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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 important
56 at first. I'll leave the description of the data directory for later.<br>
58 The very simplest call of the gedcom parser is simply the following piece
59 of code (include of the gedcom header is assumed, as everywhere in this
62 <blockquote><code>int result;<br>
64 result = <b>gedcom_parse_file</b>("myfamily.ged");<br>
66 Although this will not provide much information, one thing it does is
67 parse the entire file and return the result. The function returns 0
68 on success and 1 on failure. No other information is available using
69 this function only.<br>
71 The next sections will refine this to be able to have meaningful errors
72 and the actual data that is in the file.<br>
74 <hr width="100%" size="2">
75 <h2><a name="Error_handling"></a>Error handling</h2>
76 Since this is a relatively simple topic, it is discussed before the actual
77 callback mechanism, although it also uses a callback...<br>
79 The library can be used in several different circumstances, both terminal-based
80 as GUI-based. Therefore, it leaves the actual display of the error
81 message up to the application. For this, the application needs to register
82 a callback before parsing the GEDCOM file, which will be called by the library
83 on errors, warnings and messages.<br>
85 A typical piece of code would be:<br>
87 <blockquote><code>void <b>my_message_handler</b> (Gedcom_msg_type type,
93 <b>gedcom_set_message_handler</b>(my_message_handler);<br>
95 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
97 In the above piece of code, <code>my_message_handler</code> is the callback
98 that will be called for errors (<code>type=ERROR</code>), warnings (<code>
99 type=WARNING</code>) and messages (<code>type=MESSAGE</code>). The
100 callback must have the signature as in the example. For errors, the
101 <code> msg</code> passed to the callback will have the format:<br>
103 <blockquote><code>Error on line</code> <i><lineno></i>: <i><actual_message></i><br>
105 Note that the entire string will be properly internationalized, and encoded
106 in UTF-8 (see "Why UTF-8?" <i>LINK TBD</i>). Also, no newline
107 is appended, so that the application program can use it in any way it wants.
108 Warnings are similar, but use "Warning" instead of "Error". Messages
109 are plain text, without any prefix.<br>
111 With this in place, the resulting code will already show errors and warnings
112 produced by the parser, e.g. on the terminal if a simple <code>printf</code>
113 is used in the message handler.<br>
115 <hr width="100%" size="2">
116 <h2><a name="Data_callback_mechanism"></a>Data callback mechanism</h2>
117 The most important use of the parser is of course to get the data out
118 of the GEDCOM file. As already mentioned, the parser uses a callback
119 mechanism for that. In fact, the mechanism involves two levels.<br>
121 The primary level is that each of the sections in a GEDCOM file is notified
122 to the application code via a "start element" callback and an "end element"
123 callback (much like in a SAX interface for XML), i.e. when a line containing
124 a certain tag is parsed, the "start element" callback is called for that
125 tag, and when all its subordinate lines with their tags have been processed,
126 the "end element" callback is called for the original tag. Since GEDCOM
127 is hierarchical, this results in properly nested calls to appropriate "start
128 element" and "end element" callbacks.<br>
130 However, it would be typical for a genealogy program to support only a
131 subset of the GEDCOM standard, certainly a program that is still under development.
132 Moreover, under GEDCOM it is allowed for an application to define its
133 own tags, which will typically not be supported by another application.
