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
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>File Modules: Gwyddion Module Library Reference Manual</title>
<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
<link rel="home" href="index.html" title="Gwyddion Module Library Reference Manual">
<link rel="up" href="ModuleTutorial.html" title="Gwyddion Module Tutorial">
<link rel="prev" href="gwymodule-tutorial-process.html" title="Data Processing Modules">
<link rel="next" href="gwymodule-tutorial-graph.html" title="Graph Modules">
<meta name="generator" content="GTK-Doc V1.19 (XML mode)">
<link rel="stylesheet" href="style.css" type="text/css">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table class="navigation" id="top" width="100%" summary="Navigation header" cellpadding="2" cellspacing="5"><tr valign="middle">
<td width="100%" align="left" class="shortcuts"></td>
<td><a accesskey="h" href="index.html"><img src="home.png" width="16" height="16" border="0" alt="Home"></a></td>
<td><a accesskey="u" href="ModuleTutorial.html"><img src="up.png" width="16" height="16" border="0" alt="Up"></a></td>
<td><a accesskey="p" href="gwymodule-tutorial-process.html"><img src="left.png" width="16" height="16" border="0" alt="Prev"></a></td>
<td><a accesskey="n" href="gwymodule-tutorial-graph.html"><img src="right.png" width="16" height="16" border="0" alt="Next"></a></td>
</tr></table>
<div class="refentry">
<a name="gwymodule-tutorial-file"></a><div class="titlepage"></div>
<div class="refnamediv"><table width="100%"><tr>
<td valign="top">
<h2><span class="refentrytitle">File Modules</span></h2>
<p>File Modules —
More about file modules
</p>
</td>
<td class="gallery_image" valign="top" align="right"></td>
</tr></table></div>
<div class="refsect1">
<a name="id-1.3.6.3"></a><h2>Overview</h2>
<p>
File modules implement loading and saving of SPM files. The structure
and operation of file modules will be explained on a simple but complete
sample file module. This module imports a sample data format that was
construed for this tutorial and does not occur in practice. However,
the format resembles many (binary) data formats used in practice,
except it contains no device and manufacturer information as they
are not essential for the example.
</p>
<p>
The sample file format our module will load looks as follows, all
values are stored in little-endian:
</p>
<div class="informaltable"><table class="informaltable" border="0">
<colgroup>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>Position</th>
<th>Type</th>
<th>Name</th>
<th>Description</th>
</tr></thead>
<tbody>
<tr>
<td>0x00</td>
<td>
<span class="type">char</span>[4]</td>
<td><code class="varname">magic</code></td>
<td>Magic header <code class="literal">"SMPL"</code>
</td>
</tr>
<tr>
<td>0x04</td>
<td><span class="type">uint16</span></td>
<td><code class="varname">xres</code></td>
<td>Pixels per line</td>
</tr>
<tr>
<td>0x06</td>
<td><span class="type">uint16</span></td>
<td><code class="varname">yres</code></td>
<td>Number of lines</td>
</tr>
<tr>
<td>0x08</td>
<td><span class="type">float</span></td>
<td><code class="varname">measure</code></td>
<td>Size of one pixel [nm]</td>
</tr>
<tr>
<td>0x0c</td>
<td><span class="type">float</span></td>
<td><code class="varname">z0</code></td>
<td>Value corresponding to 0 in raw data [nm]</td>
</tr>
<tr>
<td>0x10</td>
<td><span class="type">float</span></td>
<td><code class="varname">gain</code></td>
<td>
Conversion factor from raw data to physical values [nm]
</td>
</tr>
<tr>
<td>0x14</td>
<td>
<span class="type">uint16</span>[<code class="varname">xres</code>*<code class="varname">yres</code>]</td>
<td><code class="varname">data</code></td>
<td>Raw data values</td>
</tr>
</tbody>
</table></div>
<p>
Furthermore we will assume files of this type get the extension
<code class="filename">.ssd</code> (Simple SPM Data). The complete module source
code follows, it will be described in detail below.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>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</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="preproc">#include</span><span class="normal"> </span><span class="string"><string.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><stdio.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><glib/gstdio.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><libgwyddion/gwymacros.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><libgwyddion/gwymath.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><libprocess/stats.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><libgwymodule/gwymodule-file.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string"><app/gwymoduleutils-file.h></span>
<span class="preproc">#include</span><span class="normal"> </span><span class="string">"err.h"</span>
<span class="preproc">#define</span><span class="normal"> EXTENSION </span><span class="string">".ssd"</span>
<span class="preproc">#define</span><span class="normal"> MAGIC </span><span class="string">"SMPL"</span>
<span class="preproc">#define</span><span class="normal"> </span><span class="function">MAGIC_SIZE</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">sizeof</span><span class="symbol">(</span><span class="normal">MAGIC</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">-</span><span class="normal"> </span><span class="number">1</span><span class="symbol">)</span>
<span class="keyword">enum</span><span class="normal"> </span><span class="cbracket">{</span><span class="normal"> HEADER_SIZE </span><span class="symbol">=</span><span class="normal"> MAGIC_SIZE </span><span class="symbol">+</span><span class="normal"> </span><span class="number">2</span><span class="symbol">*</span><span class="number">2</span><span class="normal"> </span><span class="symbol">+</span><span class="normal"> </span><span class="number">3</span><span class="symbol">*</span><span class="number">4</span><span class="normal"> </span><span class="cbracket">}</span><span class="symbol">;</span>
<span class="keyword">typedef</span><span class="normal"> </span><span class="keyword">struct</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="usertype">guint</span><span class="normal"> xres</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">guint</span><span class="normal"> yres</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> measure</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> z0</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> gain</span><span class="symbol">;</span>
<span class="cbracket">}</span><span class="normal"> SimpleFile</span><span class="symbol">;</span>
<span class="keyword">static</span><span class="normal"> </span><span class="usertype">gboolean</span><span class="normal"> </span><span class="function">module_register</span><span class="symbol">(</span><span class="type">void</span><span class="symbol">);</span>
<span class="keyword">static</span><span class="normal"> </span><span class="usertype">gint</span><span class="normal"> </span><span class="function">simple_detect</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">GwyFileDetectInfo</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">fileinfo</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">gboolean</span><span class="normal"> only_name</span><span class="symbol">);</span>
