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
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Ghostscript C coding guidelines</title>
<!-- $Id: C-style.htm 10732 2010-02-10 18:17:48Z giles $ -->
<!-- Originally: c-style.txt -->
<link rel="stylesheet" type="text/css" href="gs.css" title="Ghostscript Style">
</head>
<body>
<!-- [1.0 begin visible header] ============================================ -->
<!-- [1.1 begin headline] ================================================== -->
<h1>Ghostscript C coding guidelines</h1>
<!-- [1.1 end headline] ==================================================== -->
<!-- [1.2 begin table of contents] ========================================= -->
<h2>Table of contents</h2>
<blockquote><ul>
<li><a href="#Introduction">Introduction</a>
<li><a href="#C_language">C language do's and don'ts</a>
<ul>
<li>Preprocessor:
<a href="#Conditionals">Conditionals</a>,
<a href="#Macros">Macros</a>,
<a href="#Preprocessor_other">Other</a>
<li><a href="#Lexical_elements">Lexical elements</a>
<li><a href="#Scoping">Scoping</a>
<li>Data types:
<a href="#Scalars">Scalars</a>,
<a href="#Arrays">Arrays</a>,
<a href="#Typedefs">Typedefs</a>,
<a href="#Structures">Structures</a>,
<a href="#Unions">Unions</a>
<li><a href="#Expressions">Expressions</a>
<li><a href="#Statements">Statements</a>
<li><a href="#Procedures">Procedures</a> (prototypes and definitions)
<li><a href="#Standard_library">Standard library</a>
</ul>
<li><a href="#Language_extensions">Language extensions</a>
<li><a href="#Stylistic_conventions">Stylistic conventions</a>
<ul>
<li>Formatting:
<a href="#Indentation">Indentation</a>,
<a href="#Spaces">Spaces</a>,
<a href="#Parentheses">Parentheses</a>
<li><a href="#Preprocessor_style">Preprocessor</a>
<li><a href="#Naming">Naming</a>
<li><a href="#Types">Types</a>
<li><a href="#Procedures_style">Procedures</a>,
<li>Miscellany:
<a href="#Local_variables">Local variables</a>,
<a href="#Compiler_warnings">Compiler warnings</a>
</ul>
<li><a href="#File_structuring">File structuring and naming</a>
<ul>
<li><a href="#All_files">All files</a>
<li><a href="#Makefiles">Makefiles</a>
<li><a href="#General_C_code">General C Code</a>
<li><a href="#Headers">Headers (<code>.h</code> files)</a>
<li><a href="#Source">Source (<code>.c</code> files)</a>
</ul>
<li><a href="#Conventions">Ghostscript conventions</a>
<ul>
<li><a href="#Specific_names">Specific names</a>:
<a href="#code"><code>code</code></a>,
<a href="#status"><code>status</code></a>
<li><a href="#Structure_type_descriptors">Structure type descriptors</a>
<li><a href="#Objects">"Objects"</a>
<li><a href="#Error_handling">Error handling</a>
</ul>
</ul></blockquote>
<!-- [1.2 end table of contents] =========================================== -->
<!-- [1.3 begin hint] ====================================================== -->
<p>
For other information, see the <a href="Readme.htm">Ghostscript
overview</a>.
<!-- [1.3 end hint] ======================================================== -->
<hr>
<!-- [1.0 end visible header] ============================================== -->
<!-- [2.0 begin contents] ================================================== -->
<h2><a name="Introduction"></a>Introduction</h2>
<p>
This document describes Ghostscript's C coding conventions. It is primarily
<em>prescriptive</em>, documenting what developers should do when writing
new code; the companion developer documentation (<a
href="Develop.htm">Develop.htm</a>) is primarily <em>descriptive</em>,
documenting the way things are.
<p>
We encourage following the general language usage and stylistic rules for
any code that will be integrated with Ghostscript, even if the code doesn't
use Ghostscript's run-time facilities or have anything to do with
PostScript, PDF, or page description languages. Ghostscript itself follows
some additional conventions; these are documented separately under "<a
href="#Conventions">Ghostscript conventions</a>" below.
<hr>
<h2><a name="C_language"></a>C language do's and don'ts</h2>
<p>
There are several different versions of the C language, and even of the ANSI
C standard. Ghostscript versions through 7.0 were designed to compile on
pre-ANSI compilers as well as on many compilers that implemented the ANSI
standard with varying faithfulness. Ghostscript versions since 7.0 do not
cater for pre-ANSI compilers: they must conform to the ANSI 1989 standard
(ANS X3.159-1989), with certain restrictions and a few conventional
additions.
<h3>Preprocessor</h3>
<h4><a name="Conditionals"></a>Conditionals</h4>
Restrictions:
<ul>
<li>Don't assume that <code>#if</code> will treat undefined names as 0.
While the ANSI standard requires this, it may produce a warning.
<li>In <code>.c</code> files, don't use preprocessor conditionals that
test for individual platforms or compilers. Use them only in header files
named xxx<code>_.h</code>.
</ul>
<h4><a name="Macros"></a>Macros</h4>
<p>
Restrictions:
<ul>
<li>Don't redefine a macro, even with the same definition, without using
<code>#undef</code>.
<li><code>CAPITALIZE</code> macro names unless there is a good reason not
to.
<li>Even though the legacy code contains some macros which contain
control flow statments, avoid the use of this in new code and do not
create macros which contain hidden control flow, especially 'return'.
The use of control flow in macros complicates debug significantly
requiring tedious expansion of macros to build a module to be debugged
or resorting to disassembly windows to set breakpoints or to trace
flow.
<li>Don't use a macro call within a macro argument if the call expands to a
token sequence that includes any commas not within parentheses: this
produces different results depending on whether the compiler expands the
inner call before or after the argument is substituted into the macro body.
(The ANSI standard says that calls must be expanded after substitution, but
some compilers do it the other way.)
<li>Don't use macro names, even inadvertently, in string constants. Some
compilers erroneously try to expand them.
<li>Don't use macros to define shorthands for casted pointers. For
instance, avoid
<blockquote><code>
#define fdev ((gx_device_fubar *)dev)
</code></blockquote>
<p>
and instead use
<blockquote><code>
gx_device_fubar * const fdev = (gx_device_fubar *)dev;
</code></blockquote>
<p>
The use of <code>const</code> alerts the reader that this is effectively
a synonym.