134 Still, in that case, data preservation is important; it would hardly
135 be accepted that information that is not understood by a certain program
138 Therefore, the second level of callbacks involves a "default callback".
139 An application needs to subscribe to callbacks for tags it does support,
140 and need to provide a "default callback" which will be called for tags it
141 doesn't support. The application can then choose to just store the
142 information that comes via the default callback in plain textual format.<br>
144 After this introduction, let's see what the API looks like...<br>
147 <h3><a name="Start_and_end_callbacks"></a>Start and end callbacks</h3>
149 <h4><i>Callbacks for records</i> <br>
151 As a simple example, we will get some information from the header of a
152 GEDCOM file. First, have a look at the following piece of code:<br>
154 <blockquote><code>Gedcom_ctxt <b>my_header_start_cb</b> (int level,
155 Gedcom_val xref, char *tag, int parsed_tag)<br>
157 printf("The header starts\n");<br>
158 return (Gedcom_ctxt)1;<br>
161 void <b>my_header_end_cb</b> (Gedcom_ctxt self)<br>
163 printf("The header ends, context is %d\n", self); /* context
164 will print as "1" */<br>
168 <b>gedcom_subscribe_to_record</b>(REC_HEAD, my_header_start_cb,
169 my_header_end_cb);<br>
171 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
173 Using the <code>gedcom_subscribe_to_record</code> function, the application
174 requests to use the specified callbacks as start and end callback. The end
175 callback is optional: you can pass <code>NULL</code> if you are not interested
176 in the end callback. The identifiers to use as first argument to the
177 function (here <code>REC_HEAD</code>) are described in the <a href="interface.html#Record_identifiers">
178 interface details</a>.<br>
180 From the name of the function it becomes clear that this function is specific
181 to complete records. For the separate elements in records there is
182 another function, which we'll see shortly. Again, the callbacks need
183 to have the signatures as shown in the example.<br>
185 The <code>Gedcom_ctxt</code> type that is used as a result of the start
186 callback and as an argument to the end callback is vital for passing context
187 necessary for the application. This type is meant to be opaque; in
188 fact, it's a void pointer, so you can pass anything via it. The important
189 thing to know is that the context that the application returns in the start
190 callback will be passed in the end callback as an argument, and as we will
191 see shortly, also to all the directly subordinate elements of the record.<br>
193 The <code>tag</code> is the GEDCOM tag in string format, the <code>parsed_tag</code>
194 is an integer, for which symbolic values are defined as <code>TAG_HEAD,</code>
195 <code>TAG_SOUR,</code> <code>TAG_DATA,</code> ... and <code>USERTAG </code><code></code>
196 for the application-specific tags. These values are defined in the
197 header <code>gedcom-tags.h</code> that is installed, and included via <code>
198 gedcom.h</code> (so no need to include <code>gedcom-tags.h</code> yourself).<br>
200 The example passes a simple integer as context, but an application could
201 e.g. pass a <code>struct</code> that will contain the information for the
202 header. In the end callback, the application could then e.g. do some
203 finalizing operations on the <code>struct</code> to put it in its database.<br>
205 (Note that the <code>Gedcom_val</code> type for the <code>xref</code>
206 argument was not discussed, see further for this)<br>
209 <h4><i>Callbacks for elements</i></h4>
210 We will now retrieve the SOUR field (the name of the program that wrote
211 the file) from the header:<br>
213 <blockquote><code>Gedcom_ctxt <b>my_header_source_start_cb</b>(Gedcom_ctxt
215
216 int
217 level,<br>
218
219 char*
220 tag,<br>
221
222 char*
223 raw_value,<br>
224
225 int
226 parsed_tag,<br>
227
228 Gedcom_val
229 parsed_value)<br>
231 char *source = GEDCOM_STRING(parsed_value);<br>
232 printf("This file was written by %s\n", source);<br>
233 return parent;<br>
236 void <b>my_header_source_end_cb</b>(Gedcom_ctxt parent,<br>
237
238 Gedcom_ctxt self,<br>
239
240 Gedcom_val parsed_value)<br>
242 printf("End of the source description\n");<br>
246 <b>gedcom_subscribe_to_element</b>(ELT_HEAD_SOUR,<br>
247
248 my_header_source_start_cb,<br>
249
250 my_header_source_end_cb);<br>
252 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
254 The subscription mechanism for elements is similar, only the signatures
255 of the callbacks differ. The signature for the start callback shows
256 that the context of the parent line (e.g. the <code>struct</code> that describes
257 the header) is passed to this start callback. The callback itself returns
258 here the same context, but this can be its own context object of course.
259 The end callback is called with both the context of the parent and
260 the context of itself, which will be the same in the example. Again,
261 the list of identifiers to use as a first argument for the subscription function
262 are detailed in the <a href="interface.html#Element_identifiers">interface
265 If we look at the other arguments of the start callback, we see the level
266 number (the initial number of the line in the GEDCOM file), the tag (e.g.
267 "SOUR"), and then a raw value, a parsed tag and a parsed value. The
268 raw value is just the raw string that occurs as value on the line next to
269 the tag (in UTF-8 encoding). The parsed value is the meaningful value
270 that is parsed from that raw string. The parsed tag is described in
271 the section for record callbacks.<br>
273 The <code>Gedcom_val</code> type is meant to be an opaque type. The
274 only thing that needs to be known about it is that it can contain specific
275 data types, which have to be retrieved from it using pre-defined macros.