<span class="keyword">static</span><span class="normal"> <a href="../GwyContainer.html#GwyContainer-struct">GwyContainer</a></span><span class="symbol">*</span><span class="normal"> </span><span class="function">simple_load</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">gchar</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">filename</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">GwyRunType</span><span class="normal"> mode</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">GError</span><span class="normal"> </span><span class="symbol">**</span><span class="normal">error</span><span class="symbol">);</span>
<span class="keyword">static</span><span class="normal"> </span><span class="usertype">GwyModuleInfo</span><span class="normal"> module_info </span><span class="symbol">=</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> <a href="libgwymodule-gwymoduleloader.html#GWY-MODULE-ABI-VERSION:CAPS">GWY_MODULE_ABI_VERSION</a></span><span class="symbol">,</span>
<span class="normal"> </span><span class="symbol">&</span><span class="normal">module_register</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion-gwymacros.html#N-:CAPS">N_</a></span><span class="symbol">(</span><span class="string">"Imports simple data files."</span><span class="symbol">),</span>
<span class="normal"> </span><span class="string">"J. Random Hacker <hacker.jr@example.org>"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="string">"1.0"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="string">"Bit Rot Inc."</span><span class="symbol">,</span>
<span class="normal"> </span><span class="string">"2006"</span><span class="symbol">,</span>
<span class="cbracket">}</span><span class="symbol">;</span>
<span class="function"><a href="libgwymodule-gwymoduleloader.html#GWY-MODULE-QUERY:CAPS">GWY_MODULE_QUERY</a></span><span class="symbol">(</span><span class="normal">module_info</span><span class="symbol">)</span>
<span class="keyword">static</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Basic-Types.html#gboolean">gboolean</a></span>
<span class="function">module_register</span><span class="symbol">(</span><span class="type">void</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="function"><a href="libgwymodule-gwymodule-file.html#gwy-file-func-register">gwy_file_func_register</a></span><span class="symbol">(</span><span class="string">"simple"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion-gwymacros.html#N-:CAPS">N_</a></span><span class="symbol">(</span><span class="string">"Simple AFM files (.afm)"</span><span class="symbol">),</span>
<span class="normal"> </span><span class="symbol">(</span><span class="normal"><a href="libgwymodule-gwymodule-file.html#GwyFileDetectFunc">GwyFileDetectFunc</a></span><span class="symbol">)&</span><span class="normal">simple_detect</span><span class="symbol">,</span>
<span class="normal"> </span><span class="symbol">(</span><span class="normal"><a href="libgwymodule-gwymodule-file.html#GwyFileLoadFunc">GwyFileLoadFunc</a></span><span class="symbol">)&</span><span class="normal">simple_load</span><span class="symbol">,</span>
<span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">,</span>
<span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#TRUE:CAPS">TRUE</a></span><span class="symbol">;</span>
<span class="cbracket">}</span>
<span class="keyword">static</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Basic-Types.html#gint">gint</a></span>
<span class="function">simple_detect</span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">GwyFileDetectInfo</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">fileinfo</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">gboolean</span><span class="normal"> only_name</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">only_name</span><span class="symbol">)</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="function"><a href="http://developer.gnome.org/doc/API/2.0/glib/glib-String-Utility-Functions.html#g-str-has-suffix">g_str_has_suffix</a></span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">name_lowercase</span><span class="symbol">,</span><span class="normal"> EXTENSION</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">?</span><span class="normal"> </span><span class="number">20</span><span class="normal"> </span><span class="symbol">:</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">buffer_len </span><span class="symbol">></span><span class="normal"> MAGIC_SIZE</span>
<span class="normal"> </span><span class="symbol">&&</span><span class="normal"> </span><span class="function">memcmp</span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">head</span><span class="symbol">,</span><span class="normal"> MAGIC</span><span class="symbol">,</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">==</span><span class="normal"> </span><span class="number">0</span><span class="symbol">)</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="number">100</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span>
<span class="cbracket">}</span>
<span class="keyword">static</span><span class="normal"> <a href="../GwyContainer.html#GwyContainer-struct">GwyContainer</a></span><span class="symbol">*</span>
<span class="function">simple_load</span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">gchar</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">filename</span><span class="symbol">,</span>
<span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Miscellaneous-Macros.html#G-GNUC-UNUSED:CAPS">G_GNUC_UNUSED</a> </span><span class="usertype">GwyRunType</span><span class="normal"> mode</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">GError</span><span class="normal"> </span><span class="symbol">**</span><span class="normal">error</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="usertype">SimpleFile</span><span class="normal"> simple</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">GwyContainer</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">container </span><span class="symbol">=</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">GwySIUnit</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">unit</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">GwyDataField</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">dfield</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">guchar</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">buffer</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">guchar</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">p</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">GError</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">err </span><span class="symbol">=</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gsize</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> expected_size</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">guint16</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">rawdata</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">data</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gint</span><span class="normal"> i</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(!