<li>If a macro generates anything larger than a single expression (that is,
one or more statements), surround it with <code>BEGIN</code> and
<code>END</code>. These work around the fact that simple statements and
compound statements in C can't be substituted for each other syntactically.
<li>If a macro introduces local variables, only use names that end with an
underscore (<code>_</code>), such as <code>code_</code>. This avoids
clashes both with ordinary variable names (which should never end with an
underscore) and with system-defined names (which may begin with an
underscore).
</ul>
<h3><a name="Preprocessor_other"></a>Other</h3>
<p>
Restrictions:
<ul>
<li>Only use <code>#pragma</code> in files that are explicitly identified
as being platform-dependent. Many compilers complain if this is used at
all, and some complain if they don't recognize the specific pragma being
requested (both incorrect according to the ANSI standard).
</ul>
<h3><a name="Lexical_elements"></a>Lexical elements</h3>
<p>
Do not use:
<ul>
<li>ANSI trigraphs (??x)
<li>Nested comments (/* /* */ */) (not ANSI compliant, but often accepted)
<li>Multi-character character constants ('abc')
<li>Wide-character character or string constants (L'x', L"x")
</ul>
<p>
Restrictions:
<ul>
<li>Procedure and static variable names must be 31 characters or less.
<li>Externally visible procedure and variable names must be unique in the
first 23 characters.
</ul>
<h3><a name="Scoping"></a>Scoping (extern, static, ...)</h3>
<p>
Do not use:
<ul>
<li><code>register</code>
</ul>
<p>
Restrictions:
<ul>
<li>Do not allow a global variable (constant) to have more than one
non-<code>extern</code> definition, even though some ANSI C compilers
allow this. Every global constant should have exactly one definition, in a
<code>.c</code> file, and preferably just one <code>extern</code>
declaration, in a header file.
<li><code>static</code> or variables must be
<code>const</code> and initialized: non-<code>const</code> statically
allocated variables are incompatible with reentrancy, and we're in the
process of eliminating all of them.
<li>Do not use <code>extern</code> in <code>.c</code> files, only in
<code>.h</code> files, unless you have a very good reason for it (e.g.,
as in <a href="../psi/iconf.c">iconf.c</a>). There are too many such
<code>extern</code>s in the code now: we are eliminating them over time.
<li>Do not declare the same name as both <code>static</code>
and non-<code>static</code> within the same
compilation. (Some compilers complain, some do not.) This is especially a
problem for procedures: it is easy to declare a procedure as
<code>static</code> near the beginning of a file and accidentally not
declare it <code>static</code> where it is defined later in the file.
<li>Even though the ANSI standard allows initialized external declarations
(<code>extern int x = 0</code>), don't use them.
</ul>
<h3><a name="Scalars"></a>Scalars</h3>
<p>
Restrictions:
<ul>
<li>Avoid using <code>char</code>, except for <code>char *</code>
for a pointer to a string. Don't assume that <code>char</code> is
signed; also don't assume it is unsigned.
<li>Never cast a <code>float</code> to a <code>double</code>
explicitly. ANSI compilers in their default mode do all floating point
computations in double precision, and handle such casts automatically.
<li>Don't use <code>long long</code>: even though it is in the ANSI
standard, not all compilers support it. Use the
<code>(u)int64_t</code> types from <code>stdint_.h</code> instead.
<li>Don't assume anything about whether <code>sizeof(long)</code> is less
than, equal to, or greater than <code>sizeof(ptr)</code>. (However, you
can make such comparisons in preprocessor conditionals using
<code>ARCH_SIZEOF_LONG</code> and <code>ARCH_SIZEOF_PTR</code>.)
</ul>
<h3><a name="Arrays"></a>Arrays</h3>
<p>
Restrictions:
<ul>
<li>Don't declare arrays of size 0. (The newer ANSI standard allows this,
but the older one doesn't.)
<li>Don't declare an array of size 1 at the end of a structure to indicate
that a variable-size array follows.
<li>Don't declare initialized <code>auto</code> arrays.
</ul>
<h3><a name="Typedefs"></a>Typedefs</h3>
<p>
Restrictions:
<ul>
<li>Don't use <code>typedef</code> for function types, such as
<blockquote>
<code>typedef int proc_xyz_t(double, int *);</code>
</blockquote>
<p>Many compilers don't handle this correctly -- they will give errors, or
do the wrong thing, when declaring variables of type
<code>proc_xyz_t</code> and/or <code>proc_xyz_t *</code>. Instead, do
this:
<blockquote>
<code>#define PROC_XYZ(proc) int proc(double, int *)<br>
PROC_XYZ(some_proc); /* declare a procedure of this type */<br>
typedef PROC_XYZ((*proc_xyz_ptr_t)); /* define a type for procedure ptrs */<br>
<br>
proc_xyz_ptr_t pp; /* pointer to procedure */</code>
</blockquote>
<li>Don't redefine <code>typedef</code>'ed names, even with the same
definition. Some compilers complain about this, and the standard doesn't
allow it.
</ul>
<h3><a name="Structures"></a>Structures</h3>
<p>
Restrictions:
<ul>
<li>Don't use anonymous structures if you can possibly avoid it, except
occasionally as components of other structures. Ideally, use the
<code>struct</code> keyword only for declaring named structure types,
like this:
<blockquote>
<code>typedef struct xxx_s {</code><br>
... members ...<br>
<code>} xxx_t;</code>
</blockquote>
<li>Use <code>struct</code> only when declaring structure types, never
for referring to them (e.g., never declare a variable as type
<code>struct xxx_s *</code>).
<li>Don't assume that the compiler will (or won't) insert padding in
structures to align members for best performance. To preserve alignment,
only declare structure members that are narrower than an <code>int</code>
if there will be a lot of instances of that structure in memory. For such
structures, insert <code>byte</code> and/or <code>short</code> padding
members as necessary to re-establish <code>int</code> alignment.
<li>Don't declare initialized <code>auto</code> structures.
