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* qemud/internal.h, qemud/qemud.h: Rename this file so it doesn't conflict with src/internal.h. * HACKING: Document how header files should be used. * qemud/Makefile.am: Add src/ directory to includes. * qemud/event.c, qemud/mdns.c, qemud/qemud.c, qemud/remote.c, qemud/remote_protocol.c, qemud/remote_protocol.h, qemud/remote_protocol.x, src/buf.c, src/libvirt.c, src/nodeinfo.c, src/qemu_conf.c, src/qemu_driver.c, src/stats_linux.c, src/storage_backend.c, src/storage_backend_fs.c, src/storage_backend_iscsi.c, src/storage_backend_logical.c, src/storage_conf.c, src/storage_driver.c, src/util.c, src/util.h, src/virsh.c, src/virterror.c, src/xend_internal.c, src/xml.c, tests/reconnect.c, tests/xmlrpctest.c, tests/qparamtest.c: Standardize use of header files. * docs/*, po/*: Rebuild docs.
169 lines
8.6 KiB
HTML
169 lines
8.6 KiB
HTML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<!--
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This file is autogenerated from errors.html.in
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Do not edit this file. Changes will be lost.
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-->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" />
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<link rel="stylesheet" type="text/css" href="main.css" />
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<link rel="SHORTCUT ICON" href="32favicon.png" />
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<title>libvirt: Handling of errors</title>
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<meta name="description" content="libvirt, virtualization, virtualization API" />
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<div id="menu">
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<ul class="l0">
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<li>
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<div>
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<a title="Front page of the libvirt website" class="inactive" href="index.html">Home</a>
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<a title="Details of new features and bugs fixed in each release" class="inactive" href="news.html">News</a>
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<a title="Get the latest source releases, binary builds and get access to the source repository" class="inactive" href="downloads.html">Downloads</a>
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<a title="How and where to report bugs and request features" class="inactive" href="bugs.html">Bug reports</a>
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<a title="Miscellaneous links of interest related to libvirt" class="inactive" href="relatedlinks.html">Related Links</a>
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<div id="content">
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<h1>Handling of errors</h1>
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<p>The main goals of libvirt when it comes to error handling are:</p>
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<ul>
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<li>provide as much detail as possible</li>
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<li>provide the information as soon as possible</li>
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<li>dont force the library user into one style of error handling</li>
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</ul>
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<p>As result the library provide both synchronous, callback based and
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asynchronous error reporting. When an error happens in the library code the
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error is logged, allowing to retrieve it later and if the user registered an
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error callback it will be called synchronously. Once the call to libvirt ends
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the error can be detected by the return value and the full information for
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the last logged error can be retrieved.</p>
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<p>To avoid as much as possible troubles with a global variable in a
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multithreaded environment, libvirt will associate when possible the errors to
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the current connection they are related to, that way the error is stored in a
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dynamic structure which can be made thread specific. Error callback can be
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set specifically to a connection with</p>
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<p>So error handling in the code is the following:</p>
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<ol>
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<li>if the error can be associated to a connection for example when failing
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to look up a domain
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<ol><li>if there is a callback associated to the connection set with <a href="html/libvirt-virterror.html#virConnSetErrorFunc">virConnSetErrorFunc</a>,
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call it with the error information</li><li>otherwise if there is a global callback set with <a href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>,
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call it with the error information</li><li>otherwise call <a href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>
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which is the default error function of the library issuing the error
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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>
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<li>otherwise like when failing to create an hypervisor connection:
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<ol><li>if there is a global callback set with <a href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a>,
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call it with the error information</li><li>otherwise call <a href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>
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which is the default error function of the library issuing the error
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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>
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</ol>
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<p>In all cases the error information is provided as a <a href="html/libvirt-virterror.html#virErrorPtr">virErrorPtr</a> pointer to
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read-only structure <a href="html/libvirt-virterror.html#virError">virError</a> containing the
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following fields:</p>
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<ul>
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<li>code: an error number from the <a href="html/libvirt-virterror.html#virErrorNumber">virErrorNumber</a>
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enum</li>
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<li>domain: an enum indicating which part of libvirt raised the error see
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<a href="html/libvirt-virterror.html#virErrorDomain">virErrorDomain</a></li>
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<li>level: the error level, usually VIR_ERR_ERROR, though there is room for
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warnings like VIR_ERR_WARNING</li>
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<li>message: the full human-readable formatted string of the error</li>
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<li>conn: if available a pointer to the <a href="html/libvirt-libvirt.html#virConnectPtr">virConnectPtr</a>
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connection to the hypervisor where this happened</li>
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<li>dom: if available a pointer to the <a href="html/libvirt-libvirt.html#virDomainPtr">virDomainPtr</a> domain
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targeted in the operation</li>
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</ul>
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<p>and then extra raw information about the error which may be initialized
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to 0 or NULL if unused</p>
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<ul>
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<li>str1, str2, str3: string information, usually str1 is the error
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message format</li>
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<li>int1, int2: integer information</li>
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</ul>
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<p>So usually, setting up specific error handling with libvirt consist of
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registering an handler with with <a href="html/libvirt-virterror.html#virSetErrorFunc">virSetErrorFunc</a> or
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with <a href="html/libvirt-virterror.html#virConnSetErrorFunc">virConnSetErrorFunc</a>,
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check the value of the code value, take appropriate action, if needed let
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libvirt print the error on stderr by calling <a href="html/libvirt-virterror.html#virDefaultErrorFunc">virDefaultErrorFunc</a>.
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For asynchronous error handing, set such a function doing nothing to avoid
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the error being reported on stderr, and call virConnGetLastError or
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virGetLastError when an API call returned an error value. It can be a good
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idea to use <a href="html/libvirt-virterror.html#virResetLastError">virResetError</a> or <a href="html/libvirt-virterror.html#virConnResetLastError">virConnResetLastError</a>
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once an error has been processed fully.</p>
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<p>At the python level, there only a global reporting callback function at
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this point, see the error.py example about it:</p>
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<pre>def handler(ctxt, err):
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global errno
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#print "handler(%s, %s)" % (ctxt, err)
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errno = err
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libvirt.registerErrorHandler(handler, 'context') </pre>
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<p>the second argument to the registerErrorHandler function is passed as the
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first argument of the callback like in the C version. The error is a tuple
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containing the same field as a virError in C, but cast to Python.</p>
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</div>
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Sponsored by:<br /><a href="http://et.redhat.com/"><img src="et.png" alt="Project sponsored by Red Hat Emerging Technology" /></a></p>
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