</span><span class="function"><a href="../libgwyddion-gwyutils.html#gwy-file-get-contents">gwy_file_get_contents</a></span><span class="symbol">(</span><span class="normal">filename</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">size</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">err</span><span class="symbol">))</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_GET_FILE_CONTENTS</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">err</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Error-Reporting.html#g-clear-error">g_clear_error</a></span><span class="symbol">(&</span><span class="normal">err</span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="cbracket">}</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">size </span><span class="symbol"><=</span><span class="normal"> HEADER_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_TOO_SHORT</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="cbracket">}</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="function">memcmp</span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> MAGIC</span><span class="symbol">,</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">!=</span><span class="normal"> </span><span class="number">0</span><span class="symbol">)</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_FILE_TYPE</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"Simple"</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="cbracket">}</span>
<span class="normal"> p </span><span class="symbol">=</span><span class="normal"> buffer </span><span class="symbol">+</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">;</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-guint16-le">gwy_get_guint16_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-guint16-le">gwy_get_guint16_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">measure </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">z0 </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal"> expected_size </span><span class="symbol">=</span><span class="normal"> </span><span class="number">2</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">yres </span><span class="symbol">+</span><span class="normal"> HEADER_SIZE</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">size </span><span class="symbol">!=</span><span class="normal"> expected_size</span><span class="symbol">)</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_SIZE_MISMATCH</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">,</span><span class="normal"> expected_size</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="http://developer.gnome.org/doc/API/2.0/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="normal"> </span><span class="cbracket">}</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">measure </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">z0 </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal"> dfield </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwyDataField.html#gwy-data-field-new">gwy_data_field_new</a></span><span class="symbol">(</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">,</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">,</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure</span><span class="symbol">,</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure</span><span class="symbol">,</span>
<span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#FALSE:CAPS">FALSE</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion-gwyutils.html#gwy-convert-raw-data">gwy_convert_raw_data</a></span><span class="symbol">(</span><span class="normal">p</span><span class="symbol">,</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">,</span><span class="normal"> </span><span class="number">1</span><span class="symbol">,</span>
<span class="normal"> <a href="../libgwyddion-gwyutils.html#GWY-RAW-DATA-UINT16:CAPS">GWY_RAW_DATA_UINT16</a></span><span class="symbol">,</span><span class="normal"> <a href="../libgwyddion-gwyutils.html#GWY-BYTE-ORDER-LITTLE-ENDIAN:CAPS">GWY_BYTE_ORDER_LITTLE_ENDIAN</a></span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="../GwyDataField.html#gwy-data-field-get-data">gwy_data_field_get_data</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">),</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">gain</span><span class="symbol">,</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">offset</span><span class="symbol">);</span>
<span class="normal"> unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"m"</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../GwyDataField.html#gwy-data-field-set-si-unit-xy">gwy_data_field_set_si_unit_xy</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">,</span><span class="normal"> unit</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">unit</span><span class="symbol">);</span>
<span class="normal"> unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"m"</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../GwyDataField.html#gwy-data-field-set-si-unit-z">gwy_data_field_set_si_unit_z</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">,</span><span class="normal"> unit</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">unit</span><span class="symbol">);</span>
<span class="normal"> container </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwyContainer.html#gwy-container-new">gwy_container_new</a></span><span class="symbol">();</span>
<span class="normal"> </span><span class="function"><a href="../GwyContainer.html#gwy-container-set-object-by-name">gwy_container_set_object_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/data"</span><span class="symbol">,</span><span class="normal"> dfield</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../GwyContainer.html#gwy-container-set-string-by-name">gwy_container_set_string_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/data/title"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-strdup">g_strdup</a></span><span class="symbol">(</span><span class="string">"Topography"</span><span class="symbol">));</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-file-channel-import-log-add">gwy_file_channel_import_log_add</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="number">0</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"simple"</span><span class="symbol">,</span><span class="normal"> filename</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> container</span><span class="symbol">;</span>
<span class="cbracket">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1">
<a name="id-1.3.6.4"></a><h2>Administrative and Conventions</h2>
<p>
Beside standard headers our module includes a special header that
provides some common inline functions
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="preproc">#include</span><span class="normal"> </span><span class="string">"err.h"</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
It is located in <code class="filename">modules/file</code> for the use by
in-tree modules. External modules can copy it (or only the
actually used functions) to their source code.