276 These data types are described in the <a href="interface.html#Gedcom_val_types">
277 interface details</a>. <br>
279 Some extra notes:<br>
282 <li>The <code>Gedcom_val</code> argument of the end callback
283 is currently not used. It is there for future enhancements.</li>
284 <li>There is also a <code>Gedcom_val</code> argument in the
285 start callback for records. This argument is currently a string value
286 giving the pointer in string form.</li>
290 <h3><a name="Default_callbacks"></a>Default callbacks<br>
292 As described above, an application doesn't always implement the entire
293 GEDCOM spec, and application-specific tags may have been added by other applications.
294 To preserve this extra data anyway, a default callback can be registered
295 by the application, as in the following example:<br>
297 <blockquote><code>void <b>my_default_cb</b> (Gedcom_ctxt parent,
298 int level, char* tag, char* raw_value, int parsed_tag)<br>
304 <b>gedcom_set_default_callback</b>(my_default_cb);<br>
306 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
308 This callback has a similar signature as the previous ones,
309 but it doesn't contain a parsed value. However, it does contain the
310 parent context, that was returned by the application for the most specific
311 containing tag that the application supported.<br>
313 Suppose e.g. that this callback is called for some tags in the header that
314 are specific to some other application, then our application could make
315 sure that the parent context contains the struct or object that represents
316 the header, and use the default callback here to add the level, tag and
317 raw_value as plain text in a member of that struct or object, thus preserving
318 the information. The application can then write this out when the
319 data is saved again in a GEDCOM file. To make it more specific, consider
320 the following example:<br>
322 <blockquote><code>struct header {<br>
323 char* source;<br>
325 char* extra_text;<br>
328 Gedcom_ctxt my_header_start_cb(int level, Gedcom_val xref, char* tag, int
331 struct header head = my_make_header_struct();<br>
332 return (Gedcom_ctxt)head;<br>
335 void my_default_cb(Gedcom_ctxt parent, int level, char* tag, char* raw_value,
338 struct header head = (struct header)parent;<br>
339 my_header_add_to_extra_text(head, level, tag, raw_value);<br>
342 gedcom_set_default_callback(my_default_cb);<br>
343 gedcom_subscribe_to_record(REC_HEAD, my_header_start, NULL);<br>
345 result = gedcom_parse_file(filename);</code><br>
347 Note that the default callback will be called for any tag that isn't specifically
348 subscribed upon by the application, and can thus be called in various contexts.
349 For simplicity, the example above doesn't take this into account (the
350 <code>parent</code> could be of different types, depending
353 <hr width="100%" size="2">
354 <h2><a name="Other_API_functions"></a>Other API functions<br>
356 Although the above describes the basic interface of libgedcom, there are
357 some other functions that allow to customize the behaviour of the library.
358 These will be explained in the current section.<br>
360 <h3><a name="Debugging"></a>Debugging</h3>
361 The library can generate various debugging output, not only from itself,
362 but also the debugging output generated by the yacc parser. By default,
363 no debugging output is generated, but this can be customized using the following
366 <blockquote><code>void <b>gedcom_set_debug_level</b> (int level,
367 FILE* trace_output)</code><br>
369 The <code>level</code> can be one of the following values:<br>
372 <li>0: no debugging information (this is the default)</li>
373 <li>1: only debugging information from libgedcom
375 <li>2: debugging information from libgedcom and
379 If the <code>trace_output</code> is <code>NULL</code>, debugging information
380 will be written to <code>stderr</code>, otherwise the given file handle
381 is used (which must be open).<br>
384 <h3><a name="Error_treatment"></a>Error treatment</h3>
385 One of the previous sections already described the callback to be registered
386 to get error messages. The library also allows to customize what happens
387 on an error, using the following function:<br>
389 <blockquote><code>void <b>gedcom_set_error_handling</b> (Gedcom_err_mech
390 mechanism)</code><br>
392 The <code>mechanism</code> can be one of:<br>
395 <li><code>IMMED_FAIL</code>: immediately fail the parsing
396 on an error (this is the default)</li>
397 <li><code>DEFER_FAIL</code>: continue parsing after
398 an error, but return a failure code eventually</li>
399 <li><code>IGNORE_ERRORS</code>: continue parsing after
400 an error, return success always</li>
403 This doesn't influence the generation of error or warning messages, only
404 the behaviour of the parser and its return code.<br>
407 <h3><a name="Compatibility_mode"></a>Compatibility mode<br>
409 Applications are not necessarily true to the GEDCOM spec (or use a different
410 version than 5.5). The intention is that the library is resilient
411 to this, and goes in compatibility mode for files written by specific programs
412 (detected via the HEAD.SOUR tag). This compatibility mode can be enabled
413 and disabled via the following function:<br>
415 <blockquote><code>void <b>gedcom_set_compat_handling</b>
416 (int enable_compat)</code><br>
418 The argument can be:<br>
421 <li>0: disable compatibility mode</li>
422 <li>1: allow compatibility mode (this is the default)<br>
426 Note that, currently, no actual compatibility code is present, but this
427 is on the to-do list.<br>
429 <hr width="100%" size="2">
430 <pre>$Id$<br>$Name$<br></pre>