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<h1 align="center">Libvirt the virtualization API</h1>

<h1>Note: this is the flat content of the <a href="index.html">web
site</a></h1>

<h1 style="text-align: center">libvirt</h1>

<h3>what is <span class="style1">libvirt?</span></h3>

<p>Libvirt is a C toolkit to interract with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software available
under the <a href="http://www.opensource.org/licenses/lgpl-license.html">GNU
Lesser General Public License</a>. Virtualization of the Linux Operating
System means the ability to run multiple instances of Operating Systems
concurently on a single hardware system where the basic resources are driven
by a Linux instance. The library aim at providing long term stable C API
initially for the <a
href="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/index.html">Xen
paravirtualization</a> but should be able to integrate other virtualization
mechanisms if needed.</p>

<h2><a name="News">Releases</a></h2>

<p>Here is the list of official releases, however since it is early on in the
development of libvirt, it is preferable when possible to just use the <a
href="downloads.html">CVS version or snapshot</a>, contact the mailing list
and check the <a href="ChangeLog.html">ChangeLog</a> to gauge progresses.</p>

<h3>0.1.2: Jul 3 2006</h3>
<ul>
  <li> headers include paths fixup
  <li> proxy mechanism for unpriviledged read-only access by httpu
</ul>
<h3>0.1.1: Jun 21 2006</h3>
<ul>
  <li> building fixes: ncurses fallback (Jim Fehlig), VPATH builds (Daniel P.
       Berrange)
  <li> driver cleanups: new entry points, cleanup of libvirt.c (with Daniel P.
       Berrange)
  <li> Cope with API change introduced in Xen changeset 10277
  <li> new test driver for regression checks (Daniel P. Berrange)
  <li> improvements: added UUID to XML serialization, buffer usage (Karel Zak),
       --connect argument to virsh (Daniel P. Berrange), 
  <li> bug fixes: uninitialized memory access in error reporting, S-Expr
       parsing (Jim Fehlig, Jeremy Katz), virConnectOpen bug, remove a TODO
       in xs_internal.c
  <li> documentation: Python examples (David Lutterkort), new Perl binding URL,
       man page update (Karel Zak)
       
</ul>
<h3>0.1.0: Apr 10 2006</h3>
<ul>
  <li>building fixes: --with-xen-distdir option (Ronald Aigner), out of tree
    build and pkginfo cflag fix (Daniel Berrange)</li>
  <li>enhancement and fixes of the XML description format (David Lutterkort
    and Jim Fehlig)</li>
  <li>new APIs: for Node information and Reboot</li>
  <li>internal code cleanup: refactoring internals into a driver model, more
    error handling, structure sharing, thread safety and ref counting</li>
  <li>bug fixes: error message (Jim Meyering), error allocation in virsh (Jim
    Meyering), virDomainLookupByID (Jim Fehlig), </li>
  <li>documentation: updates on architecture, and format, typo fix (Jim
    Meyering)</li>
  <li>bindings: exception handling in examples (Jim Meyering), perl ones out
    of tree (Daniel Berrange)</li>
  <li>virsh: more options, create, nodeinfo (Karel Zak), renaming of some
    options (Karel Zak), use stderr only for errors (Karel Zak), man page
    (Andrew Puch)</li>
</ul>

<h3>0.0.6: Feb 28 2006</h3>
<ul>
  <li>add UUID lookup and extract API</li>
  <li>add error handling APIs both synchronous and asynchronous</li>
  <li>added minimal hook for error handling at the python level, improved the
    python bindings</li>
  <li>augment the documentation and tests to cover error handling</li>
</ul>

<h3>0.0.5: Feb 23 2006</h3>
<ul>
  <li>Added XML description parsing, dependance to libxml2, implemented the
    creation API virDomainCreateLinux()</li>
  <li>new APIs to lookup and name domain by UUID</li>
  <li>fixed the XML dump when using the Xend access</li>
  <li>Fixed a few more problem related to the name change</li>
  <li>Adding regression tests in python and examples in C</li>
  <li>web site improvement, extended the documentation to cover the XML
    format and Python API</li>
  <li>Added devhelp help for Gnome/Gtk programmers</li>
</ul>

