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4 <title>Using the GEDCOM parser library</title>
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10 <h1 align="center">Using the GEDCOM parser library</h1>
16 <li><a href="#anchor">Overview</a></li>
17 <li><a href="#Error_handling">Error handling</a></li>
18 <li><a href="#Data_callback_mechanism">Data callback mechanism</a></li>
21 <li><a href="#Start_and_end_callbacks">Start and end callbacks</a></li>
22 <li><a href="#Default_callbacks">Default callbacks</a></li>
25 <li><a href="#Other_API_functions">Other API functions</a></li>
28 <li><a href="#Debugging">Debugging</a></li>
29 <li><a href="#Error_treatment">Error treatment</a></li>
30 <li><a href="#Compatibility_mode">Compatibility mode</a></li>
33 <li><a href="interface.html">Interface details</a><br>
38 <hr width="100%" size="2">
39 <h2><a name="Overview"></a>Overview<br>
41 The GEDCOM parser library is built as a callback-based parser (comparable
42 to the SAX interface of XML). It comes with:<br>
45 <li>a library (<code>libgedcom.so</code>), to be linked in the application
47 <li>a header file (<code>gedcom.h</code>), to be used in the sources
48 of the application program</li>
49 <li>a header file (<code>gedcom-tags.h</code>) that is also installed,
50 but that is automatically included via <code>gedcom.h</code><br>
54 Next to these, there is also a data directory in <code>$PREFIX/share/gedcom-parse</code>
55 that contains some additional stuff, but which is not immediately
56 important at first. I'll leave the description of the data directory
59 The very simplest call of the gedcom parser is simply the following
60 piece of code (include of the gedcom header is assumed, as everywhere in
63 <blockquote><code>int result;<br>
65 result = <b>gedcom_parse_file</b>("myfamily.ged");<br>
67 Although this will not provide much information, one thing it does
68 is parse the entire file and return the result. The function returns
69 0 on success and 1 on failure. No other information is available using
70 this function only.<br>
72 The next sections will refine this to be able to have meaningful errors
73 and the actual data that is in the file.<br>
75 <hr width="100%" size="2">
76 <h2><a name="Error_handling"></a>Error handling</h2>
77 Since this is a relatively simple topic, it is discussed before the
78 actual callback mechanism, although it also uses a callback...<br>
80 The library can be used in several different circumstances, both terminal-based
81 as GUI-based. Therefore, it leaves the actual display of the error
82 message up to the application. For this, the application needs to register
83 a callback before parsing the GEDCOM file, which will be called by the library
84 on errors, warnings and messages.<br>
86 A typical piece of code would be:<br>
88 <blockquote><code>void <b>my_message_handler</b> (Gedcom_msg_type type,
94 <b>gedcom_set_message_handler</b>(my_message_handler);<br>
96 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
98 In the above piece of code, <code>my_message_handler</code> is the callback
99 that will be called for errors (<code>type=ERROR</code>), warnings (<code>
100 type=WARNING</code>) and messages (<code>type=MESSAGE</code>). The
101 callback must have the signature as in the example. For errors, the
102 <code> msg</code> passed to the callback will have the format:<br>
104 <blockquote><code>Error on line</code> <i><lineno></i>: <i><actual_message></i><br>
106 Note that the entire string will be properly internationalized, and
107 encoded in UTF-8 (see "Why UTF-8?" <i>LINK TBD</i>). Also,
108 no newline is appended, so that the application program can use it in any
109 way it wants. Warnings are similar, but use "Warning" instead of
110 "Error". Messages are plain text, without any prefix.<br>
112 With this in place, the resulting code will already show errors and
113 warnings produced by the parser, e.g. on the terminal if a simple <code>
114 printf</code> is used in the message handler.<br>
116 <hr width="100%" size="2">
117 <h2><a name="Data_callback_mechanism"></a>Data callback mechanism</h2>
118 The most important use of the parser is of course to get the data out
119 of the GEDCOM file. As already mentioned, the parser uses a callback
120 mechanism for that. In fact, the mechanism involves two levels.<br>
122 The primary level is that each of the sections in a GEDCOM file is notified
123 to the application code via a "start element" callback and an "end element"
124 callback (much like in a SAX interface for XML), i.e. when a line containing
125 a certain tag is parsed, the "start element" callback is called for that
126 tag, and when all its subordinate lines with their tags have been processed,
127 the "end element" callback is called for the original tag. Since GEDCOM
128 is hierarchical, this results in properly nested calls to appropriate "start
129 element" and "end element" callbacks.<br>
131 However, it would be typical for a genealogy program to support only
132 a subset of the GEDCOM standard, certainly a program that is still under
133 development. Moreover, under GEDCOM it is allowed for an application
134 to define its own tags, which will typically not be supported by another
135 application. Still, in that case, data preservation is important;
