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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>
51 Next to these, there is also a data directory in <code>$PREFIX/share/gedcom-parse</code>
52 that contains some additional stuff, but which is not immediately important
53 at first. I'll leave the description of the data directory for later.<br>
55 The very simplest call of the gedcom parser is simply the following piece
56 of code (include of the gedcom header is assumed, as everywhere in this manual):<br>
58 <blockquote><code>int result;<br>
60 result = <b>gedcom_parse_file</b>("myfamily.ged");<br>
62 Although this will not provide much information, one thing it does is
63 parse the entire file and return the result. The function returns
64 0 on success and 1 on failure. No other information is available using
65 this function only.<br>
67 The next sections will refine this to be able to have meaningful errors
68 and the actual data that is in the file.<br>
70 <hr width="100%" size="2">
71 <h2><a name="Error_handling"></a>Error handling</h2>
72 Since this is a relatively simple topic, it is discussed before the actual
73 callback mechanism, although it also uses a callback...<br>
75 The library can be used in several different circumstances, both terminal-based
76 as GUI-based. Therefore, it leaves the actual display of the error
77 message up to the application. For this, the application needs to register
78 a callback before parsing the GEDCOM file, which will be called by the library
79 on errors, warnings and messages.<br>
81 A typical piece of code would be:<br>
83 <blockquote><code>void <b>my_message_handler</b> (Gedcom_msg_type type,
89 <b>gedcom_set_message_handler</b>(my_message_handler);<br>
91 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
93 In the above piece of code, <code>my_message_handler</code> is the callback
94 that will be called for errors (<code>type=ERROR</code>), warnings (<code>
95 type=WARNING</code>) and messages (<code>type=MESSAGE</code>). The
96 callback must have the signature as in the example. For errors, the
97 <code> msg</code> passed to the callback will have the format:<br>
99 <blockquote><code>Error on line</code> <i><lineno></i>: <i><actual_message></i><br>
101 Note that the entire string will be properly internationalized, and encoded
102 in UTF-8 (see "Why UTF-8?" <i>LINK TBD</i>). Also, no newline
103 is appended, so that the application program can use it in any way it wants.
104 Warnings are similar, but use "Warning" instead of "Error". Messages
105 are plain text, without any prefix.<br>
107 With this in place, the resulting code will already show errors and warnings
108 produced by the parser, e.g. on the terminal if a simple <code>printf</code>
109 is used in the message handler.<br>
111 <hr width="100%" size="2">
112 <h2><a name="Data_callback_mechanism"></a>Data callback mechanism</h2>
113 The most important use of the parser is of course to get the data out of
114 the GEDCOM file. As already mentioned, the parser uses a callback
115 mechanism for that. In fact, the mechanism involves two levels.<br>
117 The primary level is that each of the sections in a GEDCOM file is notified
118 to the application code via a "start element" callback and an "end element"
119 callback (much like in a SAX interface for XML), i.e. when a line containing
120 a certain tag is parsed, the "start element" callback is called for that
121 tag, and when all its subordinate lines with their tags have been processed,
122 the "end element" callback is called for the original tag. Since GEDCOM
123 is hierarchical, this results in properly nested calls to appropriate "start
124 element" and "end element" callbacks.<br>
126 However, it would be typical for a genealogy program to support only a
127 subset of the GEDCOM standard, certainly a program that is still under development.
128 Moreover, under GEDCOM it is allowed for an application to define
129 its own tags, which will typically not be supported by another application.