</p>
<p>
Header <code class="filename">err.h</code> contains common error reporting
functions (called <code class="function">err_ERROR_TYPE</code>).
Its primary purpose it to ensure consistency of error
messages between file modules and to ease translators' work.
</p>
<p>
Next we define a few convenience symbols: the file extension, the magic
header and its size, and header size
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="preproc">#define</span><span class="normal"> EXTENSION </span><span class="string">".ssd"</span>
<span class="preproc">#define</span><span class="normal"> MAGIC </span><span class="string">"SMPL"</span>
<span class="preproc">#define</span><span class="normal"> </span><span class="function">MAGIC_SIZE</span><span class="normal"> </span><span class="symbol">(</span><span class="keyword">sizeof</span><span class="symbol">(</span><span class="normal">MAGIC</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">-</span><span class="normal"> </span><span class="number">1</span><span class="symbol">)</span>
<span class="keyword">enum</span><span class="normal"> </span><span class="cbracket">{</span><span class="normal"> HEADER_SIZE </span><span class="symbol">=</span><span class="normal"> MAGIC_SIZE </span><span class="symbol">+</span><span class="normal"> </span><span class="number">2</span><span class="symbol">*</span><span class="number">2</span><span class="normal"> </span><span class="symbol">+</span><span class="normal"> </span><span class="number">3</span><span class="symbol">*</span><span class="number">4</span><span class="normal"> </span><span class="cbracket">}</span><span class="symbol">;</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
We also define a structure representing the imported file. It is a bit
superfluous in our module as we do not pass this information around, but
it is useful in many modules
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">typedef</span><span class="normal"> </span><span class="keyword">struct</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="usertype">guint</span><span class="normal"> xres</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">guint</span><span class="normal"> yres</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> measure</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> z0</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">gdouble</span><span class="normal"> gain</span><span class="symbol">;</span>
<span class="cbracket">}</span><span class="normal"> SimpleFile</span><span class="symbol">;</span></pre></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1">
<a name="id-1.3.6.5"></a><h2>Registration</h2>
<p>
The feature registration is similar to data processing functions,
there are just no menu paths and stock icons, on the other hand there are
more functions the module can provide: one for each file operation. Our
module implements only file detection and import therefore we pass
<a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS"><code class="literal">NULL</code></a> for the save and export functions.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11
12</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">static</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Basic-Types.html#gboolean">gboolean</a></span>
<span class="function">module_register</span><span class="symbol">(</span><span class="type">void</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="function"><a href="libgwymodule-gwymodule-file.html#gwy-file-func-register">gwy_file_func_register</a></span><span class="symbol">(</span><span class="string">"simple"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion-gwymacros.html#N-:CAPS">N_</a></span><span class="symbol">(</span><span class="string">"Simple AFM files (.afm)"</span><span class="symbol">),</span>
<span class="normal"> </span><span class="symbol">(</span><span class="normal"><a href="libgwymodule-gwymodule-file.html#GwyFileDetectFunc">GwyFileDetectFunc</a></span><span class="symbol">)&</span><span class="normal">simple_detect</span><span class="symbol">,</span>
<span class="normal"> </span><span class="symbol">(</span><span class="normal"><a href="libgwymodule-gwymodule-file.html#GwyFileLoadFunc">GwyFileLoadFunc</a></span><span class="symbol">)&</span><span class="normal">simple_load</span><span class="symbol">,</span>
<span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">,</span>
<span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#TRUE:CAPS">TRUE</a></span><span class="symbol">;</span>
<span class="cbracket">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
It is necessary to provide at least one of load, save and export
operations, otherwise the file type would be rejected. It is highly
recommended to provide a detection function, although it is not
required. If no detection function exists for a file type the user
has to always explicitly request the type to load and/or save files
in this format.
