identify
        various objects. In [<cite id="DUBUISSON"></cite>], there is an 
        easy-to-understand description of the usage of 
        OBJECT IDENTIFIER.</p>
      <p></p>
    </section>

    <section>
      <marker id="Object Descriptor"></marker>
      <title>Object Descriptor</title>
      <p>Values of this type can be assigned a value as an ordinary string i.e.        <br></br>

        "This is the value of an Object descriptor"</p>
    </section>

    <section>
      <marker id="The TIME types"></marker>
      <title>The TIME Types</title>
      <p>Two different time types are defined within ASN.1, Generalized
        Time and UTC (Universal Time Coordinated), both are assigned a
        value as an ordinary string within double quotes i.e.
        "19820102070533.8".</p>
      <p>In case of DER encoding the compiler does not check the validity
        of the time values. The DER requirements upon those strings is 
        regarded as a matter for the application to fulfill.</p>
    </section>

    <section>
      <marker id="SEQUENCE"></marker>
      <title>SEQUENCE</title>
      <p>The structured types of ASN.1 are constructed from other types 
        in a manner similar to the concepts of array and struct in  C.
                <br></br>
 A SEQUENCE in ASN.1 is
        comparable with a struct in C and a record in Erlang. 
        A SEQUENCE may be defined as:</p>
      <pre>
Pdu ::= SEQUENCE {
   a INTEGER,
   b REAL,
   c OBJECT IDENTIFIER,
   d NULL }      </pre>
      <p>This is a 4-component structure called 'Pdu'. The major format 
        for representation of SEQUENCE in Erlang is the record format. 
        For each SEQUENCE and <c>SET</c> in an ASN.1 module an Erlang 
        record declaration is generated. For <c>Pdu</c> above, a record 
        like this is defined:</p>
      <pre>
-record('Pdu',{a, b, c, d}).      </pre>
      <p>The record declarations for a module <c>M</c> are placed in a 
        separate <c>M.hrl</c> file.</p>
      <p>Values can be assigned in Erlang as shown below:</p>
      <pre>
MyPdu = #'Pdu'{a=22,b=77.99,c={0,1,2,3,4},d='NULL'}.      </pre>
<note>
  <p>
    In very early versions of the asn1 compiler it was also possible to 
    specify the values of the components in 
    a SEQUENCE or a SET as a list of tuples <c>{ComponentName,Value}</c>. 
    This is no longer supported.
  </p>
</note>
      <p>The decode functions will return a record as result when decoding 
        a <c>SEQUENCE</c> or a <c>SET</c>.
                <marker id="DEFAULT"></marker>
</p>
      <p>A <c>SEQUENCE</c> and a <c>SET</c> may contain a component with a
        <c>DEFAULT</c> key word followed by the actual value that is the
        default value. In case of BER encoding it is optional to encode the
        value if it equals the default value. If the application uses the
        atom asn1_DEFAULT as value or if the value is a primitive value 
        that equals the default value the encoding omits the bytes for
        this value, which is more efficient and it results in fever 
        bytes to send to the receiving application.</p>
      <p>For instance, if the following types exists in a file "File.asn":</p>
      <pre>
Seq1 ::= SEQUENCE {
    a INTEGER DEFAULT 1,
    b Seq2 DEFAULT {aa TRUE, bb 15}
}

