The Gedcom parser library internals

The intention of this page is to provide some explanation of the gedcom parser, to aid development on and with it.  First, some practical issues of testing with the parser will be explained.



Basic testing

You should be able to perform a basic test using the commands:
make check

If everything goes OK, you'll see that some gedcom files are parsed, and that each parse is successful.  Note that some of the used gedcom files are made by Heiner Eichmann and are an excellent way to test gedcom parsers thoroughly.

Preparing for further testing

Some more detailed tests are possible, via the testgedcom program that is generated by make.  

However, since the output that testgedcom generates is in UTF-8 format (more on this later), some preparation is necessary to have a full view on it.  Basically, you need a terminal that understands and can display UTF-8 encoded characters, and you need to proper fonts installed to display them.  I'll give some advice on this here, based on the Red Hat 7.1 distribution that I use, with glibc 2.2 and XFree86 4.0.x.  Any other distribution that has the same or newer versions for these components should give the same results.

For the first issue, the UTF-8 capable terminal, the safest bet is to use xterm in its unicode mode (which is supported by the xterm coming with XFree86 4.0.x).  UTF-8 capabilities have only recently been added to gnome-terminal, so probably that is not in your distribution yet (it certainly isn't in Red Hat 7.1).

For the second issue, you'll need the ISO 10646-1 fonts.  These come also with XFree86 4.0.x.

The way to start xterm in unicode mode is then e.g. (put everything on 1 line !):
LANG=en_GB.UTF-8 xterm -bg 'black' -fg 'DarkGrey' -cm -fn '-Misc-Fixed-Medium-R-SemiCondensed--13-120-75-75-C-60-ISO10646-1'
This first sets the LANG variable to a locale that uses UTF-8, and then starts xterm with a proper Unicode font.  Some sample UTF-8 plain text files can be found here .  Just cat them on the command line and see the result.

Testing the parser with debugging

Given the UTF-8 capable terminal, you can now let the testgedcom program print the values that it parses.  An example of a command line is (in the top directory):
./testgedcom -dg t/input/ulhc.ged
The -dg option instructs the parser to show its own debug messages  (see ./testgedcom -h for the full set of options).  If everything is OK, you'll see the values from the gedcom file, containing a lot of special characters.

For the ANSEL test file (t/ansel.ged), you have to set the environment variable GCONV_PATH to the ansel subdirectory of the top directory:
export GCONV_PATH=./ansel
./testgedcom -dg t/input/ansel.ged
This is because for the ANSEL character set an extra module is needed for the iconv library (more on this later).  But again, this should show a lot of special characters.

Testing the lexers separately

The lexers themselves can be tested separately.  For the 1-byte lexer (i.e. supporting the encodings with 1 byte per characters, such as ASCII, ANSI and ANSEL), the command would be (in the gedcom subdirectory):
make lexer_1byte
This will generate a lexer program that can process e.g. the t/input/allged.ged test file.  Simply cat the file through the lexer on standard input and you should get all the tokens in the file.  Similar tests can be done using make lexer_hilo and make lexer_lohi (for the unicode lexers).  In each of the cases you need to know yourself which of the test files are appropriate to pass through the lexer.

This concludes the testing setup.  Now for some explanations...

Structure of the parser

I see the structure of a program using the gedcom parser as follows:

Gedcom parsing scheme

The parser is based on lex/yacc, which means that a module generated by lex takes the inputfile and determines the tokens in that file (i.e. the smallest units, such as numbers, line terminators, GEDCOM tags, characters in GEDCOM values...).  These tokens are passed to the parser module, which is generated by yacc, to parse the syntax of the file, i.e. whether the tokens appear in a sequence that is valid.  

For each recognized statement in the GEDCOM file, the parser calls some callbacks, which can be registered by the application to get the information out of the file.

This basic description ignores the problem of character encoding.

Character encoding

Refer to this page for some introduction on character encoding...

GEDCOM defines three standard encodings:
These are all supported by the parser, and converted into UTF-8 format.

$Id: parser.html,v 1.10 2002/01/20 13:32:26 verthezp Exp $
$Name: R0_90_0 $