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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /><link rel="stylesheet" type="text/css" href="libvirt.css" /><link rel="SHORTCUT ICON" href="/32favicon.png" /><title>Handling of errors</title></head><body><div id="container"><div id="intro"><div id="adjustments"></div><div id="pageHeader"></div><div id="content2"><h1 class="style1">Handling of errors</h1><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
andasynchronous error reporting. When an error happens in the library code
theerror is logged, allowing to retrieve it later and if the user registered
anerror callback it will be called synchronously. Once the call to libvirt
endsthe error can be detected by the return value and the full information
forthe last logged error can be retrieved.</p><p>To avoid as much as prossible troubles with a global variable in
amultithreaded environment, libvirt will associate when possible the errors
tothe current connection they are related to, that way the error is stored in
adynamic structure which can be made thread specific. Error callback can
beset 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
failingto 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 erroron
stderr</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 erroron
stderr</li>
<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
toread-only structure <a href="html/libvirt-virterror.html#virError">virError</a>containing
thefollowing 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
forwarnings like VIR_ERR_WARNING</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>domaintargetted
in the operation</li>
</ul><p>and then extra raw informations about the error which may be initializedto
0 or NULL if unused</p><ul><li>str1, str2, str3: string informations, usually str1 is the errormessage
format</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
ofregistering an handler with with <a href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>orwith
<a href="html/libvirt-virterror.html#virConnSetErrorFunc">virConnSetErrorFunc</a>,chech
the value of the code value, take appropriate action, if needed letlibvirt
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 avoidthe
error being reported on stderr, and call virConnGetLastError
orvirGetLastError when an API call returned an error value. It can be a
goodidea 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
atthis point, see the error.py example about it:</p><pre>def handler(ctxt, err):
</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
thefist argument of the callback like in the C version. The error is a
tuplecontaining the same field as a virError in C, but cast to Python.</p></div></div><div class="linkList2"><div class="llinks2"><h3 class="links2"><span>main menu</span></h3><ul><li><a href="index.html">Home</a></li><li><a href="news.html">Releases</a></li><li><a href="intro.html">Introduction</a></li><li><a href="architecture.html">libvirt architecture</a></li><li><a href="downloads.html">Downloads</a></li><li><a href="format.html">XML Formats</a></li><li><a href="python.html">Binding for Python</a></li><li><a href="errors.html">Handling of errors</a></li><li><a href="FAQ.html">FAQ</a></li><li><a href="bugs.html">Reporting bugs and getting help</a></li><li><a href="html/index.html">API Menu</a></li><li><a href="examples/index.html">C code examples</a></li><li><a href="ChangeLog.html">Recent Changes</a></li></ul></div><div class="llinks2"><h3 class="links2"><span>related links</span></h3><ul><li><a href="https://www.redhat.com/archives/libvir-list/">Mail archive</a></li><li><a href="https://bugzilla.redhat.com/bugzilla/buglist.cgi?product=Fedora+Core&amp;component=libvirt&amp;bug_status=NEW&amp;bug_status=ASSIGNED&amp;bug_status=REOPENED&amp;bug_status=MODIFIED&amp;short_desc_type=allwordssubstr&amp;short_desc=&amp;long_desc_type=allwordssubstr">Open bugs</a></li><li><a href="http://virt-manager.et.redhat.com/">virt-manager</a></li><li><a href="http://search.cpan.org/~danberr/Sys-Virt-0.1.0/">Perl bindings</a></li><li><a href="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/index.html">Xen project</a></li><li><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="12" value="Search..." /><input name="submit" type="submit" value="Go" /></form></li><li><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-90x34.gif" alt="Made with Libxml2 Logo" /></a></li></ul><p class="credits">Graphics and design by <a href="mail:dfong@redhat.com">Diana Fong</a></p></div></div><div id="bottom"><p class="p1"></p></div></div></body></html>
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></div></div><div class="linkList2"><div class="llinks2"><h3 class="links2"><span>main menu</span></h3><ul><li><a href="index.html">Home</a></li><li><a href="news.html">Releases</a></li><li><a href="intro.html">Introduction</a></li><li><a href="architecture.html">libvirt architecture</a></li><li><a href="downloads.html">Downloads</a></li><li><a href="format.html">XML Format</a></li><li><a href="python.html">Binding for Python</a></li><li><a href="errors.html">Handling of errors</a></li><li><a href="FAQ.html">FAQ</a></li><li><a href="bugs.html">Reporting bugs and getting help</a></li><li><a href="html/index.html">API Menu</a></li><li><a href="examples/index.html">C code examples</a></li><li><a href="ChangeLog.html">Recent Changes</a></li></ul></div><div class="llinks2"><h3 class="links2"><span>related links</span></h3><ul><li><a href="https://www.redhat.com/archives/libvir-list/">Mail archive</a></li><li><a href="https://bugzilla.redhat.com/bugzilla/buglist.cgi?product=Fedora+Core&amp;component=libvirt&amp;bug_status=NEW&amp;bug_status=ASSIGNED&amp;bug_status=REOPENED&amp;bug_status=MODIFIED&amp;short_desc_type=allwordssubstr&amp;short_desc=&amp;long_desc_type=allwordssubstr">Open bugs</a></li><li><a href="http://virt-manager.et.redhat.com/">virt-manager</a></li><li><a href="http://search.cpan.org/~danberr/Sys-Virt-0.1.0/">Perl bindings</a></li><li><a href="http://www.cl.cam.ac.uk/Research/SRG/netos/xen/index.html">Xen project</a></li><li><form action="search.php" enctype="application/x-www-form-urlencoded" method="get"><input name="query" type="text" size="12" value="Search..." /><input name="submit" type="submit" value="Go" /></form></li><li><a href="http://xmlsoft.org/"><img src="Libxml2-Logo-90x34.gif" alt="Made with Libxml2 Logo" /></a></li></ul><p class="credits">Graphics and design by <a href="mail:dfong@redhat.com">Diana Fong</a></p></div></div><div id="bottom"><p class="p1"></p></div></div></body></html>