<h3>0.0.4: Feb 10 2006</h3>
<ul>
  <li>Fix various bugs introduced in the name change</li>
</ul>

<h3>0.0.3: Feb 9 2006</h3>
<ul>
  <li>Switch name from from 'libvir' to libvirt</li>
  <li>Starting infrastructure to add code examples</li>
  <li>Update of python bindings for completeness</li>
</ul>

<h3>0.0.2: Jan 29 2006</h3>
<ul>
  <li>Update of the documentation, web site redesign (Diana Fong)</li>
  <li>integration of HTTP xend RPC based on libxend by Anthony Liquori for
    most operations</li>
  <li>Adding Save and Restore APIs</li>
  <li>extended the virsh command line tool (Karel Zak)</li>
  <li>remove xenstore transactions (Anthony Liguori)</li>
  <li>fix the Python bindings bug when domain and connections where freed</li>
</ul>

<h3>0.0.1: Dec 19 2005</h3>
<ul>
  <li>First release</li>
  <li>Basic management of existing Xen domains</li>
  <li>Minimal autogenerated Python bindings</li>
</ul>

<h2><a name="Introducti">Introduction</a></h2>

<p>Libvirt is a C toolkit to interact with the virtualization capabilities of
recent versions of Linux (and other OSes), but libvirt won't try to provide
all possible interfaces for interacting with the virtualization features.</p>

<p>To avoid ambiguity about the terms used here here are the definitions for
some of the specific concepts used in libvirt documentation:</p>
<ul>
  <li>a <strong>node</strong> is a single physical machine</li>
  <li>an <strong>hypervisor</strong> is a layer of software allowing to
    virtualize a node in a set of virtual machines with possibly different
    configurations than the node itself</li>
  <li>a <strong>domain</strong> is an instance of an operating system running
    on a virtualized machine provided by the hypervisor</li>
</ul>

<p style="text-align: center"><img
alt="Hypervisor and domains running on a node" src="node.gif"></p>

<p>Now we can define the goal of libvirt: to provide the lowest possible
generic and stable layer to manage domains on a node.</p>

<p>This implies the following:</p>
<ul>
  <li>the API should not be targetted to a single virtualization environment
    though Xen is the current default, which also means that some very
    specific capabilities which are not generic enough may not be provided as
    libvirt APIs</li>
  <li>the API should allow to do efficiently and cleanly all the operations
    needed to manage domains on a node</li>
  <li>the API will not try to provide hight level multi-nodes management
    features like load balancing, though they could be implemented on top of
    libvirt</li>
  <li>stability of the API is a big concern, libvirt should isolate
    applications from the frequent changes expected at the lower level of the
    virtualization framework</li>
</ul>

<p>So libvirt should be a building block for higher level management tools
and for applications focusing on virtualization of a single node (the only
exception being domain migration between node capabilities which may need to
be added at the libvirt level). Where possible libvirt should be extendable
to be able to provide the same API for remote nodes, however this is not the
case at the moment, the code currently handle only local node accesses.</p>

<h2><a name="architecture">libvirt architecture</a></h2>

<h3>This is in a large part Xen specific since this is the only hypervisor
supported at the moment</h3>

<p>When running in a Xen environment, programs using libvirt have to execute
in "Domain 0", which is the primary Linux OS loaded on the machine. That OS
kernel provides most if not all of the actual drivers used by the set of
domains. It also runs the Xen Store, a database of informations shared by the
hypervisor, the kernels, the drivers and the xen daemon. Xend. The xen daemon
supervise the control and execution of the sets of domains. The hypervisor,
drivers, kernels and daemons communicate though a shared system bus
implemented in the hypervisor. The figure below tries to provide a view of
this environment:</p>
<img src="architecture.gif" alt="The Xen architecture">