Seq2 ::= SEQUENCE {
    aa BOOLEAN,
    bb INTEGER
}
      </pre>
      <p>Some values and the corresponding encoding in an Erlang terminal
        is shown below:</p>
      <pre>
1> <input>asn1ct:compile('File').</input>
Erlang ASN.1 version "1.3.2" compiling "File.asn1" 
Compiler Options: []
--{generated,"File.asn1db"}--
--{generated,"File.hrl"}--
--{generated,"File.erl"}--
ok
2> <input>'File':encode('Seq1',{'Seq1',asn1_DEFAULT,asn1_DEFAULT}).</input>
{ok,["0",[0],[[],[]]]}
3> <input>lists:flatten(["0",[0],[[],[]]]).</input>
[48,0]
4> <input>'File':encode('Seq1',{'Seq1',1,{'Seq2',true,15}}).</input>
{ok,["0","\\b",[[],["\\241",[6],[[[128],[1],"\\377"],[[129],[1],[15]]]]]]}
5> <input>lists:flatten(["0","\\b",[[],["\\241",[6],[[[128],[1],"\\377"],[[129],[1],[15]]]]]]).</input>
[48,8,161,6,128,1,255,129,1,15]
6>      </pre>
      <p>The result after command line 3, in the example above,shows that the
        encoder omits the encoding of default values when they are specific 
        by asn1_DEFAULT. Line 5 shows that even primitive values that equals
        the default value are detected and not encoded. But the constructed
        value of component <c>b</c> in <c>Seq1</c> is not recognized as the
        default value. Checking of default values in <c>BER</c> is not done
        in case of complex values, because it would be to expensive.
                <marker id="DEFAULT DER"></marker>
</p>
      <p>But, the DER encoding format has stronger requirements regarding 
        default values both for SET and SEQUENCE. A more elaborate and time 
        expensive check of default values will take place. The following is
        an example with the same types and values as above but with der 
        encoding format.</p>
      <pre>
1> <input>asn1ct:compile('File',[der]).</input>
Erlang ASN.1 version "1.3.2" compiling "File.asn1" 
Compiler Options: [der]
--{generated,"File.asn1db"}--
--{generated,"File.hrl"}--
--{generated,"File.erl"}--
ok
2> <input>'File':encode('Seq1',{'Seq1',asn1_DEFAULT,asn1_DEFAULT}).</input>
{ok,["0",[0],[[],[]]]}
3> <input>lists:flatten(["0",[0],[[],[]]]).</input>
[48,0]
4> <input>'File':encode('Seq1',{'Seq1',1,{'Seq2',true,15}}).</input>
{ok,["0",[0],[[],[]]]}
5> <input>lists:flatten(["0",[0],[[],[]]]).</input>
[48,0]
6> 
      </pre>
      <p>Line 5 shows that even values of constructed types is checked and if 
        it equals the default value it will not be encoded.</p>
    </section>

    <section>
      <marker id="SET"></marker>
      <title>SET</title>
      <p>The SET type is an unusual construct and normally the SEQUENCE 
        type is more appropriate to use. Set is also inefficient compared with SEQUENCE, as the components can be in any order. Hence, it must be possible
        to distinguish every component in 'SET', both when
        encoding and decoding a value of a type defined to be a SET.
        The tags of all components must be different from each other
        in order to  be easily recognizable.</p>
      <p>A SET may be defined as:</p>
      <pre>
Pdu2 ::= SET {
    a INTEGER,
    b BOOLEAN,
    c ENUMERATED {on(0),off(1)} }      </pre>
      <p>A SET is represented as an Erlang record.
        For each SEQUENCE and <c>SET</c> in
        an ASN.1 module an Erlang record declaration is generated. For
        <c>Pdu2</c> above a record is defined like this:</p>
      <pre>
-record('Pdu2',{a, b, c}).      </pre>
      <p>The record declarations for a module <c>M</c> are placed in a 
        separate <c>M.hrl</c> file.</p>
      <p>Values can be assigned in Erlang as demonstrated below:</p>
      <pre>
V = #'Pdu2'{a=44,b=false,c=off}.      </pre>
      <p>The decode functions will return a record as result when decoding 
        a SET.
        </p>
      <p>The difference between SET and SEQUENCE is that the order of
        the components (in the BER encoded format) is undefined for SET
        and defined as the lexical order from the ASN.1 definition for
        SEQUENCE. The ASN.1 compiler for Erlang will always encode a
        SET in the lexical order. The decode routines can handle SET
        components encoded in any order but will always return the
        result as a record. Since all components of the SET must be
        distinguishable both in the encoding phase as well as the
        decoding phase the following type is not allowed in a module
        with EXPLICIT or IMPLICIT as tag-default :</p>
      <p></p>
      <pre>
Bad ::= SET {i INTEGER,
             j INTEGER }      </pre>
      <p>The  ASN.1 to Erlang compiler  rejects the above type. We
        shall not explain the concept of tag further here, we refer to
        [<cite id="X.680"></cite>].
        </p>
      <p>Encoding of a SET with components with DEFAULT values behaves
        similar as a SEQUENCE, <seealso marker="#DEFAULT">see above</seealso>. The DER encoding format restrictions on DEFAULT 
        values is the same for SET as for SEQUENCE, and is supported by 
        the compiler, <seealso marker="#DEFAULT DER">see above</seealso>.</p>
      <p>Moreover, in DER the elements of a SET will be sorted. If a 
        component is an un-tagged choice the sorting have to take place
        in run-time. This fact emphasizes the following recommendation
        if DER encoding format is used.</p>
      <p>The concept of SET is an unusual
        construct and one  cannot think of one single application
        where the set type is essential. (Imagine if someone
        "invented'' the shuffled array in 'C') People tend to think
        that 'SET' sounds nicer and more mathematical than 'SEQUENCE'
        and hence use it when  'SEQUENCE' would have been more
        appropriate. It is also most inefficient, since every correct
        implementation of SET must always be prepared to accept the
        components in any order. So, if possible use SEQUENCE instead
        of SET.</p>
    </section>

