1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><title>Using the GEDCOM parser library</title>
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6 <h1 align="center">Using the GEDCOM parser library</h1>
12 <li><a href="#anchor">Overview</a></li>
13 <li><a href="#Error_handling">Error handling</a></li>
14 <li><a href="#Data_callback_mechanism">Data callback mechanism</a></li>
17 <li><a href="#Start_and_end_callbacks">Start and end callbacks</a></li>
18 <li><a href="#Default_callbacks">Default callbacks</a></li>
21 <li><a href="#Other_API_functions">Other API functions</a></li>
24 <li><a href="#Debugging">Debugging</a></li>
25 <li><a href="#Error_treatment">Error treatment</a></li>
26 <li><a href="#Compatibility_mode">Compatibility mode</a></li>
29 <li><a href="interface.html">Interface details</a><br>
34 <hr width="100%" size="2">
35 <h2><a name="Overview"></a>Overview<br>
37 The GEDCOM parser library is built as a callback-based parser (comparable
38 to the SAX interface of XML). It comes with:<br>
41 <li>a library (<code>libgedcom.so</code>), to be linked in the application
43 <li>a header file (<code>gedcom.h</code>), to be used in the sources
44 of the application program</li>
45 <li>a header file (<code>gedcom-tags.h</code>) that is also installed,
46 but that is automatically included via <code>gedcom.h</code><br>
50 Next to these, there is also a data directory in <code>$PREFIX/share/gedcom-parse</code>
51 that contains some additional stuff, but which is not immediately
52 important at first. I'll leave the description of the data directory
55 The very simplest call of the gedcom parser is simply the following
56 piece of code (include of the gedcom header is assumed, as everywhere in
59 <blockquote><code>int result;<br>
61 result = <b>gedcom_parse_file</b>("myfamily.ged");<br>
63 Although this will not provide much information, one thing it does
64 is parse the entire file and return the result. The function returns
65 0 on success and 1 on failure. No other information is available using
66 this function only.<br>
68 The next sections will refine this to be able to have meaningful errors
69 and the actual data that is in the file.<br>
71 <hr width="100%" size="2">
72 <h2><a name="Error_handling"></a>Error handling</h2>
73 Since this is a relatively simple topic, it is discussed before the
74 actual callback mechanism, although it also uses a callback...<br>
76 The library can be used in several different circumstances, both terminal-based
77 as GUI-based. Therefore, it leaves the actual display of the error
78 message up to the application. For this, the application needs to register
79 a callback before parsing the GEDCOM file, which will be called by the library
80 on errors, warnings and messages.<br>
82 A typical piece of code would be:<br>
84 <blockquote><code>void <b>my_message_handler</b> (Gedcom_msg_type type,
90 <b>gedcom_set_message_handler</b>(my_message_handler);<br>
92 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
94 In the above piece of code, <code>my_message_handler</code> is the callback
95 that will be called for errors (<code>type=ERROR</code>), warnings (<code>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
102 encoded in UTF-8 (see "Why UTF-8?" <i>LINK TBD</i>). Also,
103 no newline is appended, so that the application program can use it in any
104 way it wants. Warnings are similar, but use "Warning" instead of
105 "Error". Messages are plain text, without any prefix.<br>
107 With this in place, the resulting code will already show errors and
108 warnings produced by the parser, e.g. on the terminal if a simple <code>
109 printf</code> 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
114 of 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
127 a subset of the GEDCOM standard, certainly a program that is still under
128 development. Moreover, under GEDCOM it is allowed for an application
129 to define its own tags, which will typically not be supported by another
130 application. Still, in that case, data preservation is important;
131 it would hardly be accepted that information that is not understood by a
132 certain program is just removed.<br>
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
137 it doesn't support. The application can then choose to just store
138 the information 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
148 a 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,
152
153 Gedcom_val xref, <br>
154
155 char *tag, <br>
156
157 char *raw_value,<br>
158
159 int parsed_tag, <br>
160
161 Gedcom_val parsed_value)<br>
163 printf("The header starts\n");<br>
164 return (Gedcom_ctxt)1;<br>
167 void <b>my_header_end_cb</b> (Gedcom_ctxt self)<br>
169 printf("The header ends, context is %d\n", (int)self); /* context
170 will print as "1" */<br>
174 <b>gedcom_subscribe_to_record</b>(REC_HEAD, my_header_start_cb,
175 my_header_end_cb);<br>
177 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
179 Using the <code>gedcom_subscribe_to_record</code> function, the application
180 requests to use the specified callbacks as start and end callback. The end