<p>The library can be initialized in 2 ways depending on the level of
priviledge of the embedding program. If it runs with root access,
virConnectOpen() can be used, it will use three different ways to connect to
the Xen infrastructure:</p>
<ul>
  <li>a connection to the Xen Daemon though an HTTP RPC layer</li>
  <li>a read/write connection to the Xen Store</li>
  <li>use Xen Hypervisor calls</li>
</ul>

<p>The library will usually interract with the Xen daemon for any operation
changing the state of the system, but for performance and accuracy reasons
may talk directly to the hypervisor when gathering state informations at
least when possible (i.e. when the running program using libvirt has root
priviledge access).</p>

<p>If it runs without root access virConnectOpenReadOnly() should be used to
connect to initialize the library. It will try to open the read-only socket
<code>/var/run/xenstored/socket_ro</code> to connect to the Xen Store and
also try to use the RPC to the Xen daemon. In this case use of hypervisor
calls and write to the Xen Store will not be possible, restraining the amount
of APIs available and slowing down information gathering about domains.</p>

<h3>Internal architecture</h3>

<p>As the previous section explains, libvirt can communicate using different
channels with the current hypervisor, and should also be able to use
different kind of hypervisor. To simplify the internal design, code, ease
maintainance and simplify the support of other virtualization engine the
internals have been structured as one core component, the libvirt.c module
acting as a front-end for the library API and a set of hypvisor drivers
defining a common set of routines. That way the Xen Daemon accces, the Xen
Store one, the Hypervisor hypercall are all isolated in separate C modules
implementing at least a subset of the common operations defined by the
drivers present in driver.h:</p>
<ul>
  <li>xend_internal: implements the driver functions though the Xen
  Daemon</li>
  <li>xs_internal: implements the subset of the driver availble though the
    Xen Store</li>
  <li>xen_internal: provide the implementation of the functions possible via
    direct hypervisor access</li>
</ul>

<p>Note that a given driver may only implement a subset of those functions,
for example saving a domain state to disk and restoring it is only possible
though the Xen Daemon, on the other hand all interfaces allow to query the
runtime state of a given domain.</p>

<p></p>

<h2><a name="Downloads">Downloads</a></h2>

<p>The latest versions of libvirt can be found on the  <a
href="ftp://libvirt.org/libvirt/">libvirt.org</a> server ( <a
href="http://libvirt.org/sources/">HTTP</a>, <a
href="ftp://libvirt.org/libvirt/">FTP</a>). You will find there the released
versions as well as <a
href="http://libvirt.org/sources/libvirt-cvs-snapshot.tar.gz">snapshot
tarballs</a> updated from CVS head every hour</p>

<p>Anonymous <a href="http://ximbiot.com/cvs/cvshome/docs/">CVS</a> is also
available, first register onto the server:</p>

<p><code>cvs -d :pserver:anoncvs@libvirt.org:2401/data/cvs login</code></p>

<p>it will request a password, enter <strong>anoncvs</strong>. Then you can
checkout the development tree with:</p>

<p><code>cvs -d :pserver:anoncvs@libvirt.org:2401/data/cvs co
libvirt</code></p>

<p>Use ./autogen.sh to configure the local checkout, then <code>make</code>
and <code>make install</code>, as usual. All normal cvs commands are now
available except commiting to the base.</p>

