wasm-libxml2

A quick experiment to build and run libxml2 as a WebAssembly module.

Building libxml2

I've used the following commands and flags (from wasienv), using the source from libxml-2.9.9.

$ wasiconfigure  ./configure --enable-static --without-http --without-ftp --without-modules --without-python --without-zlib --without-lzma --without-threads --host=x86_64

$ wasimake make

$ wasicc HTMLparser.o HTMLtree.o SAX.o SAX2.o buf.o c14n.o catalog.o chvalid.o debugXML.o dict.o encoding.o entities.o error.o globals.o hash.o legacy.o list.o parser.o parserInternals.o pattern.o relaxng.o schematron.o threads.o tree.o uri.o valid.o xinclude.o xlink.o xmlIO.o xmlcatalog.o xmlmemory.o xmlmodule.o xmlreader.o xmlregexp.o xmlsave.o xmlschemas.o xmlschemastypes.o xmlstring.o xmlunicode.o xmlwriter.o xpath.o xpointer.o xzlib.o -Wl,--whole-archive,--export-all -o libxml2.wasm

Calling libxml2

A toy program is available in main.c, it runs a slightly modified version of wasm3.

Some simple function wrappers were written on top of libxml2 in order to simplify this PoC.

wasm_new_schema_parser2 instantiates a xmlSchemaParserCtxtPtr, a xmlSchemaPtr and a xmlSchemaNewValidCtxt on top of these two. This function takes two arguments, the first one is a buffer containing the XSD contents and the second one is the buffer length. Returns a xmlSchemaValidCtxtPtr.

wasm_validate_xml takes a buffer too. Additionally the third parameter is expected to be a xmlSchemaValidCtxtPtr (as returned by wasm_new_schema_parser2). The buffer is expected to be the XML contents, this is loaded using xmlParseMemory and finally validated with xmlSchemaValidateDoc. Negative return values represent internal or API errors (source).

Building the toy program

build.sh will do the trick:

$ sh ./build.sh

The output looks as follows (note that there's an error in input.xml in order to trigger the parser error):

% ./main 
WASM module loaded
Runtime allocated memory: 458752 bytes
XSD file loaded (3231 bytes)
Allocated 458768 bytes, xsd_buf_ptr = 3231
Runtime allocated memory: 524288 bytes
XML file loaded (521 bytes)
Allocated 521 bytes, xml_buf_ptr = 489280
Entity: line 13: parser error : Opening and ending tag mismatch: namex line 13 and name
        <namex>Item 2</name>
                            ^
wasm_validate_xml = -1