181 callback is optional: you can pass <code>NULL</code> if you are not interested
182 in the end callback. The identifiers to use as first argument to
183 the function (here <code>REC_HEAD</code>) are described in the <a href="interface.html#Record_identifiers">
184 interface details</a>.<br>
186 From the name of the function it becomes clear that this function is
187 specific to complete records. For the separate elements in records
188 there is another function, which we'll see shortly. Again, the callbacks
189 need to have the signatures as shown in the example.<br>
191 The <code>Gedcom_ctxt</code> type that is used as a result of the start
192 callback and as an argument to the end callback is vital for passing context
193 necessary for the application. This type is meant to be opaque; in
194 fact, it's a void pointer, so you can pass anything via it. The important
195 thing to know is that the context that the application returns in the start
196 callback will be passed in the end callback as an argument, and as we will
197 see shortly, also to all the directly subordinate elements of the record.<br>
199 The <code>tag</code> is the GEDCOM tag in string format, the <code>parsed_tag</code>
200 is an integer, for which symbolic values are defined as <code>TAG_HEAD,</code>
201 <code>TAG_SOUR,</code> <code>TAG_DATA,</code> ... and <code>USERTAG </code><code></code>
202 for the application-specific tags. These values are defined in the
203 header <code>gedcom-tags.h</code> that is installed, and included via <code>
204 gedcom.h</code> (so no need to include <code>gedcom-tags.h</code> yourself).<br>
206 The example passes a simple integer as context, but an application could
207 e.g. pass a <code>struct</code> (or an object in a C++ application) that will contain the information for the
208 header. In the end callback, the application could then e.g. do some
209 finalizing operations on the <code>struct</code> to put it in its database.<br>
211 (Note that the <code>Gedcom_val</code> type for the <code>xref</code>
212 and <code>parsed_value</code> arguments was not discussed, see further
216 <h4><i>Callbacks for elements</i></h4>
217 We will now retrieve the SOUR field (the name of the program that wrote
218 the file) from the header:<br>
220 <blockquote><code>Gedcom_ctxt <b>my_header_source_start_cb</b>(Gedcom_ctxt
222
223 int
224 level,<br>
225
226 char*
227 tag,<br>
228
229 char*
230 raw_value,<br>
231
232 int
233 parsed_tag,<br>
234
235 Gedcom_val
236 parsed_value)<br>
238 char *source = GEDCOM_STRING(parsed_value);<br>
239 printf("This file was written by %s\n", source);<br>
240 return parent;<br>
243 void <b>my_header_source_end_cb</b>(Gedcom_ctxt parent,<br>
244
245 Gedcom_ctxt self,<br>
246
247 Gedcom_val parsed_value)<br>
249 printf("End of the source description\n");<br>
253 <b>gedcom_subscribe_to_element</b>(ELT_HEAD_SOUR,<br>
254
255 my_header_source_start_cb,<br>
256
257 my_header_source_end_cb);<br>
259 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
261 The subscription mechanism for elements is similar, only the signatures
262 of the callbacks differ. The signature for the start callback shows
263 that the context of the parent line (here e.g. the <code>struct</code> that
264 describes the header) is passed to this start callback. The callback
265 itself returns here in this example the same context, but this can be its own context object
266 of course. The end callback is called with both the context of the
267 parent and the context of itself, which in this example will be the same.
268 Again, the list of identifiers to use as a first argument for the
269 subscription function are detailed in the <a href="interface.html#Element_identifiers">
270 interface details</a> .<br>
272 If we look at the other arguments of the start callback, we see the
273 level number (the initial number of the line in the GEDCOM file), the tag
274 (e.g. "SOUR"), and then a raw value, a parsed tag and a parsed value. The
275 raw value is just the raw string that occurs as value on the line next to
276 the tag (in UTF-8 encoding). The parsed value is the meaningful value
277 that is parsed from that raw string. The parsed tag is described in
278 the section for record callbacks above.<br>
280 The <code>Gedcom_val</code> type is meant to be an opaque type. The
281 only thing that needs to be known about it is that it can contain specific
282 data types, which have to be retrieved from it using pre-defined macros.
283 These data types are described in the <a href="interface.html#Gedcom_val_types">
284 interface details</a>. <br>
286 Some extra notes:<br>
289 <li>The <code>Gedcom_val</code> argument of the end callback
290 is currently not used. It is there for future enhancements.</li>
291 <li>There are also two <code>Gedcom_val</code> arguments in
292 the start callback for records. The first one (<code>xref</code>) contains the <code>xref_value</code> corresponding to the cross-reference (or <code>NULL</code> if there isn't one), the second one (<code>parsed_value</code>) contains the value that is parsed from the <code>raw_value</code>. See the <a href="interface.html#Record_identifiers">interface details</a>.</li>