<h2><a name="Format">XML Format</a></h2>

<p>The library use an XML format to describe domains, as input to <a
href="html/libvirt-libvirt.html#virDomainCreateLinux">virDomainCreateLinux()</a>
and as the output of <a
href="html/libvirt-libvirt.html#virDomainGetXMLDesc">virDomainGetXMLDesc()</a>,
the following is an example of the format as returned by the shell command
<code>virsh xmldump fc4</code> , where fc4 was one of the running domains:</p>
<pre>&lt;domain type='xen' <span style="color: #0071FF; background-color: #FFFFFF">id='18'</span>&gt;
  &lt;name&gt;fc4&lt;/name&gt;
  <span style="color: #00B200; background-color: #FFFFFF">&lt;os&gt;
    &lt;type&gt;linux&lt;/type&gt;
    &lt;kernel&gt;/boot/vmlinuz-2.6.15-1.43_FC5guest&lt;/kernel&gt;
    &lt;initrd&gt;/boot/initrd-2.6.15-1.43_FC5guest.img&lt;/initrd&gt;
    &lt;root&gt;/dev/sda1&lt;/root&gt;
    &lt;cmdline&gt; ro selinux=0 3&lt;/cmdline&gt;
  &lt;/os&gt;</span>
  &lt;memory&gt;131072&lt;/memory&gt;
  &lt;vcpu&gt;1&lt;/vcpu&gt;
  &lt;devices&gt;
    <span style="color: #FF0080; background-color: #FFFFFF">&lt;disk type='file'&gt;
      &lt;source file='/u/fc4.img'/&gt;
      &lt;target dev='sda1'/&gt;
    &lt;/disk&gt;</span>
    <span style="color: #0000FF; background-color: #FFFFFF">&lt;interface type='bridge'&gt;
      &lt;source bridge='xenbr0'/&gt;
      &lt;mac address='</span><span style="color: #0000FF; background-color: #FFFFFF"></span><span style="color: #0000FF; background-color: #FFFFFF">aa:00:00:00:00:11'/&gt;
      &lt;script path='/etc/xen/scripts/vif-bridge'/&gt;
    &lt;/interface&gt;</span>
  &lt;/devices&gt;
&lt;/domain&gt;</pre>

<p>The root element must be called <code>domain</code> with no namespace, the
<code>type</code> attribute indicates the kind of hypervisor used, 'xen' is
the default value. The <code>id</code> attribute gives the domain id at
runtime (not however that this may change, for example if the domain is saved
to disk and restored). The domain has a few children whose order is not
significant:</p>
<ul>
  <li>name: the domain name, preferably ASCII based</li>
  <li>memory: the maximum memory allocated to the domain in kilobytes</li>
  <li>vcpu: the number of virtual cpu configured for the domain</li>
  <li>os: a block describing the Operating System, its content will be
    dependant on the OS type
    <ul>
      <li>type: indicate the OS type, always linux at this point</li>
      <li>kernel: path to the kernel on the Domain 0 filesystem</li>
      <li>initrd: an optional path for the init ramdisk on the Domain 0
        filesystem</li>
      <li>cmdline: optional command line to the kernel</li>
      <li>root: the root filesystem from the guest viewpoint, it may be
        passed as part of the cmdline content too</li>
    </ul>
  </li>
  <li>devices: a list of <code>disk</code> and <code>interface</code>
    descriptions in no special order</li>
</ul>

<p>The format of the devices and their type may grow over time, but the
following should be sufficient for basic use:</p>

<p>A disk device indicates a block device, it can have two values for the
type attribute either 'file' or 'block' corresponding to the 2 options
availble at the Xen layer. It has two mandatory children, and one optional
one in no specific order:</p>
<ul>
  <li>source with a file attribute containing the path in Domain 0 to the
    file or a dev attribute if using a block device, containing the device
    name ('hda5' or '/dev/hda5')</li>
  <li>target indicates in a dev attribute the device where it is mapped in
    the guest</li>
  <li>readonly an optional empty element indicating the device is
  read-only</li>
</ul>

<p>An interface element describes a network device mapped on the guest, it
also has a type whose value is currently 'bridge', it also have a number of
children in no specific order:</p>
<ul>
  <li>source: indicating the bridge name</li>
  <li>mac: the optional mac address provided in the address attribute</li>
  <li>ip: the optional IP address provided in the address attribute</li>
  <li>script: the script used to bridge the interfcae in the Domain 0</li>
  <li>target: and optional target indicating the device name.</li>
</ul>