    <section>
      <title>Notes about Extend-ability for SEQUENCE and SET</title>
      <p>When a SEQUENCE or SET contains an extension marker and 
        extension components like this:</p>
      <pre>
SExt ::= SEQUENCE {
           a INTEGER,
           ...,
           b BOOLEAN }
      </pre>
      <p>It means that the type may get more components in newer
        versions of the ASN.1 spec. In this case it has got a new
        component <c>b</c>. Thus, incoming messages that will be decoded
        may have more or fever components than this one.  
        </p>
      <p>The component <c>b</c> will be treated as
        an original component when encoding a message. In this case, as
        it is not an optional element, it must be encoded.
        </p>
      <p>During decoding the <c>b</c> field of the record will get the decoded 
        value of the <c>b</c> 
        component if present and otherwise the value <c>asn1_NOVALUE</c>.</p>
    </section>

    <section>
      <marker id="CHOICE"></marker>
      <title>CHOICE</title>
      <p>The CHOICE type is a space saver and is similar to the concept of a
        'union' in the C-language. As with the previous SET-type, the
        tags of all components  of a CHOICE need to be distinct. If
        AUTOMATIC TAGS are defined for the module  (which is
        preferable) the tags can be omitted completely in the ASN.1
        specification of a CHOICE.
        </p>
      <p>Assume:</p>
      <pre>
T ::= CHOICE {
        x [0] REAL,
        y [1] INTEGER,
        z [2] OBJECT IDENTIFIER }
      </pre>
      <p>It is then possible to assign values:</p>
      <pre>
TVal1 = {y,17},
TVal2 = {z,{0,1,2}},
      </pre>
      <p>A CHOICE value is  always represented as the tuple
        <c>{ChoiceAlternative, Val}</c> where <c>ChoiceAlternative</c>
        is an atom denoting the selected choice
        alternative.
        </p>
      <p>It is also allowed to have a CHOICE type tagged as follow:</p>
      <p></p>
      <pre>
C ::= [PRIVATE 111] CHOICE {
        C1,
        C2 }

C1 ::= CHOICE { 
         a [0] INTEGER,
         b [1] BOOLEAN }

C2 ::= CHOICE {
         c [2] INTEGER,
         d [3] OCTET STRING }      </pre>
      <p>In this case, the top type C appears to have no tags at all in
        its components, however, both C1 and C2 are also defined as
        CHOICE types and they have distinct tags among themselves.
        Hence, the above type C is both legal and allowed.
        </p>