296 <h3><a name="Default_callbacks"></a>Default callbacks<br>
298 As described above, an application doesn't always implement the entire
299 GEDCOM spec, and application-specific tags may have been added by other applications.
300 To preserve this extra data anyway, a default callback can be registered
301 by the application, as in the following example:<br>
303 <blockquote><code>void <b>my_default_cb</b> (Gedcom_ctxt parent,
304 int level, char* tag, char* raw_value, int parsed_tag)<br>
310 <b>gedcom_set_default_callback</b>(my_default_cb);<br>
312 result = <b>gedcom_parse_file</b>("myfamily.ged");</code><br>
314 This callback has a similar signature as the previous ones,
315 but it doesn't contain a parsed value. However, it does contain the
316 parent context, that was returned by the application for the most specific
317 containing tag that the application supported.<br>
319 Suppose e.g. that this callback is called for some tags in the header
320 that are specific to some other application, then our application could make
321 sure that the parent context contains the struct or object that represents
322 the header, and use the default callback here to add the level, tag and
323 raw_value as plain text in a member of that struct or object, thus preserving
324 the information. The application can then write this out when the
325 data is saved again in a GEDCOM file. To make it more specific, consider
326 the following example:<br>
328 <blockquote><code>struct header {<br>
329 char* source;<br>
331 char* extra_text;<br>
334 Gedcom_ctxt my_header_start_cb(int level, Gedcom_val xref, char* tag,
336
337 int parsed_tag, Gedcom_val parsed_value)<br>
339 struct header head = my_make_header_struct();<br>
340 return (Gedcom_ctxt)head;<br>
343 void my_default_cb(Gedcom_ctxt parent, int level, char* tag, char* raw_value,
346 struct header head = (struct header)parent;<br>
347 my_header_add_to_extra_text(head, level, tag, raw_value);<br>
350 gedcom_set_default_callback(my_default_cb);<br>
351 gedcom_subscribe_to_record(REC_HEAD, my_header_start, NULL);<br>
353 result = gedcom_parse_file(filename);</code><br>
355 Note that the default callback will be called for any tag that isn't
356 specifically subscribed upon by the application, and can thus be called
357 in various contexts. For simplicity, the example above doesn't take
358 this into account (the <code>parent</code> could be of different
359 types, depending on the context).<br>
361 Note also that the default callback is not called when the parent context is <code>NULL</code><code></code>. This is e.g. the case if none of the "upper" tags has been subscribed upon.<br>
363 <hr width="100%" size="2">
365 <h2><a name="Other_API_functions"></a>Other API functions<br>
367 Although the above describes the basic interface of libgedcom, there
368 are some other functions that allow to customize the behaviour of the library.
369 These will be explained in the current section.<br>
371 <h3><a name="Debugging"></a>Debugging</h3>
372 The library can generate various debugging output, not only from itself,
373 but also the debugging output generated by the yacc parser. By default,
374 no debugging output is generated, but this can be customized using the
375 following function:<br>
377 <blockquote><code>void <b>gedcom_set_debug_level</b> (int level,
378 FILE* trace_output)</code><br>
380 The <code>level</code> can be one of the following values:<br>
383 <li>0: no debugging information (this is the
385 <li>1: only debugging information from libgedcom
387 <li>2: debugging information from libgedcom and
391 If the <code>trace_output</code> is <code>NULL</code>, debugging information
392 will be written to <code>stderr</code>, otherwise the given file handle
393 is used (which must be open).<br>
396 <h3><a name="Error_treatment"></a>Error treatment</h3>
397 One of the previous sections already described the callback to be registered
398 to get error messages. The library also allows to customize what
399 happens on an error, using the following function:<br>
401 <blockquote><code>void <b>gedcom_set_error_handling</b> (Gedcom_err_mech
402 mechanism)</code><br>
404 The <code>mechanism</code> can be one of:<br>
408 <li><code>IMMED_FAIL</code>: immediately fail the
409 parsing on an error (this is the default)</li>
410 <li><code>DEFER_FAIL</code>: continue parsing after
411 an error, but return a failure code eventually</li>
412 <li><code>IGNORE_ERRORS</code>: continue parsing
413 after an error, return success always</li>
417 This doesn't influence the generation of error or warning messages, only
418 the behaviour of the parser and its return code.<br>
422 <h3><a name="Compatibility_mode"></a>Compatibility mode<br>
424 Applications are not necessarily true to the GEDCOM spec (or use a different
425 version than 5.5). The intention is that the library is resilient
426 to this, and goes in compatibility mode for files written by specific programs
427 (detected via the HEAD.SOUR tag). This compatibility mode can be
428 enabled and disabled via the following function:<br>
431 <blockquote><code>void <b>gedcom_set_compat_handling</b>
432 (int enable_compat)</code><br>
434 The argument can be:<br>
438 <li>0: disable compatibility mode</li>
439 <li>1: allow compatibility mode (this is the default)<br>
444 Note that, currently, no actual compatibility code is present, but this
445 is on the to-do list.<br>
448 <hr width="100%" size="2">
450 <pre><font size="-1">$Id$<br>$Name$</font><br></pre>