</p>
</div>
<div class="refsect1">
<a name="id-1.3.6.6"></a><h2>Detection</h2>
<p>
Two types of detection exist: on load when we have the file
available, and on save when the file does not exist and we know only
the requested file name. They are differentiated by
<code class="varname">only_name</code> argument. In both cases the detection
function gets a <a class="link" href="libgwymodule-gwymodule-file.html#GwyFileDetectInfo" title="GwyFileDetectInfo"><span class="type">GwyFileDetectInfo</span></a> structure, but if
<code class="varname">only_name</code> is <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#FALSE:CAPS"><code class="literal">FALSE</code></a>, some fields are unset.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11
12
13</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">static</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Basic-Types.html#gint">gint</a></span>
<span class="function">simple_detect</span><span class="symbol">(</span><span class="keyword">const</span><span class="normal"> </span><span class="usertype">GwyFileDetectInfo</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">fileinfo</span><span class="symbol">,</span>
<span class="normal"> </span><span class="usertype">gboolean</span><span class="normal"> only_name</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">only_name</span><span class="symbol">)</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-str-has-suffix">g_str_has_suffix</a></span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">name_lowercase</span><span class="symbol">,</span><span class="normal"> EXTENSION</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">?</span><span class="normal"> </span><span class="number">20</span><span class="normal"> </span><span class="symbol">:</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">buffer_len </span><span class="symbol">></span><span class="normal"> MAGIC_SIZE</span>
<span class="normal"> </span><span class="symbol">&&</span><span class="normal"> </span><span class="function">memcmp</span><span class="symbol">(</span><span class="normal">fileinfo</span><span class="symbol">-></span><span class="normal">head</span><span class="symbol">,</span><span class="normal"> MAGIC</span><span class="symbol">,</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">==</span><span class="normal"> </span><span class="number">0</span><span class="symbol">)</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="number">100</span><span class="symbol">;</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span>
<span class="cbracket">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
The latter case is usually trivial, we just compare the extension with
the normal extension of our file type.
</p>
<p>
The former case can be more complex. File name does not tell us much
about file type: consider just the number of formats using extension
<code class="filename">.afm</code> (not talking about <code class="filename">.dat</code>
which can be essentially anything). Therefore we use file names only
as the last resort and base the detection on actual file contents.
</p>
<p>
Many formats have well defined magic headers, precisely for the puropose
of easy detection. If out format has one, we are lucky and the detection
code can look as simple as in the example. To detect a particular
version or type of a file format a more detailed analysis is necessary,
but not substantially different from the simple case.
</p>
<p>
The most problematic case arises when the format has no detectable magic
header at all. Often we can employ the following approach: read the
header data and check whether it makes sense if interpreted as our
file format. For example we read the number of lines and pixels per
line, calculate expected file size and compare it to the real file size.
Or we read the physical dimensions and check they are positive (and
approximately of expected magnitude). If our detection routine requires
significant processing or <em class="structfield"><code>head</code></em> and
<em class="structfield"><code>tail</code></em> are not sufficient and we have to open
the file ourselves, it is advisable to perform a quick check that can
eliminate most files that are <span class="emphasis"><em>not</em></span> of our type first,
and then perform the sophisticated test only on the likely candidates.
</p>
</div>
<div class="refsect1">
<a name="id-1.3.6.7"></a><h2>Loading</h2>
<p>
The task of the load function is to read the file and create
a <a href="../GwyContainer.html#GwyContainer-struct"><span class="type">GwyContainer</span></a> containing all importable data. It must not assume
anything about the file properties and contents, not even that the file
passed the detection because the user can request a file type
explicitly, bypassing detection. It has also deal with broken files,
checking whether the file is long enough to contain the data it claims
to and performing further parameter validation if necessary.
</p>
<p>
The file contents can be often read with <a href="../libgwyddion-gwyutils.html#gwy-file-get-contents"><code class="function">gwy_file_get_contents()</code></a> which
uses <code class="function">mmap()</code> (if available). The returned memory
is read-only. Sometimes destructive header parsing is useful, in such
a case it is possible to use <a href="/usr/share/gtk-doc/html/glib/glib-Memory-Allocation.html#g-memdup"><code class="function">g_memdup()</code></a> to duplicate the header,
keeping the data part mapped read-only.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">size </span><span class="symbol"><=</span><span class="normal"> HEADER_SIZE</span><span class="symbol">)</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="function">err_TOO_SHORT</span><span class="symbol">(</span><span class="normal">error</span><span class="symbol">);</span>
<span class="normal"> </span><span class="function"><a href="../libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>
<span class="cbracket">}</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
The header is parsed using the
<a class="link" href="gwymodule-tutorial-file.html#module-tutorial-file-portability" title="Portability">portable</a>
binary buffer reading functions.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">p </span><span class="symbol">=</span><span class="normal"> buffer </span><span class="symbol">+</span><span class="normal"> MAGIC_SIZE</span><span class="symbol">;</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">xres </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-guint16-le">gwy_get_guint16_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">yres </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-guint16-le">gwy_get_guint16_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">z0 </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-get-gfloat-le">gwy_get_gfloat_le</a></span><span class="symbol">(&</span><span class="normal">p</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
Then we can create the data field and read the data into it. Since
<span class="application">Gwyddion</span> always works with values in base
SI units, we have to convert the raw data values to physical values.