<p>Life cycle actions for the domain can also be expressed in the XML format,
they drive what should be happening if the domain crashes, is rebooted or is
poweroff. There is various actions possible when this happen:</p>
<ul>
  <li>destroy: The domain is cleaned up (that's the default normal processing
    in Xen)</li>
  <li>restart: A new domain is started in place of the old one with the same
    configuration parameters</li>
  <li>preserve: The domain will remain in memory until it is destroyed
    manually, it won't be running but allows for post-mortem debugging</li>
  <li>rename-restart: a variant of the previous one but where the old domain
    is renamed before being saved to allow a restart</li>
</ul>

<p>The following could be used for a Xen production system:</p>
<pre>&lt;domain&gt;
  ...
  &lt;on_reboot&gt;restart&lt;/on_reboot&gt;
  &lt;on_poweroff&gt;destroy&lt;/on_poweroff&gt;
  &lt;on_crash&gt;rename-restart&lt;/on_crash&gt;
  ...
&lt;/domain&gt;</pre>

<p>While the format may be extended in various ways as support for more
hypervisor types and features are added, it is expected that this core subset
will remain functional in spite of the evolution of the library.</p>

<h2><a name="Python" id="Python">Binding for Python</a></h2>

<p>Libvirt comes with direct support for the Python language (just make sure
you installed the libvirt-python package if not compiling from sources). Also
note that Daniel Berrange provides <a
href="http://search.cpan.org/~danberr/Sys-Virt-0.1.0/">bindings for Perl</a>
too.</p>

<p>The Python binding should be complete and are mostly automatically
generated from the formal description of the API in xml. The bindings are
articulated around 2 classes <code>virConnect</code> and virDomain mapping to
the C types. Functions in the C API taking either type as argument then
becomes methods for the classes, their name is just stripped from the
virConnect or virDomain(Get) prefix and the first letter gets converted to
lower case, for example the C functions:</p>

<p><code>int <a
href="html/libvirt-libvirt.html#virConnectNumOfDomains">virConnectNumOfDomains</a>
(virConnectPtr conn);</code></p>

<p><code>int <a
href="html/libvirt-libvirt.html#virDomainSetMaxMemory">virDomainSetMaxMemory</a>
(virDomainPtr domain, unsigned long memory);</code></p>

<p>become</p>

<p><code>virConn::numOfDomains(self)</code></p>

<p><code>virDomain::setMaxMemory(self, memory)</code></p>

<p>This process is fully automated, you can get a summary of the conversion
in the file libvirtclass.txt present in the python dir or in the docs.There
is a couple of function who don't map directly to their C counterparts due to
specificities in their argument conversions:</p>
<ul>
  <li><code><a
    href="html/libvirt-libvirt.html#virConnectListDomains">virConnectListDomains</a></code>
    is replaced by <code>virDomain::listDomainsID(self)</code> which returns
    a list of the integer ID for the currently running domains</li>
  <li><code><a
    href="html/libvirt-libvirt.html#virDomainGetInfo">virDomainGetInfo</a></code>
    is replaced by <code>virDomain::info()</code> which returns a list of
    <ol>
      <li>state: one of the state values (virDomainState)</li>
      <li>maxMemory: the maximum memory used by the domain</li>
      <li>memory: the current amount of memory used by the domain</li>
      <li>nbVirtCPU: the number of virtual CPU</li>
      <li>cpuTime: the time used by the domain in nanoseconds</li>
    </ol>
  </li>
</ul>