130 Still, in that case, data preservation is important; it would hardly
131 be accepted that information that is not understood by a certain program
134 Therefore, the second level of callbacks involves a "default callback".
135 An application needs to subscribe to callbacks for tags it does support,
136 and need to provide a "default callback" which will be called for tags it
137 doesn't support. The application can then choose to just store the information
138 that comes via the default callback in plain textual format.<br>
140 After this introduction, let's see what the API looks like...<br>
143 <h3><a name="Start_and_end_callbacks"></a>Start and end callbacks</h3>
145 <h4><i>Callbacks for records</i> <br>
147 As a simple example, we will get some information from the header of a
148 GEDCOM file. First, have a look at the following piece of code:<br>
150 <blockquote><code>Gedcom_ctxt <b>my_header_start_cb</b> (int level,
151 Gedcom_val xref, char *tag)<br>
153 printf("The header starts\n");<br>
154 return (Gedcom_ctxt)1;<br>
157 void <b>my_header_end_cb</b> (Gedcom_ctxt self)<br>
159 printf("The header ends, context is %d\n", self); /* context
160 will print as "1" */<br>
164 <b>gedcom_subscribe_to_record</b>(REC_HEAD, my_header_start_cb,
165 my_header_end_cb);<br>
167 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
169 Using the <code>gedcom_subscribe_to_record</code> function, the application
170 requests to use the specified callbacks as start and end callback. The end
171 callback is optional: you can pass <code>NULL</code> if you are not interested
172 in the end callback. The identifiers to use as first argument to the
173 function (here <code>REC_HEAD</code>) are described in the <a href="interface.html#Record_identifiers">
174 interface details</a>.<br>
176 From the name of the function it becomes clear that this function is specific
177 to complete records. For the separate elements in records there is
178 another function, which we'll see shortly. Again, the callbacks need
179 to have the signatures as shown in the example.<br>
181 The <code>Gedcom_ctxt</code> type that is used as a result of the start
182 callback and as an argument to the end callback is vital for passing context
183 necessary for the application. This type is meant to be opaque; in fact,
184 it's a void pointer, so you can pass anything via it. The important
185 thing to know is that the context that the application returns in the start
186 callback will be passed in the end callback as an argument, and as we will
187 see shortly, also to all the directly subordinate elements of the record.<br>
189 The example passes a simple integer as context, but an application could
190 e.g. pass a <code>struct</code> that will contain the information for the
191 header. In the end callback, the application could then e.g. do some
192 finalizing operations on the <code>struct</code> to put it in its database.<br>
194 (Note that the <code>Gedcom_val</code> type for the <code>xref</code> argument
195 was not discussed, see further for this)<br>
198 <h4><i>Callbacks for elements</i></h4>
199 We will now retrieve the SOUR field (the name of the program that wrote
200 the file) from the header:<br>
202 <blockquote><code>Gedcom_ctxt <b>my_header_source_start_cb</b>(Gedcom_ctxt
204
205 int
206 level,<br>
207
208 char*
210
211 char*
212 raw_value,<br>
213
214 Gedcom_val parsed_value)<br>
216 char *source = GEDCOM_STRING(parsed_value);<br>
217 printf("This file was written by %s\n", source);<br>
218 return parent;<br>
221 void <b>my_header_source_end_cb</b>(Gedcom_ctxt parent,<br>
222
223 Gedcom_ctxt self,<br>
224
225 Gedcom_val parsed_value)<br>
227 printf("End of the source description\n");<br>
231 <b>gedcom_subscribe_to_element</b>(ELT_HEAD_SOUR,<br>
232
233 my_header_source_start_cb,<br>
234
235 my_header_source_end_cb);<br>
237 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
239 The subscription mechanism for elements is similar, only the signatures
240 of the callbacks differ. The signature for the start callback shows
241 that the context of the parent line (e.g. the <code>struct</code> that describes
242 the header) is passed to this start callback. The callback itself
243 returns here the same context, but this can be its own context object of
244 course. The end callback is called with both the context of the parent
245 and the context of itself, which will be the same in the example. Again,
246 the list of identifiers to use as a first argument for the subscription function
247 are detailed in the <a href="interface.html#Element_identifiers">interface
250 If we look at the other arguments of the start callback, we see the level
251 number (the initial number of the line in the GEDCOM file), the tag (e.g.
252 "SOUR"), and then a raw value and a parsed value. The raw value is
253 just the raw string that occurs as value on the line next to the tag (in
254 UTF-8 encoding). The parsed value is the meaningful value that is parsed
255 from that raw string.<br>
257 The <code>Gedcom_val</code> type is meant to be an opaque type. The
258 only thing that needs to be known about it is that it can contain specific
259 data types, which have to be retrieved from it using pre-defined macros.