</ul>
<h3><a name="Unions"></a>Unions</h3>
<p>
Restrictions:
<ul>
<li>Use unions only as components of structures, not as typedefs in their
own right.
<li>Don't attempt to initialize unions: not all compilers support this, even
though it is in the 1989 ANSI standard.
</ul>
<h3><a name="Expressions"></a>Expressions</h3>
<p>
Restrictions:
<ul>
<li>Don't assign a larger integer data type to a smaller one without a cast
(<code>int_x = long_y</code>).
<li>It's OK to use the address of a structure or array element
(<code>&p->e</code>, <code>&a[i]</code>) locally, or pass it to a
procedure that won't store it, but don't store such an address in allocated
storage unless you're very sure of what you're doing.
<li>Don't use conditional expressions with structure or union values.
(Pointers to structures or unions are OK.)
<li>For calling a variable or parameter procedure, use
<code>ptr->func(...)</code>. Some old code uses explicit indirection,
<code>(*ptr->func)(...)</code>: don't use this in new code.
<li>Don't write expressions that depend on order of evaluation, unless the
order is created explicitly by use of <code>||</code>,
<code>&&</code>, <code>?:</code>, <code>,</code>, or
function nesting (the arguments of a function must be evaluated before the
function is called). In particular, don't assume that the arguments of a
function will be evaluated left-to-right, or that the left side of an
assignment will be evaluated before the right.
<li>Don't mix integer and enumerated types ad lib: treat enumerated types as
distinct from integer types, and use casts to convert between the two.
(Some compilers generate warnings if you do not do this.)
</ul>
<h3><a name="Statements"></a>Statements</h3>
<p>
Restrictions:
<ul>
<li>If you use an expression as a statement, other than an assignment or a
function call with <code>void</code> return value, enclose it explicitly
in <code>DISCARD()</code>.
<li>The type of the operand of a <code>switch</code> must match the type
of the case labels, whether the labels are <code>int</code>s or the
members of an <code>enum</code> type. (Use a cast if necessary.)
<li>It is OK for one case of a switch to "fall through" into another (i.e.,
for the statement just before a case label not to be a control transfer),
but a comment <code>/* falls through */</code> is
required.
<li>If you are returning an error code specified explicitly (e.g.,
<code>return gs_error_rangecheck</code> or
<code>return e_rangecheck</code>), use
<code>return_error()</code> rather than plain <code>return</code>.
However, if the program is simply propagating an error code generated
elsewhere, as opposed to generating the error, use <code>return</code>
(e.g., <code>if (code < 0) return code</code>).
</ul>
<h3><a name="Procedures"></a>Procedures</h3>
<p>
Restrictions:
<ul>
<li>Provide a prototype for every procedure, and make sure the prototype is
available at every call site. If the procedure is local to a file
(<code>static</code>), the prototype should precede the procedure, in
the same file; if the procedure is global, the prototype should be in a
header file.
<li>If a procedure parameter is itself a procedure, do list its parameter
types rather than just using <code>()</code>. For example,
<blockquote><code>
int foo(int (*callback)(int, int));
</code></blockquote>
<p>
rather than just
<blockquote><code>
int foo(int (*callback)());
</code></blockquote>
<li>Don't use the <code>P</code>* macros in new code. (See the
Procedures section of <a href="#Language_extensions">Language extensions</a>
below for more information.)
<li>Always provide an explicit return type for procedures, in both the
prototype and the definition: don't rely on the implicit declaration as
<code>int</code>.
<li>Don't use <code>float</code> as the return type of a procedure,
unless there's a special reason. Floating point hardware typically does
everything in double precision internally and has to do extra work to
convert between double and single precision.
<li>Don't declare parameters as being of type <code>float</code>,
<code>short</code>, or <code>char</code>. If you do this and forget
to include the prototype at a call site, ANSI compilers will generate
incompatible calling sequences. Use <code>floatp</code> (a synonym for
<code>double</code>, mnemonic for "float parameter") instead of
<code>float</code>, and use <code>int</code> or <code>uint</code>
instead of <code>short</code> or <code>char</code>.
</ul>
<h3><a name="Standard_library"></a>Standard library</h3>
<p>
Restrictions:
<ul>
<li>Only use library features that are documented in the established ANSI
standard (e.g., Harbison & Steele's book). Do not use procedures that are
"standards" promulgated by Microsoft (e.g., <code>stricmp</code>), or
originate in BSD Unix (e.g., <code>strcasecmp</code>), or were added in
later versions of the standard such as C 9X.
<li>Do not use any features from <code>stdio.h</code> that assume the
existence of <code>stdin</code>, <code>stdout</code>, or
<code>stderr</code>. See <a href="../base/gsio.h">gsio.h</a> for the full
list. Instead, use <code>gs_stdin</code> et al.
</ul>
<hr>
<h2><a name="Language_extensions"></a>Language extensions</h2>
<h3>Scoping</h3>
<dl>
<dt><code>static</code>
<dd>Ghostscript assumes the compiler supports the <code>static</code>
keyword for declaring variables and procedures as local to a particular
source file.
<dt><code>inline</code>
<dd><code>inline</code> is available even if the compiler does not
support it. Be aware, however, that it may have no effect. In particular,
do not use <code>inline</code> in header files. Instead, use the
<code>extern_inline</code> facility described just below.
<dt><code>extern_inline</code>
<dd>Compilers that do support <code>inline</code> vary in how they decide
whether to (also) compile a closed-code copy of the procedure. Because of
this, putting an <code>inline</code> procedure in a header file may
produce multiple closed copies, causing duplicate name errors at link time.
<code>extern_inline</code> provides a safe way to put
<code>inline</code> procedures in header files, regardless of compiler.
Unfortunately, the only way we've found to make this fully portable involves
a fair amount of boilerplate. For details, please see <a
href="../base/stdpre.h">stdpre.h</a>.
</dl>
<h3>Scalar types</h3>
<dl>
<dt><code>bool, true, false</code>
<dd><code>bool</code> is intended as a Boolean type, with canonical
values <code>true</code> and <code>false</code>. In a more reasonable
language, such as Java, <code>bool</code> is an enumerated type requiring
an explicit cast to or from <code>int</code>; however, because C's
conditionals are defined as producing <code>int</code> values, we can't
even define <code>bool</code> as a C <code>enum</code> without
provoking compiler warnings.