<p>So let's look at a simple example inspired from the <code>basic.py</code>
test found in <code>python/tests/</code> in the source tree:</p>
<pre>import <span style="color: #0071FF; background-color: #FFFFFF">libvirt</span>
import sys

conn = <span style="color: #0071FF; background-color: #FFFFFF">libvirt</span>.openReadOnly(None)
if conn == None:
    print 'Failed to open connection to the hypervisor'
    sys.exit(1)

try:
    dom0 = conn.<span style="color: #007F00; background-color: #FFFFFF">lookupByName</span>("Domain-0")
except:
    print 'Failed to find the main domain'
    sys.exit(1)

print "Domain 0: id %d running %s" % (dom0.<span style="color: #FF0080; background-color: #FFFFFF">ID</span>(), dom0.<span style="color: #FF0080; background-color: #FFFFFF">OSType</span>())
print dom0.<span style="color: #FF0080; background-color: #FFFFFF">info</span>()</pre>

<p>There is not much to comment about it, it really is a straight mapping
from the C API, the only points to notice are:</p>
<ul>
  <li>the import of the module called <code><span
    style="color: #0071FF; background-color: #FFFFFF">libvirt</span></code></li>
  <li>getting a connection to the hypervisor, in that case using the
    openReadOnly function allows the code to execute as a normal user.</li>
  <li>getting an object representing the Domain 0 using <span
    style="color: #007F00; background-color: #FFFFFF">lookupByName</span></li>
  <li>if the domain is not found a libvirtError exception will be raised</li>
  <li>extracting and printing some informations about the domain using
    various <span
    style="color: #E50073; background-color: #FFFFFF">methods</span>
    associated to the virDomain class.</li>
</ul>

<h2><a name="Errors" id="Errors">Handling of errors</a></h2>

<p>The main goals of libvirt when it comes to error handling are:</p>
<ul>
  <li>provide as much detail as possible</li>
  <li>provide the informations as soon as possible</li>
  <li>dont force the library user into one style of error handling</li>
</ul>

<p>As result the library provide both synchronous, callback based and
asynchronous error reporting. When an error happens in the library code the
error is logged, allowing to retrieve it later and if the user registered an
error callback it will be called synchronously. Once the call to libvirt ends
the error can be detected by the return value and the full information for
the last logged error can be retrieved.</p>

<p>To avoid as much as prossible troubles with a global variable in a
multithreaded environment, libvirt will associate when possible the errors to
the current connection they are related to, that way the error is stored in a
dynamic structure which can be made thread specific. Error callback can be
set specifically to a connection with</p>

<p>So error handling in the code is the following:</p>
<ol>
  <li>if the error can be associated to a connection for example when failing
    to look up a domain
    <ol>
      <li>if there is a callback associated to the connection set with <a
        href="html/libvirt-virterror.html#virConnSetErrorFunc">virConnSetErrorFunc</a>,
        call it with the error informations</li>
      <li>otherwise if there is a global callback set with <a
        href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>,
        call it with the error information</li>
      <li>otherwise call <a
        href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>
        which is the default error function of the library issuing the error
        on stderr</li>
      <li>save the error in the connection for later retrieval with <a
        href="html/libvirt-virterror.html#virConnGetLastError">virConnGetLastError</a></li>
    </ol>
  </li>
  <li>otherwise like when failing to create an hypervisor connection:
    <ol>
      <li>if there is a global callback set with <a
        href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>,
        call it with the error information</li>
      <li>otherwise call <a
        href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>
        which is the default error function of the library issuing the error
        on stderr</li>
      <li>save the error in the connection for later retrieval with <a
        href="html/libvirt-virterror.html#virGetLastError">virGetLastError</a></li>
    </ol>
  </li>
</ol>