136 it would hardly be accepted that information that is not understood by a
137 certain program is just removed.<br>
139 Therefore, the second level of callbacks involves a "default callback".
140 An application needs to subscribe to callbacks for tags it does support,
141 and need to provide a "default callback" which will be called for tags
142 it doesn't support. The application can then choose to just store
143 the information that comes via the default callback in plain textual format.<br>
145 After this introduction, let's see what the API looks like...<br>
148 <h3><a name="Start_and_end_callbacks"></a>Start and end callbacks</h3>
150 <h4><i>Callbacks for records</i> <br>
152 As a simple example, we will get some information from the header of
153 a GEDCOM file. First, have a look at the following piece of code:<br>
155 <blockquote><code>Gedcom_ctxt <b>my_header_start_cb</b> (int level,
157
158 Gedcom_val xref, <br>
159
160 char *tag, <br>
161
162 char *raw_value,<br>
163
164 int parsed_tag, <br>
165
166 Gedcom_val parsed_value)<br>
168 printf("The header starts\n");<br>
169 return (Gedcom_ctxt)1;<br>
172 void <b>my_header_end_cb</b> (Gedcom_ctxt self)<br>
174 printf("The header ends, context is %d\n", self); /* context
175 will print as "1" */<br>
179 <b>gedcom_subscribe_to_record</b>(REC_HEAD, my_header_start_cb,
180 my_header_end_cb);<br>
182 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
184 Using the <code>gedcom_subscribe_to_record</code> function, the application
185 requests to use the specified callbacks as start and end callback. The end
186 callback is optional: you can pass <code>NULL</code> if you are not interested
187 in the end callback. The identifiers to use as first argument to
188 the function (here <code>REC_HEAD</code>) are described in the <a href="interface.html#Record_identifiers">
189 interface details</a>.<br>
191 From the name of the function it becomes clear that this function is
192 specific to complete records. For the separate elements in records
193 there is another function, which we'll see shortly. Again, the callbacks
194 need to have the signatures as shown in the example.<br>
196 The <code>Gedcom_ctxt</code> type that is used as a result of the start
197 callback and as an argument to the end callback is vital for passing context
198 necessary for the application. This type is meant to be opaque; in
199 fact, it's a void pointer, so you can pass anything via it. The important
200 thing to know is that the context that the application returns in the start
201 callback will be passed in the end callback as an argument, and as we will
202 see shortly, also to all the directly subordinate elements of the record.<br>
204 The <code>tag</code> is the GEDCOM tag in string format, the <code>parsed_tag</code>
205 is an integer, for which symbolic values are defined as <code>TAG_HEAD,</code>
206 <code>TAG_SOUR,</code> <code>TAG_DATA,</code> ... and <code>USERTAG </code><code></code>
