Using the GEDCOM parser library


Index

Overview

The GEDCOM parser library is built as a callback-based parser (comparable to the SAX interface of XML).  It comes with:
Next to these, there is also a data directory in $PREFIX/share/gedcom-parse that contains some additional stuff, but which is not immediately important at first.  I'll leave the description of the data directory for later.

The very simplest call of the gedcom parser is simply the following piece of code (include of the gedcom header is assumed, as everywhere in this manual):
int result;
...
result = gedcom_parse_file("myfamily.ged");
Although this will not provide much information, one thing it does is parse the entire file and return the result.  The function returns 0 on success and 1 on failure.  No other information is available using this function only.

The next sections will refine this to be able to have meaningful errors and the actual data that is in the file.

Error handling

Since this is a relatively simple topic, it is discussed before the actual callback mechanism, although it also uses a callback...

The library can be used in several different circumstances, both terminal-based as GUI-based.  Therefore, it leaves the actual display of the error message up to the application.  For this, the application needs to register a callback before parsing the GEDCOM file, which will be called by the library on errors, warnings and messages.

A typical piece of code would be:
void my_message_handler (Gedcom_msg_type type, char *msg)
{
  ...
}
...
gedcom_set_message_handler(my_message_handler);
...
result = gedcom_parse_file("myfamily.ged");
In the above piece of code, my_message_handler is the callback that will be called for errors (type=ERROR), warnings ( type=WARNING) and messages (type=MESSAGE).  The callback must have the signature as in the example.  For errors, the msg passed to the callback will have the format:
Error on line <lineno>: <actual_message>
Note that the entire string will be properly internationalized, and encoded in UTF-8 (see "Why UTF-8?"  LINK TBD).  Also, no newline is appended, so that the application program can use it in any way it wants.  Warnings are similar, but use "Warning" instead of "Error".  Messages are plain text, without any prefix.

With this in place, the resulting code will already show errors and warnings produced by the parser, e.g. on the terminal if a simple printf is used in the message handler.

Data callback mechanism

The most important use of the parser is of course to get the data out of the GEDCOM file.  As already mentioned, the parser uses a callback mechanism for that.  In fact, the mechanism involves two levels.

The primary level is that each of the sections in a GEDCOM file is notified to the application code via a "start element" callback and an "end element" callback (much like in a SAX interface for XML), i.e. when a line containing a certain tag is parsed, the "start element" callback is called for that tag, and when all its subordinate lines with their tags have been processed, the "end element" callback is called for the original tag.  Since GEDCOM is hierarchical, this results in properly nested calls to appropriate "start element" and "end element" callbacks.

However, it would be typical for a genealogy program to support only a subset of the GEDCOM standard, certainly a program that is still under development.  Moreover, under GEDCOM it is allowed for an application to define its own tags, which will typically not  be supported by another application.  Still, in that case, data preservation is important; it would hardly be accepted that information that is not understood by a certain program is just removed.

Therefore, the second level of callbacks involves a "default callback".  An application needs to subscribe to callbacks for tags it does support, and need to provide a "default callback" which will be called for tags it doesn't support.  The application can then choose to just store the information that comes via the default callback in plain textual format.

After this introduction, let's see what the API looks like...

Start and end callbacks

Callbacks for records

As a simple example, we will get some information from the header of a GEDCOM file.  First, have a look at the following piece of code:
Gedcom_ctxt my_header_start_cb (int level, Gedcom_val xref, char *tag)
{
  printf("The header starts\n");
  return (Gedcom_ctxt)1;
}

void my_header_end_cb (Gedcom_ctxt self)
{
  printf("The header ends, context is %d\n", self);   /* context will print as "1" */
}

...
gedcom_subscribe_to_record(REC_HEAD, my_header_start_cb, my_header_end_cb);
...
result = gedcom_parse_file("myfamily.ged");
Using the gedcom_subscribe_to_record function, the application requests to use the specified callbacks as start and end callback. The end callback is optional: you can pass NULL if you are not interested in the end callback.  The identifiers to use as first argument to the function (here REC_HEAD) are described in TBD (use the header file for now...).

From the name of the function it becomes clear that this function is specific to complete records.  For the separate elements in records there is another function, which we'll see shortly.  Again, the callbacks need to have the signatures as shown in the example.

The Gedcom_ctxt type that is used as a result of the start callback and as an argument to the end callback is vital for passing context necessary for the application.  This type is meant to be opaque; in fact, it's a void pointer, so you can pass anything via it.  The important thing to know is that the context that the application returns in the start callback will be passed in the end callback as an argument, and as we will see shortly, also to all the directly subordinate elements of the record.

The example passes a simple integer as context, but an application could e.g. pass a struct that will contain the information for the header.  In the end callback, the application could then e.g. do some finalizing operations on the struct to put it in its database.

(Note that the Gedcom_val type for the xref argument was not discussed, see further for this)

Callbacks for elements

We will now retrieve the SOUR field (the name of the program that wrote the file) from the header:
Gedcom_ctxt my_header_source_start_cb(Gedcom_ctxt parent,
                                      int         level,
                                      char*       tag,
                                      char*       raw_value,
                                      Gedcom_val  parsed_value)
{
  char *source = GEDCOM_STRING(parsed_value);
  printf("This file was written by %s\n", source);
  return parent;
}

void my_header_source_end_cb(Gedcom_ctxt parent,
                             Gedcom_ctxt self,
                             Gedcom_val  parsed_value)
{
  printf("End of the source description\n");
}

...
gedcom_subscribe_to_element(ELT_HEAD_SOUR,
                            my_header_source_start_cb,
                            my_header_source_end_cb);
...
result = gedcom_parse_file("myfamily.ged");
The subscription mechanism for elements is similar, only the signatures of the callbacks differ.  The signature for the start callback shows that the context of the parent line (e.g. the struct that describes the header) is passed to this start callback.  The callback itself returns here the same context, but this can be its own context object of course.  The end callback is called with both the context of the parent and the context of itself, which will be the same in the example.

If we look at the other arguments of the start callback, we see the level number (the initial number of the line in the GEDCOM file), the tag (e.g. "SOUR"), and then a raw value and a parsed value.  The raw value is just the raw string that occurs as value on the line next to the tag (in UTF-8 encoding).  The parsed value is the meaningful value that is parsed from that raw string.

The Gedcom_val type is meant to be an opaque type.  The only thing that needs to be known about it is that it can contain specific data types, which have to be retrieved from it using pre-defined macros.  Currently, the specific types are (with val of type Gedcom_val):


 type checker
 cast operator
null value
 GEDCOM_IS_NULL(val)
 N/A
string
 GEDCOM_IS_STRING(val)
 char* str = GEDCOM_STRING(val);
date
 GEDCOM_IS_DATE(val)
 struct date_value dv = GEDCOM_DATE(val) ;

The null value is used for when the GEDCOM spec doesn't allow a value, or when an optional value is allowed but none is given.
 
The string value is the most general used value currently, for all those values that don't have a more specific meaning.  In essence, the value that is returned by GEDCOM_STRING is always the same as the raw_value passed to the start callback, and is thus in fact redundant.

The date value is used for all elements that return a date.  (Description of struct date_value TBD: look in the header file for the moment).

The type checker returns a true or a false value according to the type of the value, but this is in principle only necessary in the rare circumstances that two types are possible, or where an optional value can be provided.  In most cases, the type is fixed for a specific tag (types per tag to be described).

Some extra notes:

Default callbacks

TO BE COMPLETED
$Id$
$Name$