<p>In all cases the error informations are provided as a <a
href="html/libvirt-virterror.html#virErrorPtr">virErrorPtr</a> pointer to
read-only structure <a
href="html/libvirt-virterror.html#virError">virError</a> containing the
following fields:</p>
<ul>
  <li>code: an error number from the <a
    href="html/libvirt-virterror.html#virErrorNumber">virErrorNumber</a>
  enum</li>
  <li>domain: an enum indicating which part of libvirt raised the error see
    <a
  href="html/libvirt-virterror.html#virErrorDomain">virErrorDomain</a></li>
  <li>level: the error level, usually VIR_ERR_ERROR, though there is room for
    warnings like VIR_ERR_WARNING</li>
  <li>message: the full human-readable formatted string of the error</li>
  <li>conn: if available a pointer to the <a
    href="html/libvirt-libvirt.html#virConnectPtr">virConnectPtr</a>
    connection to the hypervisor where this happened</li>
  <li>dom: if available a pointer to the <a
    href="html/libvirt-libvirt.html#virDomainPtr">virDomainPtr</a> domain
    targetted in the operation</li>
</ul>

<p>and then extra raw informations about the error which may be initialized
to 0 or NULL if unused</p>
<ul>
  <li>str1, str2, str3: string informations, usually str1 is the error
    message format</li>
  <li>int1, int2: integer informations</li>
</ul>

<p>So usually, setting up specific error handling with libvirt consist of
registering an handler with with <a
href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a> or
with <a
href="html/libvirt-virterror.html#virConnSetErrorFunc">virConnSetErrorFunc</a>,
chech the value of the code value, take appropriate action, if needed let
libvirt print the error on stderr by calling <a
href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>.
For asynchronous error handing, set such a function doing nothing to avoid
the error being reported on stderr, and call virConnGetLastError or
virGetLastError when an API call returned an error value. It can be a good
idea to use <a
href="html/libvirt-virterror.html#virResetLastError">virResetError</a> or <a
href="html/libvirt-virterror.html#virConnResetLastError">virConnResetLastError</a>
once an error has been processed fully.</p>

<p>At the python level, there only a global reporting callback function at
this point, see the error.py example about it:</p>
<pre>def handler(ctxt, err):
    global errno

    #print "handler(%s, %s)" % (ctxt, err)
    errno = err

libvirt.registerErrorHandler(handler, 'context') </pre>

<p>the second argument to the registerErrorHandler function is passed as the
fist argument of the callback like in the C version. The error is a tuple
containing the same field as a virError in C, but cast to Python.</p>

<h2><a name="FAQ" id="FAQ">FAQ</a></h2>

<p>Table of Contents:</p>
<ul>
  <li><a href="FAQ.html#License">License(s)</a></li>
  <li><a href="FAQ.html#Installati">Installation</a></li>
  <li><a href="FAQ.html#Compilatio">Compilation</a></li>
  <li><a href="FAQ.html#Developer">Developer corner</a></li>
</ul>

<h3><a name="License">License</a>(s)</h3>
<ol>
  <li><em>Licensing Terms for libvirt</em>
    <p>libvirt is released under the <a
    href="http://www.opensource.org/licenses/lgpl-license.html">GNU Lesser
    General Public License</a>, see the file COPYING.LIB in the distribution
    for the precise wording. The only library that libvirt depends upon is
    the Xen store access library which is also licenced under the LGPL.</p>
  </li>
  <li><em>Can I embed libvirt in a proprietary application ?</em>
    <p>Yes. The LGPL allows you to embed libvirt into a proprietary
    application. It would be graceful to send-back bug fixes and improvements
    as patches for possible incorporation in the main development tree. It
    will decrease your maintainance costs anyway if you do so.</p>
  </li>
</ol>