      <section>
        <title>Extendable CHOICE</title>
        <p>When a CHOICE contains an extension marker and the decoder detects
          an unknown alternative of the CHOICE the value is represented as:</p>
        <pre>
{asn1_ExtAlt, BytesForOpenType}
        </pre>
        <p>Where <c>BytesForOpenType</c> is a list of bytes constituting the 
          encoding of the "unknown" CHOICE alternative. </p>
      </section>
    </section>

    <section>
      <marker id="SOF"></marker>
      <title>SET OF and SEQUENCE OF</title>
      <p>The SET OF and SEQUENCE OF types correspond to the concept of an array
        found in several programming languages. The Erlang syntax for
        both of these types is straight forward. For example:</p>
      <pre>
Arr1 ::= SET SIZE (5) OF INTEGER (4..9) 
Arr2 ::= SEQUENCE OF OCTET STRING      </pre>
      <p>We may have the following in Erlang:</p>
      <pre>
Arr1Val = [4,5,6,7,8],
Arr2Val = ["abc",[14,34,54],"Octets"],      </pre>
      <p>Please note that the definition of the SET OF type implies that 
        the order of the components is undefined, but in practice there is 
        no difference between SET OF and SEQUENCE OF. The ASN.1 compiler 
        for Erlang does not randomize the order of the SET OF components 
        before encoding.</p>
      <p>However, in case of a value of the type <c>SET OF</c>, the DER 
        encoding format requires the elements to be sent in ascending 
        order of their encoding, which implies an expensive sorting 
        procedure in run-time. Therefore it is strongly recommended to 
        use <c>SEQUENCE OF</c> instead of <c>SET OF</c> if it is possible.</p>
    </section>

    <section>
      <marker id="ANY"></marker>
      <title>ANY and ANY DEFINED BY</title>
      <p>The types <c>ANY</c> and <c>ANY DEFINED BY</c> have been removed 
        from the standard since 1994. It is recommended not to use
        these types any more. They may, however, exist in some old ASN.1
        modules.
        The idea with this type was to leave a "hole" in a definition where
        one could put unspecified data of any kind, even non ASN.1 data.</p>
      <p>A value of this type is encoded as an <c>open type</c>.</p>
      <p>Instead of <c>ANY</c>/<c>ANY DEFINED BY</c> one should use
        <c>information object class</c>, <c>table constraints</c> and
        <c>parameterization</c>. In particular the construct
        <c>TYPE-IDENTIFIER.@Type</c> accomplish the same as the
        deprecated <c>ANY</c>.</p>
      <p>See also <seealso marker="#Information Object">Information object</seealso></p>
    </section>

    <section>
      <marker id="NegotiationTypes"></marker>
      <title>EXTERNAL, EMBEDDED PDV and CHARACTER STRING</title>
      <p>These types are used in presentation layer negotiation. They are
        encoded according to their associated type, see [<cite id="X.680"></cite>].</p>
      <p>The <c>EXTERNAL</c> type had a slightly different associated type
        before 1994. [<cite id="X.691"></cite>] states that encoding shall follow
        the older associate type. Therefore does generated encode/decode
        functions convert values of the newer format to the older format
        before encoding. This implies that it is allowed to use 
        <c>EXTERNAL</c> type values of either format for encoding. Decoded
        values are always returned on the newer format.</p>
    </section>

    <section>
      <title>Embedded Named Types</title>
      <p>The structured types previously described may very well have other named types
        as their components. The general syntax to assign a value to the component C
        of a named ASN.1 type T in Erlang is the record syntax
        <c>#'T'{'C'=Value}</c>. 
        Where <c>Value</c> may be a value of yet another type T2.</p>
      <p>For example:</p>
      <pre>
B ::= SEQUENCE {
        a Arr1,
        b [0] T }

Arr1 ::= SET SIZE (5) OF INTEGER (4..9) 

T ::= CHOICE {
        x [0] REAL,
        y [1] INTEGER,
        z [2] OBJECT IDENTIFIER }      </pre>
      <p>The above example can be assigned like this in Erlang:</p>
      <pre>
V2 = #'B'{a=[4,5,6,7,8], b={x,7.77}}.
      </pre>
    </section>
  </section>

