libxml2

libxml2

Interface to libxml2, with DOM interface.

Index

• Why?
• FAQ

Why?

I needed to write go-xmlsec. This means we need to build trees using libxml2, and then muck with it in xmlsec: Two separate packages in Go means we cannot (safely) pass around C.xmlFooPtr objects (also, you pay a penalty for pointer types). This package carefully avoid references to C.xmlFooPtr types and uses uintptr to pass data around, so other libraries that needs to interact with libxml2 can safely interact with it.

Status

• This library should be considered alpha grade. API may still change.
• Much of commonly used functionalities from libxml2 that I use are there already, and are known to be functional

Package Layout:

Features

Create XML documents using DOM-like interface:

  d := dom.CreateDocument()
  e, err := d.CreateElement("foo")
  if err != nil {
    println(err)
    return
  }
  d.SetDocumentElement(e)
  ...

Parse documents:

  d, err := libxml2.ParseString(xmlstring)
  if err != nil {
    println(err)
    return
  }

Use XPath to extract node values:

  text := xpath.String(node.Find("//xpath/expression"))

Examples

Basic XML Example

import (
  "log"
  "net/http"

  "github.com/lestrrat-go/libxml2"
  "github.com/lestrrat-go/libxml2/parser"
  "github.com/lestrrat-go/libxml2/types"
  "github.com/lestrrat-go/libxml2/xpath"
)

func ExampleXML() {
  res, err := http.Get("http://blog.golang.org/feed.atom")
  if err != nil {
    panic("failed to get blog.golang.org: " + err.Error())
  }

  p := parser.New()
  doc, err := p.ParseReader(res.Body)
  defer res.Body.Close()

  if err != nil {
    panic("failed to parse XML: " + err.Error())
  }
  defer doc.Free()

  doc.Walk(func(n types.Node) error {
    log.Printf(n.NodeName())
    return nil
  })

  root, err := doc.DocumentElement()
  if err != nil {
    log.Printf("Failed to fetch document element: %s", err)
    return
  }

  ctx, err := xpath.NewContext(root)
  if err != nil {
    log.Printf("Failed to create xpath context: %s", err)
    return
  }
  defer ctx.Free()

  ctx.RegisterNS("atom", "http://www.w3.org/2005/Atom")
  title := xpath.String(ctx.Find("/atom:feed/atom:title/text()"))
  log.Printf("feed title = %s", title)
}

Basic HTML Example

func ExampleHTML() {
  res, err := http.Get("http://golang.org")
  if err != nil {
    panic("failed to get golang.org: " + err.Error())
  }

  doc, err := libxml2.ParseHTMLReader(res.Body)
  if err != nil {
    panic("failed to parse HTML: " + err.Error())
  }
  defer doc.Free()

  doc.Walk(func(n types.Node) error {
    log.Printf(n.NodeName())
    return nil
  })

  nodes := xpath.NodeList(doc.Find(`//div[@id="menu"]/a`))
  for i := 0; i < len(nodes); i++ {
    log.Printf("Found node: %s", nodes[i].NodeName())
  }
}

XSD Validation

import (
  "io/ioutil"
  "log"
  "os"
  "path/filepath"

  "github.com/lestrrat-go/libxml2"
  "github.com/lestrrat-go/libxml2/xsd"
)

func ExampleXSD() {
  xsdfile := filepath.Join("test", "xmldsig-core-schema.xsd")
  f, err := os.Open(xsdfile)
  if err != nil {
    log.Printf("failed to open file: %s", err)
    return
  }
  defer f.Close()

  buf, err := ioutil.ReadAll(f)
  if err != nil {
    log.Printf("failed to read file: %s", err)
    return
  }

  s, err := xsd.Parse(buf)
  if err != nil {
    log.Printf("failed to parse XSD: %s", err)
    return
  }
  defer s.Free()

  d, err := libxml2.ParseString(`<foo></foo>`)
  if err != nil {
    log.Printf("failed to parse XML: %s", err)
    return
  }
  defer d.Free()

  if err := s.Validate(d); err != nil {
    for _, e := range err.(xsd.SchemaValidationError).Errors() {
      log.Printf("error: %s", e.Error())
    }
    return
  }

  log.Printf("validation successful!")
}

Caveats

Other libraries

There exists many similar libraries. I want speed, I want DOM, and I want XPath.When all of these are met, I'd be happy to switch to another library.

For now my closest contender was xmlpath, but as of this writing it suffers in the speed (for xpath) area a bit:

shoebill% go test -v -run=none -benchmem -benchtime=5s -bench .
PASS
BenchmarkXmlpathXmlpath-4     500000         11737 ns/op         721 B/op          6 allocs/op
BenchmarkLibxml2Xmlpath-4    1000000          7627 ns/op         368 B/op         15 allocs/op
BenchmarkEncodingXMLDOM-4    2000000          4079 ns/op        4560 B/op          9 allocs/op
BenchmarkLibxml2DOM-4        1000000         11454 ns/op         264 B/op          7 allocs/op
ok      github.com/lestrrat-go/libxml2  37.597s

FAQ

"It won't build"

The very first thing you need to be aware is that this is a C binding to libxml2. You should understand how to build C programs, how to debug them, or at least be able to ask the right questions and deal with a great deal more than Go alone.

Having said that, the most common causes for build errors are:

1. You have not installed libxml2 / You installed it incorrectly

The first one is obvious, but I get this a lot. You have to install libxml2. If you are installing via some sort of package manager like apt/apk, remember that you need to install the "development" files as well. The name of the package differs in each environment, but it's usually something like "libxml2-dev".

The second is more subtle, and tends to happen when you install your libxml2 in a non-standard location. This causes problems for other tools such as your C compiler or pkg-config. See more below

1. Your header files are not in the search path

If you don't understand what header files are or how they work, this is where you should either look for your local C-guru, or study how these things work before filing an issue on this repository.

Your C compiler, which is invoked via Go, needs to be able to find the libxml2 header files. If you installed them in a non-standard location, for example, such as outside of /usr/include and /usr/local/include, you may have to configure them yourself.

How to configure them depends greatly on your environment, and again, if you don't understand how you can fix it, you should consult your local C-guru about it, not this repository.

1. Your pkg-config files are not in the search path

If you don't understand what pkg-config does, this is where you should either look for your local sysadmin friend, or study how these things work before filing an issue on this repository.

pkg-config provides metadata about a installed components, such as build flags that are required. Go uses it to figure out how to build and link Go programs that needs to interact with things written in C.

However, pkg-config is merely a thin frontend to extract information from file(s) that each component provided upon installation. pkg-config itself needs to know where to find these files.

Make sure that the output of the following command contains libxml-2.0. If not, and you don't understand how to fix this yourself, you should consult your local sysadmin friend about it, not this repository

pkg-config --list-all

"Fatal error: 'libxml/HTMLparser.h' file not found"

See the first FAQ entry.

I can't statically link this module to libxml2

Use the static_build tag when building this module, for example:

go build -tags static_build

See Also

• https://github.com/lestrrat-go/xmlsec

Credits

• Work on this library was generously sponsored by HDE Inc (https://www.hde.co.jp)