<h3><a name="Installati">Installation</a></h3>
<ol>
  <li><em>Where can I get libvirt</em> ?
    <p>The original distribution comes from <a
    href="ftp://libvirt.org/libvirt/">ftp://libvirt.org/libvirt/</a>.</p>
  </li>
  <li><em>I can't install the libvirt/libvirt-devel RPM packages due to
    failed dependencies</em>
    <p>The most generic solution is to re-fetch the latest src.rpm , and
    rebuild it locally with</p>
    <p><code>rpm --rebuild libvirt-xxx.src.rpm</code>.</p>
    <p>If everything goes well it will generate two binary rpm packages (one
    providing the shared libs and virsh, and the other one, the -devel
    package, providing includes, static libraries and scripts needed to build
    applications with libvirt that you can install locally.</p>
    <p>One can also rebuild the RPMs from a tarball:</p>
    <p><code>rpmbuild -ta libdir-xxx.tar.gz</code></p>
    <p>Or from a configured tree with:</p>
    <p><code>make rpm</code></p>
  </li>
  <li><em>Failure to use the API for non-root users</em>
    <p>Large parts of the API may only be accessible with root priviledges,
    however the read only access to the xenstore data doesnot have to be
    forbidden to user, at least for monitoring purposes. If "virsh dominfo"
    fails to run as an user, change the mode of the xenstore read-only socket
    with:</p>
    <p><code>chmod 666 /var/run/xenstored/socket_ro</code></p>
    <p>and also make sure that the Xen Daemon is running correctly with local
    HTTP server enabled, this is defined in
    <code>/etc/xen/xend-config.sxp</code> which need the following line to be
    enabled:</p>
    <p><code>(xend-http-server yes)</code></p>
    <p>If needed restart the xend daemon after making the change with the
    following command run as root:</p>
    <p><code>service xend restart</code></p>
  </li>
</ol>

<h3><a name="Compilatio">Compilation</a></h3>
<ol>
  <li><em>What is the process to compile libvirt ?</em>
    <p>As most UNIX libraries libvirt follows the "standard":</p>
    <p><code>gunzip -c libvirt-xxx.tar.gz | tar xvf -</code></p>
    <p><code>cd libvirt-xxxx</code></p>
    <p><code>./configure --help</code></p>
    <p>to see the options, then the compilation/installation proper</p>
    <p><code>./configure [possible options]</code></p>
    <p><code>make</code></p>
    <p><code>make install</code></p>
    <p>At that point you may have to rerun ldconfig or a similar utility to
    update your list of installed shared libs.</p>
  </li>
  <li><em>What other libraries are needed to compile/install libvirt ?</em>
    <p>Libvirt requires libxenstore, which is usually provided by the xen
    packages as well as the public headers to compile against libxenstore.</p>
  </li>
  <li><em>I use the CVS version and there is no configure script</em>
    <p>The configure script (and other Makefiles) are generated. Use the
    autogen.sh script to regenerate the configure script and Makefiles,
    like:</p>
    <p><code>./autogen.sh --prefix=/usr --disable-shared</code></p>
  </li>
</ol>

<h3><a name="Developer">Developer</a> corner</h3>
<ol>
  <li><em>Troubles compiling or linking programs using libvirt</em>
    <p>To simplify the process of reusing the library, libvirt comes with
    pkgconfig support, which can be used directly from autoconf support or
    via the pkg-config command line tool, like:</p>
    <p><code>pkg-config libvirt --libs</code></p>
  </li>
</ol>

<h2><a name="Reporting">Reporting bugs and getting help</a></h2>

<p>There is a mailing-list <a
href="mailto:libvir-list@redhat.com">libvir-list@redhat.com</a> for libvirt,
with an  <a href="https://www.redhat.com/archives/libvir-list/">on-line
archive</a>. Please subscribe to this list before posting by visiting the <a
href="https://www.redhat.com/mailman/listinfo/libvir-list">associated Web</a>
page and follow the instructions. Patches with explanations and provided as
attachments are really appreciated and will be discussed on the mailing list.
If possible generate the patches by using cvs diff -u in a CVS checkout.</p>

<p>We expect to use <a href="https://bugzilla.redhat.com/">Red Hat
Bugzilla</a> to track bugs for libvirt, though there isn't a libvirt software
module defined yet, in the meantime use the mailing-list, thanks !.</p>
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