libstemmer_c

This document pertains to the C version of the libstemmer distribution, available for download from:

http://snowball.tartarus.org/dist/libstemmer_c.tgz

Compiling the library

A simple makefile is provided for Unix style systems. On such systems, it should be possible simply to run "make", and the file "libstemmer.o" and the example program "stemwords" will be generated.

If this doesn't work on your system, you need to write your own build system (or call the compiler directly). The files to compile are all contained in the "libstemmer", "runtime" and "src_c" directories, and the public header file is contained in the "include" directory.

The library comes in two flavours; UTF-8 only, and UTF-8 plus other character sets. To use the utf-8 only flavour, compile "libstemmer_utf8.c" instead of "libstemmer.c".

For convenience "mkinc.mak" is a makefile fragment listing the source files and header files used to compile the standard version of the library. "mkinc_utf8.mak" is a comparable makefile fragment listing just the source files for the UTF-8 only version of the library.

Using the library

The library provides a simple C API. Essentially, a new stemmer can be obtained by using "sb_stemmer_new". "sb_stemmer_stem" is then used to stem a word, "sb_stemmer_length" returns the stemmed length of the last word processed, and "sb_stemmer_delete" is used to delete a stemmer.

Creating a stemmer is a relatively expensive operation - the expected usage pattern is that a new stemmer is created when needed, used to stem many words, and deleted after some time.

Stemmers are re-entrant, but not threadsafe. In other words, if you wish to access the same stemmer object from multiple threads, you must ensure that all access is protected by a mutex or similar device.

libstemmer does not currently incorporate any mechanism for caching the results of stemming operations. Such caching can greatly increase the performance of a stemmer under certain situations, so suitable patches will be considered for inclusion.

The standard libstemmer sources contain an algorithm for each of the supported languages. The algorithm may be selected using the english name of the language, or using the 2 or 3 letter ISO 639 language codes. In addition, the traditional "Porter" stemming algorithm for english is included for backwards compatibility purposes, but we recommend use of the "English" stemmer in preference for new projects.

(Some minor algorithms which are included only as curiosities in the snowball website, such as the Lovins stemmer and the Kraaij Pohlmann stemmer, are not included in the standard libstemmer sources. These are not really supported by the snowball project, but it would be possible to compile a modified libstemmer library containing these if desired.)

The stemwords example

The stemwords example program allows you to run any of the stemmers compiled into the libstemmer library on a sample vocabulary. For details on how to use it, run it with the "-h" command line option.

Using the library in a larger system

If you are incorporating the library into the build system of a larger program, I recommend copying the unpacked tarball without modification into a subdirectory of the sources of your program. Future versions of the library are intended to keep the same structure, so this will keep the work required to move to a new version of the library to a minimum.

As an additional convenience, the list of source and header files used in the library is detailed in mkinc.mak - a file which is in a suitable format for inclusion by a Makefile. By including this file in your build system, you can link the snowball system into your program with a few extra rules.

Using the library in a system using GNU autotools

The libstemmer_c library can be integrated into a larger system which uses the GNU autotool framework (and in particular, automake and autoconf) as follows:

1) Unpack libstemmer_c.tgz in the top level project directory so that there is a libstemmer_c subdirectory of the top level directory of the project.

2) Add a file "Makefile.am" to the unpacked libstemmer_c folder, containing:

noinst_LTLIBRARIES = libstemmer.la include $(srcdir)/mkinc.mak noinst_HEADERS = $(snowball_headers) libstemmer_la_SOURCES = $(snowball_sources)

(You may also need to add other lines to this, for example, if you are using compiler options which are not compatible with compiling the libstemmer library.)

3) Add libstemmer_c to the AC_CONFIG_FILES declaration in the project's configure.ac file.

4) Add to the top level makefile the following lines (or modify existing assignments to these variables appropriately):

AUTOMAKE_OPTIONS = subdir-objects AM_CPPFLAGS = -I$(top_srcdir)/libstemmer_c/include SUBDIRS=libstemmer_c _LIBADD = libstemmer_c/libstemmer.la

(Where is the name of the library or executable which links against libstemmer.)