If the file uses non-base units (which is common in SPM), we have to
multiply the data by the corresponding factor: in this case with
10<sup>-9</sup> as the physcial quantities are stored
in nanometers in the file.
Raw data conversion can be most easily performed with the convenience
function <a href="../libgwyddion-gwyutils.html#gwy-convert-raw-data"><code class="function">gwy_convert_raw_data()</code></a>. Sometimes it may be not sufficient
so you can resort to the low-level GLib macros such as <a href="/usr/share/gtk-doc/html/glib/glib-Byte-Order-Macros.html#GUINT16-FROM-LE:CAPS"><code class="function">GUINT16_FROM_LE()</code></a>
or Gwyddion get-functions <a href="../libgwyapp-file-module-utils.html#gwy-get-gdouble-be"><code class="function">gwy_get_gdouble_be()</code></a>.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">z0 </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal">simple</span><span class="symbol">.</span><span class="normal">gain </span><span class="symbol">*=</span><span class="normal"> </span><span class="number">1e-9</span><span class="symbol">;</span>
<span class="normal">dfield </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwyDataField.html#gwy-data-field-new">gwy_data_field_new</a></span><span class="symbol">(</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">,</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">,</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure</span><span class="symbol">,</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">measure</span><span class="symbol">,</span>
<span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#FALSE:CAPS">FALSE</a></span><span class="symbol">);</span>
<span class="function"><a href="../libgwyddion-gwyutils.html#gwy-convert-raw-data">gwy_convert_raw_data</a></span><span class="symbol">(</span><span class="normal">p</span><span class="symbol">,</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">xres</span><span class="symbol">*</span><span class="normal">simple</span><span class="symbol">.</span><span class="normal">yres</span><span class="symbol">,</span><span class="normal"> </span><span class="number">1</span><span class="symbol">,</span>
<span class="normal"> <a href="../libgwyddion-gwyutils.html#GWY-RAW-DATA-UINT16:CAPS">GWY_RAW_DATA_UINT16</a></span><span class="symbol">,</span><span class="normal"> <a href="../libgwyddion-gwyutils.html#GWY-BYTE-ORDER-LITTLE-ENDIAN:CAPS">GWY_BYTE_ORDER_LITTLE_ENDIAN</a></span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="../GwyDataField.html#gwy-data-field-get-data">gwy_data_field_get_data</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">),</span>
<span class="normal"> simple</span><span class="symbol">.</span><span class="normal">gain</span><span class="symbol">,</span><span class="normal"> simple</span><span class="symbol">.</span><span class="normal">offset</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
We have to set also the lateral and value units of the created data
field (by default, the dimensions and values are unitless).
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"m"</span><span class="symbol">);</span>
<span class="function"><a href="../GwyDataField.html#gwy-data-field-set-si-unit-xy">gwy_data_field_set_si_unit_xy</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">,</span><span class="normal"> unit</span><span class="symbol">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">unit</span><span class="symbol">);</span>
<span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"m"</span><span class="symbol">);</span>
<span class="function"><a href="../GwyDataField.html#gwy-data-field-set-si-unit-z">gwy_data_field_set_si_unit_z</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">,</span><span class="normal"> unit</span><span class="symbol">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">unit</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
Our sample file format can only store topography channels, therefore
the units are always meters. For different types of channels the units
have to be set accordingly. Note that
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"nA"</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
is exactly the same as
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwySIUnit.html#gwy-si-unit-new">gwy_si_unit_new</a></span><span class="symbol">(</span><span class="string">"A"</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
In other words, the prefixes are ignored because <a href="../GwySIUnit.html#GwySIUnit-struct"><span class="type">GwySIUnit</span></a> represents
the unit type and contains no magnitude information. However, if the
format stores the units as a string including the prefix we can use
<a href="../GwySIUnit.html#gwy-si-unit-new-parse"><code class="function">gwy_si_unit_new_parse()</code></a> to obtain the corresponding power of 10 (and
subsequently multiply data field values with the correct multiplier).