207 for the application-specific tags. These values are defined in the
208 header <code>gedcom-tags.h</code> that is installed, and included via <code>
209 gedcom.h</code> (so no need to include <code>gedcom-tags.h</code> yourself).<br>
211 The example passes a simple integer as context, but an application could
212 e.g. pass a <code>struct</code> that will contain the information for the
213 header. In the end callback, the application could then e.g. do some
214 finalizing operations on the <code>struct</code> to put it in its database.<br>
216 (Note that the <code>Gedcom_val</code> type for the <code>xref</code>
217 and <code>parsed_value</code> arguments was not discussed, see further
221 <h4><i>Callbacks for elements</i></h4>
222 We will now retrieve the SOUR field (the name of the program that wrote
223 the file) from the header:<br>
225 <blockquote><code>Gedcom_ctxt <b>my_header_source_start_cb</b>(Gedcom_ctxt
227
228 int
229 level,<br>
230
231 char*
232 tag,<br>
233
234 char*
235 raw_value,<br>
236
237 int
238 parsed_tag,<br>
239
240 Gedcom_val
241 parsed_value)<br>
243 char *source = GEDCOM_STRING(parsed_value);<br>
244 printf("This file was written by %s\n", source);<br>
245 return parent;<br>
248 void <b>my_header_source_end_cb</b>(Gedcom_ctxt parent,<br>
249
250 Gedcom_ctxt self,<br>
251
252 Gedcom_val parsed_value)<br>
254 printf("End of the source description\n");<br>
258 <b>gedcom_subscribe_to_element</b>(ELT_HEAD_SOUR,<br>
259
260 my_header_source_start_cb,<br>
261
262 my_header_source_end_cb);<br>
264 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
266 The subscription mechanism for elements is similar, only the signatures
267 of the callbacks differ. The signature for the start callback shows
268 that the context of the parent line (e.g. the <code>struct</code> that
269 describes the header) is passed to this start callback. The callback
270 itself returns here the same context, but this can be its own context object
271 of course. The end callback is called with both the context of the
272 parent and the context of itself, which will be the same in the example.
273 Again, the list of identifiers to use as a first argument for the
274 subscription function are detailed in the <a href="interface.html#Element_identifiers">
275 interface details</a> .<br>
277 If we look at the other arguments of the start callback, we see the
278 level number (the initial number of the line in the GEDCOM file), the tag
279 (e.g. "SOUR"), and then a raw value, a parsed tag and a parsed value. The
280 raw value is just the raw string that occurs as value on the line next to
281 the tag (in UTF-8 encoding). The parsed value is the meaningful value
282 that is parsed from that raw string. The parsed tag is described in
283 the section for record callbacks.<br>
285 The <code>Gedcom_val</code> type is meant to be an opaque type. The
286 only thing that needs to be known about it is that it can contain specific
287 data types, which have to be retrieved from it using pre-defined macros.