  <section>
    <title>Naming of Records in .hrl Files</title>
    <p>When an asn1 specification is compiled all defined types of
      type SET or SEQUENCE will result in a corresponding record in the
      generated hrl file. This is because the values for SET/SEQUENCE
      as mentioned in sections above are represented as records.</p>
    <p>Though there are some special cases of this functionality that
      are presented below.</p>

    <section>
      <title>Embedded Structured Types</title>
      <p>It is also possible in ASN.1 to have components that are  themselves
        structured types.
        For example, it is possible to have:</p>
      <pre>
Emb ::= SEQUENCE {
    a SEQUENCE OF OCTET STRING,
    b SET {
       a [0] INTEGER,
       b [1] INTEGER DEFAULT 66},
    c CHOICE {
       a INTEGER,
       b FooType } }

FooType ::= [3] VisibleString      </pre>
      <p>The following records are generated because of the type <c>Emb</c>:</p>
      <pre>
-record('Emb,{a, b, c}).
-record('Emb_b',{a, b = asn1_DEFAULT}). % the embedded SET type
      </pre>
      <p>Values of the <c>Emb</c> type can be assigned like this:</p>
      <code type="none">
V = #'Emb'{a=["qqqq",[1,2,255]], 
           b = #'Emb_b'{a=99}, 
           c ={b,"Can you see this"}}.
      </code>
      <p>For an embedded type of type SEQUENCE/SET in a SEQUENCE/SET
        the record name is extended with an underscore and the component
        name. If the embedded structure is deeper with SEQUENCE, SET or
        CHOICE types in the line, each component-/alternative-name will
        be added to the record-name.</p>
      <p>For example:</p>
      <pre>
Seq ::= SEQUENCE{
    a CHOICE{
        b SEQUENCE {
           c  INTEGER
        }
    }
}      </pre>
      <p>will result in the following record:</p>
      <pre>
-record('Seq_a_b',{c}).      </pre>
      <p>If the structured type has a component with an embedded
        SEQUENCE OF/SET OF which embedded type in turn is a
        SEQUENCE/SET it will give a record with the SEQOF/SETOF
        addition as in the following example:</p>
      <pre>
Seq ::= SEQUENCE {
    a SEQUENCE OF SEQUENCE {
           b
               }
    c SET OF SEQUENCE {
           d
               }
}      </pre>
      <p>This results in the records:</p>
      <pre>
-record('Seq_a_SEQOF'{b}).
-record('Seq_c_SETOF'{d}).      </pre>
      <p>A parameterized type should be considered as an embedded
        type. Each time a such type is referenced an instance of it is
        defined. Thus in the following example a record with name
        <c>'Seq_b'</c> is generated in the .hrl file and used to hold
        values.</p>
      <pre>
Seq ::= SEQUENCE {
    b PType{INTEGER}
}

PType{T} ::= SEQUENCE{
    id T
}      </pre>
    </section>

    <section>
      <title>Recursive Types</title>
      <p>Types may refer to themselves. Suppose:</p>
      <pre>
Rec ::= CHOICE {
     nothing [0] NULL,
     something SEQUENCE {
          a INTEGER,
          b OCTET STRING,
          c Rec }}      </pre>
      <p>This type is recursive; that is, it refers to itself. This is allowed 
        in ASN.1 and the  ASN.1-to-Erlang compiler  supports this recursive
        type. A value for this type is assigned in Erlang as shown below:</p>
      <pre>
V = {something,#'Rec_something'{a = 77, 
                                b = "some octets here", 
                                c = {nothing,'NULL'}}}.      </pre>
    </section>
  </section>