<p>
Even though <code>bool</code> is a synonym for <code>int</code>, treat
them as conceptually different types:
<ul>
<li>Initialize or set <code>bool</code> variables to <code>true</code>
or <code>false</code>, not 0 or 1.
<li>Use the Boolean operators <code>!</code>, <code>&&</code>,
and <code>||</code> only with Booleans. Don't use the idiom
<code>!!x</code> to create a Boolean that is true iff <code>x</code>
!= 0: use <code>x != 0</code>.
<li>Use an explicit <code>(int)</code> cast to convert a Boolean to an
integer.
</ul>
<dt><code>byte, ushort, uint, ulong</code>
<dd>These types are simply shorthands for <code>unsigned char, short, int,
long</code>.
<p>
In addition, the use of <code>byte *</code> indicates a
Ghostscript-style string, with explicit length given separately, as
opposed to a null terminated C-style string, which is <code>char
*</code>.
<dt><code>floatp</code>
<dd>This is a synonym for <code>double</code>. It should be used for,
and only for, procedure parameters that would otherwise be
<code>float</code>. (As noted above, procedure parameters should not be
declared as <code>float</code>.)
<dt><code>bits8, bits16, bits32</code>
<dd>These are unsigned integer types of the given width. Use them wherever
the actual width matters: do <em>not</em>, for example, use
<code>short</code> assuming that it is 16 bits wide. New code can use
the C99 fixed-width types from <code>stdint_.h</code>.
</dl>
<hr>
<h2><a name="Stylistic_conventions"></a>Stylistic conventions</h2>
<p>
Ghostscript's coding rules cover not only the use of the language, but also
many stylistic issues like the choice of names and the use of whitespace.
The stylistic rules are meant to produce code that is easy to read. It's
important to observe them as much as possible in order to maintain a
consistent style, but if you find these rules getting in your way or
producing ugly-looking results once in a while, it's OK to break it.
<h3><a name="Formatting"></a>Formatting</h3>
<h4><a name="Indentation"></a>Indentation</h4>
<p>
We've formatted all of our code using the GNU <code>indent</code> program.
<blockquote><code>
indent -bad -nbap -nsob -br -ce -cli4 -npcs -ncs \<br>
-i4 -di0 -psl -lp -lps somefile.c
</code></blockquote>
<p>
does a 98% accurate job of producing our preferred style. Unfortunately,
there are bugs in all versions of GNU <code>indent</code>, requiring
both pre- and post-processing of the code.
<p>
Put indentation points every 4 spaces, with 8 spaces = 1 tab stop.
<p>
Don't indent the initial <code>#</code> of preprocessor commands.
However, for nested preprocessor commands, do use indentation between the
<code>#</code> and the command itself. Use 2 spaces per level of
nesting, e.g.:
<blockquote>
<code>#ifndef xyz</code><br>
<code># define xyz 0</code><br>
<code>#endif</code>
</blockquote>
<p>
For assignments (including chain assignments), put the entire statement on
one line if it will fit; if not, break it after a <code>=</code> and
indent all the following lines. I.e., format like this:
<blockquote>
var1 <code>=</code> value<code>;</code><br>
var1 <code>=</code> var2 <code>=</code> value<code>;</code><br>
var1 <code>=</code><br>
value<code>;</code><br>
var1 <code>=</code><br>
var2 <code>=</code> value<code>;</code><br>
var1 <code>=</code> var2 <code>=</code><br>
value<code>;</code>
</blockquote>
<p>
But not like this:
<blockquote>
var1 <code>=</code><br>
var2 <code>=</code> value<code>;</code>
</blockquote>
<p>
Indent in-line blocks thus:
<blockquote>
<code>{</code><br>
... declarations ...<br>
{{ blank line if any declarations above }}<br>
... statements ...<br>
<code>}</code>
</blockquote>
<p>
Similarly, indent procedures thus:
<blockquote>
return_type<br>
proc_name(... arguments ...)<br>
<code>{</code><br>
... declarations ...<br>
{{ blank line if any declarations above }}<br>
... statements ...<br>
<code>}</code>
</blockquote>
<p>
If a control construct (<code>if</code>, <code>do</code>,
<code>while</code>, or <code>for</code>) has a one-line body, use
this:
<blockquote>
... control construct ...<br>
... subordinate simple statement ...
</blockquote>
This is considered particularly important so that a breakpoint can be
set inside the conditional easily.
<p>
If it has a multi-line body, use this:
<blockquote>
... control construct ... <code>{</code><br>
... subordinate code ...<br>
<code>}</code>
</blockquote>
<p>
If the subordinate code has declarations, see blocks above.
<p>
For if-else statements, do this:
<blockquote>
<code>if (</code> ...<code> ) {</code><br>
... subordinate code ...<br>
<code>} else if (</code> ...<code> ) {</code><br>
... subordinate code ...<br>
<code>} else {</code><br>
... subordinate code ...<br>
<code>}</code>
</blockquote>
<p>
When there are more than two alternatives, as in the example above, use the
above ("parallel") syntax rather than the following ("nested") syntax:
<blockquote>
<code>if (</code> ...<code> ) {</code><br>
... subordinate code ...<br>
<code>} else {</code><br>
<code> if (</code> ...<code> ) {</code><br>
... subordinate code ...<br>
<code> } else {</code><br>
... subordinate code ...<br>
<code> }</code><br>
<code>}</code>
</blockquote>
<p>
Similarly, for do-while statements, do this:
<blockquote>
<code>do {</code><br>
... body ...<br>
<code>} while (</code> ... condition ... <code>);</code>
</blockquote>
<h4><a name="Spaces"></a>Spaces</h4>
<p>
Do put a space:
<ul>
<li>after every comma and semicolon, unless it ends a line;
<li>around every binary operator other than "<code>-></code>" and
"<code>.</code>", although you can omit the spaces around the innermost
operator in a nested expression if you like;
<li>on both sides of the parentheses of an <code>if</code>, <code>for</code>, or <code>while</code>.
</ul>
<p>
Don't put a space:
<ul>
<li>at the end of a line;
<li>before a comma or semicolon;
<li>after unary prefix operators;
<li>before the parenthesis of a macro or procedure call.