260 These data types are described in the <a href="interface.html#Gedcom_val_types">
261 interface details</a>. <br>
263 Some extra notes:<br>
266 <li>The <code>Gedcom_val</code> argument of the end callback
267 is currently not used. It is there for future enhancements.</li>
268 <li>There is also a <code>Gedcom_val</code> argument in the
269 start callback for records. This argument is currently a string value
270 giving the pointer in string form.</li>
274 <h3><a name="Default_callbacks"></a>Default callbacks<br>
276 As described above, an application doesn't always implement the entire
277 GEDCOM spec, and application-specific tags may have been added by other applications.
278 To preserve this extra data anyway, a default callback can be registered
279 by the application, as in the following example:<br>
281 <blockquote><code>void <b>my_default_cb</b> (Gedcom_ctxt parent,
282 int level, char* tag, char* raw_value)<br>
288 <b>gedcom_set_default_callback</b>(my_default_cb);<br>
290 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
292 This callback has a similar signature as the previous ones, but
293 it doesn't contain a parsed value. However, it does contain the parent
294 context, that was returned by the application for the most specific containing
295 tag that the application supported.<br>
297 Suppose e.g. that this callback is called for some tags in the header that
298 are specific to some other application, then our application could make sure
299 that the parent context contains the struct or object that represents the
300 header, and use the default callback here to add the level, tag and raw_value
301 as plain text in a member of that struct or object, thus preserving the information.
302 The application can then write this out when the data is saved again
303 in a GEDCOM file. To make it more specific, consider the following example:<br>
305 <blockquote><code>struct header {<br>
306 char* source;<br>
308 char* extra_text;<br>
311 Gedcom_ctxt my_header_start_cb(int level, Gedcom_val xref, char* tag)<br>
313 struct header head = my_make_header_struct();<br>
314 return (Gedcom_ctxt)head;<br>
317 void my_default_cb(Gedcom_ctxt parent, int level, char* tag, char* raw_value)<br>
319 struct header head = (struct header)parent;<br>
320 my_header_add_to_extra_text(head, level, tag, raw_value);<br>
323 gedcom_set_default_callback(my_default_cb);<br>
324 gedcom_subscribe_to_record(REC_HEAD, my_header_start, NULL);<br>
326 result = gedcom_parse_file(filename);</code><br>
328 Note that the default callback will be called for any tag that isn't specifically
329 subscribed upon by the application, and can thus be called in various contexts.
330 For simplicity, the example above doesn't take this into account (the
331 <code>parent</code> could be of different types, depending on
334 <hr width="100%" size="2">
335 <h2><a name="Other_API_functions"></a>Other API functions<br>
337 Although the above describes the basic interface of libgedcom, there are
338 some other functions that allow to customize the behaviour of the library.
339 These will be explained in the current section.<br>
341 <h3><a name="Debugging"></a>Debugging</h3>
342 The library can generate various debugging output, not only from itself,
343 but also the debugging output generated by the yacc parser. By default,
344 no debugging output is generated, but this can be customized using the following
347 <blockquote><code>void <b>gedcom_set_debug_level</b> (int level,
348 FILE* trace_output)</code><br>
350 The <code>level</code> can be one of the following values:<br>
353 <li>0: no debugging information (this is the default)</li>
354 <li>1: only debugging information from libgedcom
356 <li>2: debugging information from libgedcom and
360 If the <code>trace_output</code> is <code>NULL</code>, debugging information
361 will be written to <code>stderr</code>, otherwise the given file handle is
362 used (which must be open).<br>
365 <h3><a name="Error_treatment"></a>Error treatment</h3>
366 One of the previous sections already described the callback to be registered
367 to get error messages. The library also allows to customize what happens
368 on an error, using the following function:<br>
370 <blockquote><code>void <b>gedcom_set_error_handling</b> (Gedcom_err_mech
371 mechanism)</code><br>
373 The <code>mechanism</code> can be one of:<br>
376 <li><code>IMMED_FAIL</code>: immediately fail the parsing
377 on an error (this is the default)</li>
378 <li><code>DEFER_FAIL</code>: continue parsing after
379 an error, but return a failure code eventually</li>
380 <li><code>IGNORE_ERRORS</code>: continue parsing after
381 an error, return success always</li>
384 This doesn't influence the generation of error or warning messages, only
385 the behaviour of the parser and its return code.<br>
388 <h3><a name="Compatibility_mode"></a>Compatibility mode<br>
390 Applications are not necessarily true to the GEDCOM spec (or use a different
391 version than 5.5). The intention is that the library is resilient to
392 this, and goes in compatibility mode for files written by specific programs
393 (detected via the HEAD.SOUR tag). This compatibility mode can be enabled
394 and disabled via the following function:<br>
396 <blockquote><code>void <b>gedcom_set_compat_handling</b>
397 (int enable_compat)</code><br>
399 The argument can be:<br>
402 <li>0: disable compatibility mode</li>
403 <li>1: allow compatibility mode (this is the default)<br>
407 Note that, currently, no actual compatibility code is present, but this
408 is on the to-do list.<br>
410 <hr width="100%" size="2">
411 <pre>$Id$<br>$Name$<br></pre>