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">unit </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwySIUnit.html#gwy-si-unit-new-parse">gwy_si_unit_new_parse</a></span><span class="symbol">(</span><span class="string">"nA"</span><span class="symbol">,</span><span class="normal"> </span><span class="symbol">&</span><span class="normal">power10</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
Finally we create a new <a href="../GwyContainer.html#GwyContainer-struct"><span class="type">GwyContainer</span></a>, put the data field at
<code class="literal">"/0/data"</code> and set the channel title
<code class="literal">"/0/data/title"</code> (if the file contains explicit
channel names, they are good candidates for a channel title, otherwise
we settle for channel type as Topography or Current), and return the
container. The application then takes care of adding it to the data
browser and showing it in a data window. Calling
<a href="../libgwyapp-file-module-utils.html#gwy-file-channel-import-log-add"><code class="function">gwy_file_channel_import_log_add()</code></a> for channel 0 adds a data operation
log entry, noting that the data originated by import from the
<code class="literal">"simple"</code> format.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11
12</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">container </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwyContainer.html#gwy-container-new">gwy_container_new</a></span><span class="symbol">();</span>
<span class="function"><a href="../GwyContainer.html#gwy-container-set-object-by-name">gwy_container_set_object_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/data"</span><span class="symbol">,</span><span class="normal"> dfield</span><span class="symbol">);</span>
<span class="function"><a href="../GwyContainer.html#gwy-container-set-string-by-name">gwy_container_set_string_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/data/title"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-strdup">g_strdup</a></span><span class="symbol">(</span><span class="string">"Topography"</span><span class="symbol">));</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">dfield</span><span class="symbol">);</span>
<span class="function"><a href="../libgwyapp-file-module-utils.html#gwy-file-channel-import-log-add">gwy_file_channel_import_log_add</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="number">0</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"simple"</span><span class="symbol">,</span><span class="normal"> filename</span><span class="symbol">);</span>
<span class="function"><a href="../libgwyddion-gwyutils.html#gwy-file-abandon-contents">gwy_file_abandon_contents</a></span><span class="symbol">(</span><span class="normal">buffer</span><span class="symbol">,</span><span class="normal"> size</span><span class="symbol">,</span><span class="normal"> <a href="/usr/share/gtk-doc/html/glib/glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">);</span>
<span class="keyword">return</span><span class="normal"> container</span><span class="symbol">;</span></pre></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1">
<a name="id-1.3.6.8"></a><h2>Loading More</h2>
<p>
Real file formats often contain more information we would like to
import. First, they can contain several data fields (channels or even
unrelated images). We store the second data at
<code class="literal">"/1/data"</code> and its title at
<code class="literal">"/1/data/title"</code>, the third channel data at
<code class="literal">"/2/data"</code>, etc. The number sequence does not have
to be contignuous but it is customary to make it so (in any case
small integers should be used, do not start counting from 12758933).
</p>
<p>
Auxiliary information about the data as date and time of scanning, scan
speed, tunneling current, tip oscillation frequency or user-defined
comments can be stored to metadata. Metadata is a nested <a href="../GwyContainer.html#GwyContainer-struct"><span class="type">GwyContainer</span></a>
at <code class="literal">"/0/meta"</code> (for the first channel, the number of
course corresponds to channel id). The keys are simply names of the
values, the values are always strings. Note both keys and values have
to be converted to UTF-8.
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="normal">meta </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="../GwyContainer.html#gwy-container-new">gwy_container_new</a></span><span class="symbol">();</span>
<span class="function"><a href="../GwyContainer.html#gwy-container-set-string-by-name">gwy_container_set_string_by_name</a></span><span class="symbol">(</span><span class="normal">meta</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"Comment"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-strdup">g_strdup</a></span><span class="symbol">(</span><span class="normal">myfile</span><span class="symbol">-></span><span class="normal">user_comment</span><span class="symbol">));</span>
<span class="function"><a href="../GwyContainer.html#gwy-container-set-string-by-name">gwy_container_set_string_by_name</a></span><span class="symbol">(</span><span class="normal">meta</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"Tip oscillation frequency"</span><span class="symbol">,</span>
<span class="normal"> </span><span class="function"><a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-strdup-printf">g_strdup_printf</a></span><span class="symbol">(</span><span class="string">"%g Hz"</span><span class="symbol">,</span><span class="normal"> myfile</span><span class="symbol">-></span><span class="normal">freq</span><span class="symbol">));</span>
<span class="function"><a href="../GwyContainer.html#gwy-container-set-object-by-name">gwy_container_set_object_by_name</a></span><span class="symbol">(</span><span class="normal">container</span><span class="symbol">,</span><span class="normal"> </span><span class="string">"/0/meta"</span><span class="symbol">,</span><span class="normal"> meta</span><span class="symbol">);</span>
<span class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></span><span class="symbol">(</span><span class="normal">meta</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1">
<a name="module-tutorial-file-portability"></a><h2>Portability</h2>
<p>
Gwyddion runs on many platforms and can be compiled with various
compilers. File import is the area most affected by the differences
between them, therefore certain care must be taken to make file modules
work across all platforms.