  <section>
    <title>ASN.1 Values</title>
    <p>Values can be assigned to ASN.1 type within the ASN.1 code
      itself, as opposed to the actions taken in the previous chapter where
      a value was assigned to an ASN.1 type in Erlang. The full value
      syntax of ASN.1 is supported and [X.680] describes in detail how
      to assign values in ASN.1. Below is a short example:</p>
    <pre>
TT ::= SEQUENCE {
   a INTEGER,
   b SET OF OCTET STRING }

tt TT ::= {a 77,b {"kalle","kula"}}    </pre>
    <p>The value defined here could be used in several ways. 
      Firstly, it could be used as the value in some DEFAULT component:</p>
    <pre>
SS ::= SET {
    s [0] OBJECT IDENTIFIER,
    val TT DEFAULT tt }    </pre>
    <p>It could also be used from inside an Erlang program. If the above ASN.1
      code was defined in ASN.1 module <c>Values</c>, then the ASN.1 value
      <c>tt</c> can be reached from Erlang as
      a function call to <c>'Values':tt()</c> as in the example below.</p>
    <pre>
1> <input>Val = 'Values':tt().</input>
{'TT',77,["kalle","kula"]}
2> <input>{ok,Bytes} = 'Values':encode('TT',Val).</input>
{ok,["0",
     [18],
     [[[128],[1],"M"],["\\241","\\r",[[[4],[5],"kalle"],[[4],[4],"kula"]]]]]}
3> <input>FlatBytes = lists:flatten(Bytes).</input>
[48,18,128,1,77,161,13,4,5,107,97,108,108,101,4,4,107,117,108,97]
4> <input>'Values':decode('TT',FlatBytes).</input>
{ok,{'TT',77,["kalle","kula"]}}
5>
    </pre>
    <p>The above example shows that a function is generated by the compiler 
      that returns a valid Erlang representation of the value, even though
      the value is of a complex type.</p>
    <p>Furthermore, there is a macro generated for each value in the .hrl 
      file. So, the defined value <c>tt</c> can also be extracted by 
      <c>?tt</c> in application code.</p>
  </section>

  <section>
    <title>Macros</title>
    <p>MACRO is not supported as the the type is no longer part of the 
      ASN.1 standard.</p>
  </section>

  <section>
    <marker id="Information Object"></marker>
    <title>ASN.1 Information Objects (X.681)</title>
    <p>Information Object Classes, Information Objects and Information
      Object Sets, (in the following called classes, objects and
      object sets respectively), are defined in the standard
      definition [<cite id="X.681"></cite>]. In the following only a brief
      explanation is given. </p>
    <p>These constructs makes it possible to define open types,
      i.e. values of that type can be of any ASN.1 type. It is also
      possible to define relationships between different types and
      values, since classes can hold types, values, objects, object
      sets and other classes in its fields.
      An Information Object Class may be defined in ASN.1 as:</p>
    <pre>
GENERAL-PROCEDURE ::= CLASS {
      &amp;Message,
      &amp;Reply               OPTIONAL,
      &amp;Error               OPTIONAL,
      &amp;id          PrintableString UNIQUE
}
WITH SYNTAX {
      NEW MESSAGE     &amp;Message
      [REPLY           &amp;Reply]
      [ERROR           &amp;Error]
      ADDRESS          &amp;id
}    </pre>
    <p>An object is an instance of a class and an object set is a set
      containing objects of one specified class. A definition may look like
      below.</p>
    <p>The object <c>object1</c> is an instance of the CLASS 
      GENERAL-PROCEDURE and has one type field and one fixed type value 
      field. The object <c>object2</c> also has an OPTIONAL field ERROR,
      which is a type field.</p>
    <pre>
object1 GENERAL-PROCEDURE ::= {
    NEW MESSAGE      PrintableString
    ADDRESS          "home"
}