</ul>
<h4><a name="Parentheses"></a>Parentheses</h4>
<p>
Parentheses are important in only a few places:
<ul>
<li>Around the inner subexpressions in expressions that mix
<code>&&</code> and <code>||</code>, even if they are not
required by precedence, for example
<blockquote><code>
(xx && yy) || zz
</code></blockquote>
<li>Similarly around inner subexpressions in expressions that mix
<code>&</code>, <code>|</code>, or shifts, especially if mixing
these with other operators, for instance
<blockquote><code>
(x << 3) | (y >> 5)
</code></blockquote>
<li>In macro definitions around every use of an argument that logically
could be an expression, for example
<blockquote><code>
((x) * (x) + (y) * (y))
</code></blockquote>
</ul>
<p>
Anywhere else, given the choice, use fewer parentheses.
<p>
For stylistic consistency with the existing Ghostscript code, put
parentheses around conditional expressions even if they aren't
syntactically required, unless you really dislike doing this. Note that
the parentheses should go around the entire expression, not the condition.
For instance, instead of
<blockquote><code>
hpgl_add_point_to_path(pgls, arccoord_x, arccoord_y,<br>
(pgls->g.pen_down) ? gs_lineto : gs_moveto);
</code></blockquote>
<p>
use
<blockquote><code>
hpgl_add_point_to_path(pgls, arccoord_x, arccoord_y,<br>
(pgls->g.pen_down ? gs_lineto : gs_moveto));
</code></blockquote>
<h3><a name="Preprocessor_style"></a>Preprocessor</h3>
<h4>Conditionals</h4>
<p>
Using preprocessor conditionals can easily lead to unreadable code, since
the eye really wants to read linearly rather than having to parse the
conditionals just to figure out what code is relevant. It's OK to use
conditionals that have small scope and that don't change the structure or
logic of the program (typically, they select between different sets of
values depending on some configuration parameter), but where possible, break
up source modules rather than use conditionals that affect the structure or
logic.
<h4>Macros</h4>
<p>
Ghostscript code uses macros heavily to effectively extend the rather
weak abstraction capabilities of the C language, specifically in the area of
memory management and garbage collection: in order to read Ghostscript code
effectively, you simply have to learn some of these macros as though they
were part of the language. The current code also uses macros heavily for
other purposes, but we are trying to phase these out as rapidly as possible,
because they make the code harder to read and debug, and to use the
rules that follow consistently in new code.
<p>
Define macros in the smallest scope you can manage (procedure, file, or
<code>.h</code> file), and <code>#undef</code> them at the end of
that scope: that way, someone reading the code can see the definitions
easily when reading the uses. If that isn't appropriate, define them in as
large a scope as possible, so that they effectively become part of the
language. This places an additional burden on the reader, but it can be
amortized over reading larger amounts of code.
<p>
Try hard to use procedures instead of macros. Use "<code>inline</code>"
if you really think the extra speed is needed, but only within a
<code>.c</code> file: don't put inline procedures in <code>.h</code>
files, because most compilers don't honor "<code>inline</code>" and
you'll wind up with a copy of the procedure in every <code>.c</code>
file that includes the <code>.h</code> file.
<p>
If you define a macro that looks like a procedure, make sure it will work
wherever a procedure will work. In particular, put parentheses around every
use of an argument within the macro body, so that the macro will parse
correctly if some of the arguments are expressions, and put parentheses
around the entire macro body. (This is still subject to the problem that an
argument may be evaluated more than once, but there is no way around this in
C, since C doesn't provide for local variables within expressions.)
<p>
If you define macros for special loop control structures, make their uses
look somewhat like ordinary loops, for instance:
<blockquote>
<code>BEGIN_RECT(xx, yy) {</code><br>
... body indented one position ...<br>
<code>} END_RECT(xx, yy);</code>
</blockquote>
<p>
If at all possible, don't use free variables in macros -- that is, variables
that aren't apparent as arguments of the macro. If you must use free
variables, list them all in a comment at the point where the macro is
defined.
<p>
If you define new macros or groups of macros, especially if they aren't
simply inline procedures or named constant values, put some extra effort
into documenting them, to compensate for the fact that macros are
intrinsically harder to understand than procedures.
<h3><a name="Comments"></a>Comments</h3>
<p>
The most important descriptive comments are ones in header files that
describe structures, including invariants; but every procedure or structure
declaration, or group of other declarations, should have a comment. Don't
spend a lot of time commenting executable code unless something unusual or
subtle is going on.
<h3><a name="Naming"></a>Naming</h3>
<p>
Use fully spelled-out English words in names, rather than contractions.
This is most important for procedure and macro names, global variables and
constants, values of <code>#define</code> and <code>enum</code>,
<code>struct</code> and other <code>typedef</code> names, and
structure member names, and for argument and variable names which have
uninformative types like <code>int</code>. It's not very important for
arguments or local variables of distinctive types, or for local index or
count variables.
<p>
Avoid names that run English words together:
"<code>hpgl_compute_arc_center</code>" is better than
"<code>hpgl_compute_arccenter</code>". However, for terms drawn from
some predefined source, like the names of PostScript operators, use a term
in its well-known form (for instance, <code>gs_setlinewidth</code>
rather than <code>gs_set_line_width</code>).
<p>
Procedures, variables, and structures visible outside a single
<code>.c</code> file should generally have prefixes that indicate what
subsystem they belong to (in the case of Ghostscript, <code>gs_</code>
or <code>gx_</code>). This rule isn't followed very consistently.
<h3><a name="Types"></a>Types</h3>
<p>
Many older structure names don't have <code>_t</code> on the end, but
this suffix should be used in all new code. (The <code>_s</code>
structure name is needed only to satisfy some debuggers. No code other than
the structure declaration should refer to it.)