</p>
<div class="refsect2">
<a name="id-1.3.6.9.3"></a><h3>Text Data</h3>
<p>
Text files can encode <span class="emphasis"><em>line ends</em></span>
differently from the platform Gwyddion is running on. Therefore
opening files as text may lead to unexpected results. One usually
opens all files as binary and uses <a href="../libgwyddion-gwyutils.html#gwy-str-next-line"><code class="function">gwy_str_next_line()</code></a> to parse text
files (or text parts of files) to lines.
</p>
<p>
The text representation of <span class="emphasis"><em>real numbers</em></span> is
locale-dependent. User's locale is unpredictable and a module must not
set locale to arbitrary values because it would affect the rest of the
application. Therefore one has to use functions as <a href="/usr/share/gtk-doc/html/glib/glib-String-Utility-Functions.html#g-ascii-strtod"><code class="function">g_ascii_strtod()</code></a>
to read real numbers portably.
</p>
<p>
Many text strings, like comments and remarks, are stored in an
arbitrary <span class="emphasis"><em>character encoding</em></span>.
Typically it is ISO 8895-1 (Latin1) or UTF-16, but sometimes more
exotic encodings are used. Since Gwyddion uses UTF-8 for all text data
(like Gtk+ does) they must be converted, e.g., with <a href="/usr/share/gtk-doc/html/glib/glib-Character-Set-Conversion.html#g-convert"><code class="function">g_convert()</code></a>.
</p>
</div>
<hr>
<div class="refsect2">
<a name="id-1.3.6.9.4"></a><h3>Binary Data</h3>
<p>
The canonical bad example is
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">struct</span><span class="normal"> </span><span class="cbracket">{</span>
<span class="normal"> </span><span class="type">char</span><span class="normal"> c</span><span class="symbol">;</span>
<span class="normal"> </span><span class="usertype">LONG</span><span class="normal"> i</span><span class="symbol">;</span>
<span class="normal"> </span><span class="type">wchar_t</span><span class="normal"> remark</span><span class="symbol">[</span><span class="number">20</span><span class="symbol">];</span>
<span class="cbracket">}</span><span class="normal"> header</span><span class="symbol">;</span>
<span class="function">fread</span><span class="symbol">(&</span><span class="normal">header</span><span class="symbol">,</span><span class="normal"> </span><span class="keyword">sizeof</span><span class="symbol">(</span><span class="normal">header</span><span class="symbol">),</span><span class="normal"> </span><span class="number">1</span><span class="symbol">,</span><span class="normal"> filehandle</span><span class="symbol">);</span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
For start, <span class="type">LONG</span> is unlikely to exist on non-Microsoft
platforms. So should we fix it to <span class="type">long int</span>? No, the
<span class="emphasis"><em>type size</em></span> of classic C types (<span class="type">short</span>,
<span class="type">long</span>, etc.) is different on different architectures (more
precisely ABIs) and the standard prescribes only the lower limit. If we
mean `32bit integer' we also have to express it in our code:
<a href="/usr/share/gtk-doc/html/glib/glib-Basic-Types.html#gint32"><span class="type">gint32</span></a>.
</p>
<p>
Likewise, <span class="type">wchar_t</span> size is not well-defined. If the
file comes from a Microsoft platform, where <span class="type">wchar_t</span> is
used a lot, it is probably 16bit, so we write <a href="/usr/share/gtk-doc/html/glib/glib-Unicode-Manipulation.html#gunichar2"><span class="type">gunichar2</span></a> instead
and use <a href="/usr/share/gtk-doc/html/glib/glib-Unicode-Manipulation.html#g-utf16-to-utf8"><code class="function">g_utf16_to_utf8()</code></a> to get UTF-8.
</p>
<p>
Then we have <span class="emphasis"><em>byte order</em></span>. If we are on a platform
with different native byte order than the file uses, we have to reverse
the bytes in each multi-byte value. GLib provides macros such as
<a href="/usr/share/gtk-doc/html/glib/glib-Byte-Order-Macros.html#GUINT16-FROM-LE:CAPS"><code class="function">GUINT16_FROM_LE()</code></a> to simplify this task.
</p>
<p>
And last but not least, different ABIs prescribe different
<span class="emphasis"><em>structure padding</em></span>. Typically there can be
from 0 to 7 unused bytes between <em class="structfield"><code>c</code></em> and
<em class="structfield"><code>i</code></em> fields. While most compilers provide
some packing <code class="literal">#pragma</code>s or other means to
specify desired structure padding, the portability of these
constructs is questionable at best.
</p>
<p>
The answer to (almost) all the problems above is: Don't do that then.
Avoid direct reading of structures, read the data into a flat byte
buffer instead and use the
file module utils
functions to obtain individual values.
</p>
</div>
</div>
</div>
<div class="footer">
<hr>Generated by GTK-Doc V1.19</div>
</body>
</html>
|