object2 GENERAL-PROCEDURE ::= {
    NEW MESSAGE INTEGER
    ERROR INTEGER
    ADDRESS "remote"
}    </pre>
    <p>The field ADDRESS is a UNIQUE field. Objects in an object set must
      have unique values in their UNIQUE field, as in GENERAL-PROCEDURES: </p>
    <pre>
GENERAL-PROCEDURES GENERAL-PROCEDURE ::= {
    object1 | object2}    </pre>
    <p>One can not encode a class, object or object set, only referring to
      it when defining other ASN.1 entities. Typically one refers to a
      class and to object sets by table constraints and component
      relation constraints [<cite id="X.682"></cite>] in ASN.1 types, as in: </p>
    <pre>
StartMessage  ::= SEQUENCE {
    msgId  GENERAL-PROCEDURE.&amp;id  ({GENERAL-PROCEDURES}),
    content GENERAL-PROCEDURE.&amp;Message ({GENERAL-PROCEDURES}{@msgId}),
    }    </pre>
    <p>In the type <c>StartMessage</c> the constraint following the
      <c>content</c> field tells that in a value of type
      <c>StartMessage</c> the value in the <c>content</c> field must
      come from the same object that is chosen by the <c>msgId</c>
      field.</p>
    <p>So, the value <c>#'StartMessage'{msgId="home",content="Any Printable String"}</c> is legal to encode as a StartMessage
      value, while the value <c>#'StartMessage'{msgId="remote", content="Some String"}</c> is illegal since the constraint
      in StartMessage tells that when you have chosen a value from a 
      specific object in the object set GENERAL-PROCEDURES in the 
      msgId field you have to choose a value from that same object in
      the content field too. In this second case it should have been 
      any INTEGER value.</p>
    <p><c>StartMessage</c> can in the <c>content</c> field be
      encoded with a value of any type that an object in the
      <c>GENERAL-PROCEDURES</c> object set has in its <c>NEW MESSAGE</c> field. This field refers to a type field
      <c><![CDATA[&amp;Message]]></c> in the class. The <c>msgId</c> field is always
      encoded as a PrintableString, since the field refers to a fixed type
      in the class.</p>
  </section>

  <section>
    <title>Parameterization (X.683)</title>
    <p>Parameterization, which is defined in the standard [<cite id="X.683"></cite>], can be used when defining types, values, value
      sets, information object classes, information objects or
      information object sets. 
      A part of a definition can be supplied as a parameter. For
      instance, if a Type is used in a definition with certain
      purpose, one want the type-name to express the intention. This
      can be done with parameterization.</p>
    <p>When many types (or an other ASN.1 entity) only differs in some
      minor cases, but the structure of the types are similar, only
      one general type can be defined and the differences may be supplied
      through parameters. </p>
    <p>One example of use of parameterization is:</p>
    <pre>
General{Type} ::= SEQUENCE
{
     number     INTEGER,
     string     Type
}
      
T1 ::= General{PrintableString}

T2 ::= General{BIT STRING}
    </pre>
    <p>An example of a value that can be encoded as type T1 is {12,"hello"}.</p>
    <p>Observe that the compiler not generates encode/decode functions for
      parameterized types, only for the instances of the parameterized 
      types. So, if a file contains the types General{}, T1 and T2 above,
      encode/decode functions will only be generated for T1 and T2.
      </p>
  </section>

  <section>
    <title>Tags</title>
    <p>Every built-in ASN.1 type, except CHOICE and ANY have a universal tag.
      This is a unique number that clearly identifies the type.      <br></br>
 