<p>
Declare structure types that contain pointers to other instances of
themselves like this:
<blockquote>
<code>typedef struct xxx_s xxx_t;</code><br>
<code>struct xxx_s {</code><br>
... members ...<br>
<code>xxx_t *</code>ptr_member_name;<br>
... members ...<br>
<code>};</code>
</blockquote>
<p>
If, to maintain data abstraction and avoid including otherwise unnecessary
header files, you find that you want the type <code>xxx_t</code> to be
available in a header file that doesn't include the definition of the
structure <code>xxx_s</code>, use this approach:
<blockquote>
<code>#ifndef xxx_DEFINED</code><br>
<code># define xxx_DEFINED</code><br>
<code>typedef struct xxx_s xxx_t;</code><br>
<code>#endif</code><br>
<code>struct xxx_s {</code><br>
... members ...<br>
<code>};</code>
</blockquote>
<p>
You can then copy the first 4 lines in other header files. (Don't ever
include them in an executable code file.)
<p>
Don't bother using <code>const</code> for anything other than with
pointers as described below. However, in those places where it is necessary
to cast a pointer of type <code>const T *</code> to type
<code>T *</code>, always include a comment that explains why you are
"breaking const".
<h4>Pointers</h4>
<p>
Use <code>const</code> for pointer referents (that is,
<code>const T *</code>) wherever possible and appropriate.
<p>
If you find yourself wanting to use <code>void *</code>, try to
find an alternative using unions or (in the case of super- and subclasses)
casts, unless you're writing something like a memory manager that really
treats memory as opaque.
<h3><a name="Procedures_style"></a>Procedures</h3>
<p> In general, don't create procedures that are private and only called
from one place. However, if a compound statement (especially an arm of
a conditional) is too long for the eye easily to match its enclosing
braces "<code>{...}</code>" -- that is, longer than 10 or 15 lines --
and it doesn't use or set a lot of state held in outer local variables,
it may be more readable if you put it in a procedure.
<h3>Miscellany</h3>
<h4><a name="Local_variables"></a>Local variables</h4>
<p>
Don't assign new values to procedure parameters. It makes debugging very
confusing when the parameter values printed for a procedure are not the
ones actually supplied by the caller. Instead use a separate local
variable initialized to the value of the parameter.
<p>
If a local variable is only assigned a value once, assign it that value at
its declaration, if possible. For example,
<blockquote>
<code>int x = </code>some expression <code>;</code>
</blockquote>
<p>
rather than
<blockquote>
<code>int x;</code><br>
...<br>
<code>x = </code> some expression <code>;</code>
</blockquote>
<p>
Use a local pointer variable like this to "narrow" pointer types:
<blockquote>
<code>int</code><br>
someproc(... <code>gx_device *dev</code> ...)<br>
<code>{<br>
gx_device_printer *const pdev = (gx_device_printer *)dev;</code><br>
...<br>
<code>}</code>
</blockquote>
<p>
Don't "shadow" a local variable or procedure parameter with an inner local
variable of the same name. I.e., don't do this:
<blockquote>
<code>int</code><br>
someproc(... <code>int x</code> ...)<br>
<code>{</code><br>
...<br>
<code> int x;</code><br>
...<br>
<code>}</code>
</blockquote>
<h4><a name="Compiler_warnings"></a>Compiler warnings</h4>
<p>
We want Ghostscript to compile with no warnings. This is a constant
battle as compilers change and new code is added. Work hard to eliminate
warnings by improving the code structure instead of patching over them.
If the compiler can't figure out that a variable is always initialized,
a future reader will probably have trouble as well.
<hr>
<h2><a name="File_structuring"></a>File structuring</h2>
<h3><a name="All_files"></a>All files</h3>
<p>
Keep file names within the "8.3" format for portability:
<ul>
<li>Use only letters, digits, dash, and underscore in file names.
<li>Don't assume upper and lower case letters are distinct.
<li>Put no more than 8 characters before the ".", if any.
<li>If there is a ".", put between 1 and 3 characters after the ".".
</ul>
<p>
For files other than documentation files, use only lower case letters
in the names; for HTML documentation files, capitalize the first letter.
<p>
Every code file should start with comments containing
<ol>
<li>a copyright notice,
<li>the name of the file in the form of an RCS Id:
<blockquote><code>
/* $<!-- -->Id: filename.ext $*/
</code></blockquote>
<p>
(using the comment convention appropriate to the language of the file), and
<li>a summary, no more than one line, of what the file contains.
</ol>
<p>
If you create a file by copying the beginning of another file, be sure to
update the copyright year and change the file name.
<h3><a name="Makefiles"></a>Makefiles</h3>
<p>
Use the extension <code>.mak</code> for makefiles.
<p>
For each
<blockquote><code>
#include "xxx.h"
</code></blockquote>
<p>
make sure there is a dependency on <code>$(xxx_h)</code> in the
makefile. If xxx ends with a "<code>_</code>", this rule still holds,
so that if you code
<blockquote><code>
#include "math_.h"
</code></blockquote>
<p>
the makefile must contain a dependency on "<code>$(math__h)</code>"
(note the two underscores "<code>__</code>").
<p>
List the dependencies bottom-to-top, like the <code>#include</code>
statements themselves; within each level, list them alphabetically. Do
this also with <code>#include</code> statements themselves whenever
possible (but sometimes there are inter-header dependencies that require
bending this rule).
<p>
For compatibility with the build utilities on OpenVMS, always put a space
before the colon that separates the target(s) of a rule from the dependents.
<h3><a name="General_C_code"></a>General C code</h3>
<p>
List <code>#include</code> statements from "bottom" to "top", that is,
in the following order:
<blockquote><ol>
<li>System includes (<code>"xxx_.h"</code>)
<li><code>gs*.h</code>
<li><code>gx*.h</code> (yes, <code>gs</code> and <code>gx</code>
are in the wrong order.)
<li><code>s*.h</code>
<li><code>i*.h</code> (or other interpreter headers that don't start
with "<code>i</code>")
</ol></blockquote>
<h3><a name="Headers"></a>Headers (<code>.h</code> files)</h3>
<p>
In header files, always use the following at the beginning of a header file
to prevent double inclusion:
<blockquote>
{{ Copyright notice etc. }}<br><br>
<code>#ifndef </code><filename><code>_INCLUDED</code><br>
<code>#define </code><filename><code>_INCLUDED</code><br><br>
{{ The contents of the file }}<br><br>
<code>#endif /* </code><filename><code>_INCLUDED */</code>
</blockquote>
<p>
The header file is the first place that a reader goes for
information about procedures, structures, constants, etc., so ensure that
every procedure and structure has a comment that says what it does. Divide
procedures into meaningful groups set off by some distinguished form of
comment.