288 These data types are described in the <a href="interface.html#Gedcom_val_types">
289 interface details</a>. <br>
291 Some extra notes:<br>
294 <li>The <code>Gedcom_val</code> argument of the end callback
295 is currently not used. It is there for future enhancements.</li>
296 <li>There is also a <code>Gedcom_val</code> argument in
297 the start callback for records. This argument is currently a string
298 value giving the pointer in string form.</li>
302 <h3><a name="Default_callbacks"></a>Default callbacks<br>
304 As described above, an application doesn't always implement the entire
305 GEDCOM spec, and application-specific tags may have been added by other applications.
306 To preserve this extra data anyway, a default callback can be registered
307 by the application, as in the following example:<br>
309 <blockquote><code>void <b>my_default_cb</b> (Gedcom_ctxt parent,
310 int level, char* tag, char* raw_value, int parsed_tag)<br>
316 <b>gedcom_set_default_callback</b>(my_default_cb);<br>
318 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
320 This callback has a similar signature as the previous ones,
321 but it doesn't contain a parsed value. However, it does contain the
322 parent context, that was returned by the application for the most specific
323 containing tag that the application supported.<br>
325 Suppose e.g. that this callback is called for some tags in the header
326 that are specific to some other application, then our application could make
327 sure that the parent context contains the struct or object that represents
328 the header, and use the default callback here to add the level, tag and
329 raw_value as plain text in a member of that struct or object, thus preserving
330 the information. The application can then write this out when the
331 data is saved again in a GEDCOM file. To make it more specific, consider
332 the following example:<br>
334 <blockquote><code>struct header {<br>
335 char* source;<br>
337 char* extra_text;<br>
340 Gedcom_ctxt my_header_start_cb(int level, Gedcom_val xref, char* tag,
342
343 int parsed_tag, Gedcom_val parsed_value)<br>
345 struct header head = my_make_header_struct();<br>
346 return (Gedcom_ctxt)head;<br>
349 void my_default_cb(Gedcom_ctxt parent, int level, char* tag, char* raw_value,
352 struct header head = (struct header)parent;<br>
353 my_header_add_to_extra_text(head, level, tag, raw_value);<br>
356 gedcom_set_default_callback(my_default_cb);<br>
357 gedcom_subscribe_to_record(REC_HEAD, my_header_start, NULL);<br>
359 result = gedcom_parse_file(filename);</code><br>
361 Note that the default callback will be called for any tag that isn't
362 specifically subscribed upon by the application, and can thus be called
363 in various contexts. For simplicity, the example above doesn't take
364 this into account (the <code>parent</code> could be of different
365 types, depending on the context).<br>
367 <hr width="100%" size="2">
369 <h2><a name="Other_API_functions"></a>Other API functions<br>
371 Although the above describes the basic interface of libgedcom, there
372 are some other functions that allow to customize the behaviour of the library.
373 These will be explained in the current section.<br>
375 <h3><a name="Debugging"></a>Debugging</h3>
376 The library can generate various debugging output, not only from itself,
377 but also the debugging output generated by the yacc parser. By default,
378 no debugging output is generated, but this can be customized using the
379 following function:<br>
381 <blockquote><code>void <b>gedcom_set_debug_level</b> (int level,
382 FILE* trace_output)</code><br>
384 The <code>level</code> can be one of the following values:<br>
387 <li>0: no debugging information (this is the
389 <li>1: only debugging information from libgedcom
391 <li>2: debugging information from libgedcom and
395 If the <code>trace_output</code> is <code>NULL</code>, debugging information
396 will be written to <code>stderr</code>, otherwise the given file handle
397 is used (which must be open).<br>
400 <h3><a name="Error_treatment"></a>Error treatment</h3>
401 One of the previous sections already described the callback to be registered
402 to get error messages. The library also allows to customize what
403 happens on an error, using the following function:<br>
405 <blockquote><code>void <b>gedcom_set_error_handling</b> (Gedcom_err_mech
406 mechanism)</code><br>
408 The <code>mechanism</code> can be one of:<br>
412 <li><code>IMMED_FAIL</code>: immediately fail the
413 parsing on an error (this is the default)</li>
414 <li><code>DEFER_FAIL</code>: continue parsing after
415 an error, but return a failure code eventually</li>
416 <li><code>IGNORE_ERRORS</code>: continue parsing
417 after an error, return success always</li>
421 This doesn't influence the generation of error or warning messages, only
422 the behaviour of the parser and its return code.<br>
426 <h3><a name="Compatibility_mode"></a>Compatibility mode<br>
428 Applications are not necessarily true to the GEDCOM spec (or use a different
429 version than 5.5). The intention is that the library is resilient
430 to this, and goes in compatibility mode for files written by specific programs
431 (detected via the HEAD.SOUR tag). This compatibility mode can be
432 enabled and disabled via the following function:<br>
435 <blockquote><code>void <b>gedcom_set_compat_handling</b>
436 (int enable_compat)</code><br>
438 The argument can be:<br>
442 <li>0: disable compatibility mode</li>
443 <li>1: allow compatibility mode (this is the default)<br>
448 Note that, currently, no actual compatibility code is present, but this
449 is on the to-do list.<br>
452 <hr width="100%" size="2">
454 <pre>$Id$<br>$Name$<br></pre>