      It is essential for all users of ASN.1  to
      understand  all the details about tags.</p>
    <p>Tags are implicitly encoded in the BER encoding as shown below, but 
      are hardly not accounted for in the PER encoding. In PER tags are
      used for instance to sort the components of a SET.</p>
    <p>There are four different types of tags.</p>
    <taglist>
      <tag><em>universal</em></tag>
      <item>
        <p>For types whose meaning is the same in all 
          applications. Such as integers, sequences and so on; that is, all the built in
          types.</p>
      </item>
      <tag><em>application</em></tag>
      <item>
        <p>For application specific types for example, the types in
          X.400 Message handling service have this sort of tag.</p>
      </item>
      <tag><em>private</em></tag>
      <item>
        <p>For your own private types.</p>
      </item>
      <tag><em>context</em></tag>
      <item>
        <p>This is used to distinguish otherwise indistinguishable
          types in a specific context. For example, if we have two
          components of a 
          CHOICE type that are both <c>INTEGER</c> values, there is no
          way for the decoder to 
          decipher which component was actually chosen, since both
          components will be
          tagged as <c>INTEGER</c>. When this or similar situations occur,  
          one or both of the components should be given a  context specific
          to resolve the ambiguity.</p>
      </item>
    </taglist>
    <p>The tag in the case of the 'Apdu' type [PRIVATE 1] is encoded to a
      sequence of bytes  making it possible for  a
      decoder to  look at the (initial) bytes that arrive and determine
      whether the rest of the bytes  must be of the type associated
      with that particular sequence of bytes.  This means that each
      tag must be uniquely associated with <em>only</em> one  ASN.1
      type.
      </p>
    <p>Immediately following the tag is a sequence of bytes
      informing the  decoder of the length of the instance. This is
      sometimes referred to  as TLV  (Tag length value) encoding.
      Hence, the structure of a BER encoded series of bytes is as shown in the table below.</p>
    <p></p>
    <table>
      <row>
        <cell align="left" valign="middle">Tag</cell>
        <cell align="left" valign="middle">Len</cell>
        <cell align="left" valign="middle">Value</cell>
      </row>
      <tcaption>Structure of a BER encoded series of bytes</tcaption>
    </table>
  </section>

  <section>
    <title>Encoding Rules</title>
    <p>When the first recommendation on ASN.1 was released 1988 it was
      accompanied with the Basic Encoding Rules, BER, as the only 
      alternative for encoding.
      BER is a somewhat verbose protocol. It adopts a so-called TLV (type, 
      length, value) approach to encoding in which every element of the 
      encoding carries some type information, some length information and 
      then the value of that element. Where the element is itself 
      structured, then the Value part of the element is itself a series of 
      embedded TLV components, to whatever depth is necessary. In summary, 
      BER is not a compact encoding but is relatively fast and easy to 
      produce.</p>
    <p>The DER (Distinguished Encoding Rule) encoding format was included in
      the standard in 1994. It is a specialized form of BER, which gives 
      the encoder the option to encode some entities differently. For
      instance, is the value for TRUE any octet with any bit set to one. But,
      DER does not leave any such choices. The value for TRUE in the DER 
      case is encoded as the octet <c>11111111</c>. So, the same value 
      encoded by two different DER encoders must result in the same bit 
      stream.</p>
    <p>A more compact encoding is achieved with the Packed Encoding
    Rules PER which was introduced together with the revised
    recommendation in 1994. PER takes a rather different approach from
    that taken by BER. The first difference is that the tag part in
    the TLV is omitted from the encodings, and any tags in the
    notation are not encoded. The potential ambiguities are resolved
    as follows:</p>
    <list type="bulleted">
      <item>
        <p>A CHOICE is encoded by first encoding a choice index which
          identifies the chosen
          alternative by its position in the notation.</p>
      </item>
      <item>
        <p>The elements of a SEQUENCE are transmitted in textual
        order. OPTIONAL or DEFAULT elements are preceded by a bit map
        to identify which elements are present. After sorting the
        elements of a SET in the "canonical tag order" as defined in
        X.680 8.6 they are treated as a SEQUENCE regarding OPTIONAL
        and DEFAULT elements. A SET is transferred in the sorted
        order.</p>
      </item>
    </list>
    <p>A second difference is that PER takes full account of the sub-typing
      information in that the encoded bytes are affected by the constraints.
      The BER encoded bytes are unaffected by the constraints.
      PER uses the sub-typing information to for example omit length fields 
      whenever possible. </p>
    <p>The run-time functions, sometimes take the constraints into account
      both for BER and PER. For instance are SIZE constrained strings checked.</p>
    <p>There are two variants of PER, <em>aligned</em> and <em>unaligned</em>.
      In summary, PER results in compact encodings which require much more
      computation to produce than  BER.
      </p>
  </section>
</chapter>