<h3><a name="Source"></a>Source (<code>.c</code> files)</h3>
<p>
After the initial comments, arrange C files in the following order:
<blockquote><ol>
<li><code>#include</code> statements
<li>Exported data declarations
<li>Explicit externs (if necessary)
<li>Forward declarations of procedures
<li>Private data declarations
<li>Exported procedures
<li>Private procedures
</ol></blockquote>
<p>
Be flexible about the order of the declarations if necessary to improve
readability. Many older files don't follow this order, often without good
reason.
<hr>
<h2><a name="Conventions"></a>Ghostscript conventions</h2>
<h3><a name="Specific_names"></a>Specific names</h3>
<p>
The Ghostscript code uses certain names consistently for certain kinds of
values. Some of the commonest and least obvious are these two:
<h4><a name="code"></a><code>code</code></h4>
<blockquote>
A value to be returned from a procedure:
<table cellpadding=0 cellspacing=0>
<tr valign=top> <td align=right>< 0
<td>
<td>An error code defined in
<a href="../base/gserrors.h">gserrors.h</a>
(or <a href="../psi/ierrors.h">ierrors.h</a>)
<tr valign=top> <td align=right>0
<td>
<td>Normal return
<tr valign=top> <td align=right>> 0
<td>
<td>A non-standard but successful return (which must be documented, preferably with the procedure's prototype)
</table>
</blockquote>
<h4><a name="status"></a><code>status</code></h4>
<blockquote>
A value returned from a stream procedure:
<table cellpadding=0 cellspacing=0>
<tr valign=top> <td align=right>< 0
<td>
<td>An exceptional condition as defined in
<a href="../base/scommon.h">scommon.h</a>
<tr valign=top> <td align=right>0
<td>
<td>Normal return (or, from the "<code>process</code>" procedure, means that more input is needed)
<tr valign=top> <td align=right>1
<td>
<td>More output space is needed (from the "<code>process</code>" procedure)
</table>
</blockquote>
<h3><a name="Structure_type_descriptors"></a>Structure type descriptors</h3>
<p>
The Ghostscript memory manager requires run-time type information for every
structure. (We don't document this in detail here: see the <a
href="Develop.htm#Structure_descriptors">Structure descriptors</a> section
of the developer documentation for details.) Putting the descriptor for a
structure next to the structure definition will help keep the two
consistent, so immediately after the definition of a structure
<code>xxx_s</code>, define its structure descriptor:
<blockquote>
<code>struct xxx_s {</code><br>
... members ...<br>
<code>};</code><br>
<code>#define private_st_xxx() /* in </code><filename><code>.c */\</code><br>
<code> gs_private_st_</code><whatever><code>(st_xxx, xxx_t,\</code><br>
<code> "xxx_t", xxx_enum_ptrs, xxx_reloc_ptrs,\</code><br>
<code> </code>... additional parameters as needed ...<code>)</code>
</blockquote>
<p>
The file that implements operations on this structure
(<filename><code>.c</code>) should then include, near the
beginning, the line:
<blockquote>
<code>private_st_xxx();</code>
</blockquote>
<p>
In much existing code, structure descriptors are declared as
<code>public</code>, which allows clients to allocate instances of the
structure directly. We now consider this bad design. Instead, structure
descriptors should always be <code>static</code>; the implementation
file should provide one or more procedures for allocating instances, e.g.,
<blockquote>
<code>xxx_t *gs_xxx_alloc(P1(gs_memory_t *mem));</code>
</blockquote>
<p>
If it is necessary to make a structure descriptor public, it should be
declared in its clients as
<blockquote>
<code>extern_st(st_xxx);</code>
</blockquote>
<h3><a name="Objects"></a>"Objects"</h3>
<p>
Ghostscript makes heavy use of object-oriented constructs, including
analogues of classes, instances, subclassing, and class-associated
procedures. However, these constructs are implemented in C rather than C++,
for two reasons:
<ul>
<li>The first Ghostscript code was written in 1986, long before C++ was
codified or was well supported by tools. Even today, C++ tools rarely
support C++ as well as C tools support C.
<li>C++ imposes its own implementations for virtual procedures, inheritance,
run-time type information, and (to some extent) memory management.
Ghostscript requires use of its own memory manager, and also sometimes
requires the ability to change the virtual procedures of an object
dynamically.
</ul>
<h4>Classes</h4>
<p>
The source code representation of a class is simply a
<code>typedef</code> for a C <code>struct</code>. See <a
href="C-style.htm#Structures">Structures</a>, above, for details.
<h4>Procedures</h4>
<p>
Ghostscript has no special construct for non-virtual procedures associated
with a class. In some cases, the <code>typedef</code> for the class is
in a header file but the <code>struct</code> declaration is in the
implementation code file: this provides an extra level of opaqueness, since
clients then can't see the representation and must make all accesses through
procedures. You should use this approach in new code, if it doesn't
increase the size of the code too much or require procedure calls for very
heavily used accesses.
<p>
Ghostscript uses three different approaches for storing and accessing
virtual procedures, plus a fourth one that is recommended but not currently
used. For exposition, we assume the class (type) is named
<code>xxx_t</code>, it has a virtual procedure
<code>void (*virtu)(P1(xxx_t *))</code>, and we have a variable
declared as <code>xxx_t *pxx</code>.
<ol>
<li>The procedures are stored in a separate, constant structure of type
<code>xxx_procs</code>, of which <code>virtu</code> is a member. The
structure definition of <code>xxx_t</code> includes a member defined as
<code>const xxx_procs *procs</code> (always named
<code>procs</code>). The construct for calling the virtual procedure is
<code>pxx->procs->virtu(pxx)</code>.
<li>The procedures are defined in a structure of type
<code>xxx_procs</code> as above. The structure definition of
<code>xxx_t</code> includes a member defined as
<code>xxx_procs procs</code> (always named <code>procs</code>).
The construct for calling the virtual procedure is
<code>pxx->procs.virtu(pxx)</code>.
<li>The procedures are not defined in a separate structure: each procedure
is a separate member of <code>xxx_t</code>. The construct for calling
the virtual procedure is <code>pxx->virtu(pxx)</code>.
<li>The procedures are defined in a structure of type
<code>xxx_procs</code> as above. The structure definition of
<code>xxx_t</code> includes a member defined as
<code>xxx_procs procs[1]</code> (always named
<code>procs</code>). The construct for calling the virtual procedure is
again <code>pxx->procs->virtu(pxx)</code>.
</ol>
<p>
Note that in approach 1, the procedures are in a shared constant structure;
in approaches 2 - 4, they are in a per-instance structure that can be
changed dynamically, which is sometimes important.
<p>
In the present Ghostscript code, approach 1 is most common, followed by 2
and 3; 4 is not used at all. For new code, you should use 1 or 4: that way,
all virtual procedure calls have the same form, regardless of whether the
procedures are shared and constant or per-instance and mutable.
<h4>Subclassing</h4>
<p>
Ghostscript's class mechanism allows for subclasses that can add data
members, or can add procedure members if approach 1 or 3 (above) is used.
Since C doesn't support subclassing, we use a convention to accomplish it.
In the example below, <code>gx_device</code> is the root class; it has a
subclass <code>gx_device_forward</code>, which in turn has a subclass
<code>gx_device_null</code>. First we define a macro for all the members
of the root class, and the root class type. (As for structures in general,
classes need a structure descriptor, as discussed in <a
href="#Structures">Structures</a> above: we include these in the examples
below.)
<blockquote><code>
#define gx_device_common\<br>
type1 member1;\<br>
</code>...<code><br>
typeN memberN<br>
<br>
typedef struct gx_device_s {<br>
gx_device_common;<br>
} gx_device;<br>
<br>
#define private_st_gx_device() /* in gsdevice.c */\<br>
gs_private_st_</code><whatever><code>(st_gx_device, gx_device,\<br>
"gx_device", device_enum_ptrs, device_reloc_ptrs,\<br>
</code>... additional parameters as needed ...<code>)
</code></blockquote>
<p>
We then define a similar macro and type for the subclass.
<blockquote><code>
#define gx_device_forward_common\<br>
gx_device_common;\<br>
gx_device *target<br>
<br>
typedef struct gx_device_forward_s {<br>
gx_device_forward_common;<br>
} gx_device_forward;<br>
<br>
#define private_st_device_forward() /* in gsdevice.c */\<br>
gs_private_st_suffix_add1(st_device_forward, gx_device_forward,\<br>
"gx_device_forward", device_forward_enum_ptrs, device_forward_reloc_ptrs,\<br>
gx_device, target)
</code></blockquote>
<p>
Finally, we define a leaf class, which doesn't need a macro because we don't
currently subclass it. (We can create the macro later if needed, with no
changes anywhere else.) In this particular case, the leaf class has no
additional data members, but it could have some.
<blockquote><code>
typedef struct gx_device_null_s {<br>
gx_device_forward_common;<br>
};<br>
<br>
#define private_st_device_null() /* in gsdevice.c */\<br>
gs_private_st_suffix_add0_local(st_device_null, gx_device_null,\<br>
"gx_device_null", device_null_enum_ptrs, device_null_reloc_ptrs,\<br>
gx_device_forward)
</code></blockquote>
<p>
Note that the above example is <strong>not</strong> the actual definition of
the <code>gx_device</code> structure type: the actual type has some
additional complications because it has a finalization procedure. See <a
href="../base/gxdevcli.h">base/gxdevcli.h</a> for the details.
<p>
If you add members to a root class (such as <code>gx_device</code> in
this example), or change existing members, do this in the
<code>gx_device_common</code> macro, not the <code>gx_device</code>
structure definition. Similarly, to change the
<code>gx_device_forward</code> class, modify the
<code>gx_device_forward_common</code> macro, not the structure
definition. Only change the structure definition if the class is a leaf
class (one with no <code>_common</code> macro and no possibility of
subclassing), like <code>gx_device_null</code>.
<h3><a name="Error_handling"></a>Error handling</h3>
<p>
Every caller should check for error returns and, in general, propagate them
to <b>its</b> callers. By convention, nearly every procedure returns an
<code>int</code> to indicate the outcome of the call:
<blockquote><table cellpadding=0 cellspacing=0>
<tr valign=top> <td align=right>< 0
<td>
<td>Error return
<tr valign=top> <td align=right>0
<td>
<td>Normal return
<tr valign=top> <td align=right>> 0
<td>
<td>Non-error return other than the normal case
</table></blockquote>
<p>
To make a procedure generate an error and return it, as opposed to
propagating an error generated by a lower procedure, you should use
<blockquote>
<code>return_error(</code><em>error_number</em><code>);</code>
</blockquote>
<p>
Sometimes it is more convenient to generate the error in one place and
return it in another. In this case, you should use
<blockquote>
<code>code = gs_note_error(</code><em>error_number</em><code>);</code><br>
...<br>
<code>return code;</code>
</blockquote>
<p>
In executables built for debugging, the <code>-E</code> (or
<code>-Z#</code>) command line switch causes <code>return_error</code>
and <code>gs_note_error</code> to print the error number and the source
file and line: this is often helpful for identifying the original cause of
an error.
<p>
See the file <a href="../base/gserrors.h">base/gserrors.h</a> for the error
return codes used by the graphics library, most of which correspond directly
to PostScript error conditions.
<!-- [2.0 end contents] ==================================================== -->
<!-- [3.0 begin visible trailer] =========================================== -->
<hr>
<p>
<small>Copyright © 2000-2006 Artifex Software, Inc. All rights reserved.</small>
<p>
This software is provided AS-IS with no warranty, either express or
implied.
This software is distributed under license and may not be copied, modified
or distributed except as expressly authorized under the terms of that
license. Refer to licensing information at http://www.artifex.com/
or contact Artifex Software, Inc., 7 Mt. Lassen Drive - Suite A-134,
San Rafael, CA 94903, U.S.A., +1(415)492-9861, for further information.
<p>
<small>Ghostscript version 8.71, 10 February 2010
<!-- [3.0 end visible trailer] ============================================= -->
</small></body>
</html>
|
