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    <h1 >Logging in the library and the daemon</h1>
    <p>Libvirt includes logging facilities starting from version 0.6.0,
       this complements the <a href="errors.html">error handling</a>
       mechanism and APIs to allow tracing though the execution of the
       library as well as in the libvirtd daemon.</p>
    <p>The logging functionalities in libvirt are based on 3 key concepts,
       similar to the one present in other generic logging facilities like
       log4j:</p>
    <ul>
      <li>log messages: they are information generated at runtime by
          the libvirt code, it includes a priority level (DEBUG = 1,
          INFO = 2, WARNING = 3, ERROR = 4), a category, function name and
          line number, indicating where it originated from, and finally
          a formatted message, in addition the library adds a timestamp
          at the begining of the message</li>
      <li>log filters: it's a set of patter and priorities allowing to acept
          or reject a log message, if the message category matches a filter,
          the message priority is compared to the filter priority, if lower
          the message is discarded, if higher the message is output. If
          no filter matches, then a general priority level is applied to
          all remaining messages. This allows to capture for example all
          debug messages for the QEmu driver, but otherwise only allow
          errors to show up from other parts</li>
      <li>log outputs: once a message has gone though filtering a set of
          output defines where to send the message, they can also filter
          based on the priority, for example it may be useful to output
          all messages to a debugging file but only allow errors to be
          logged though syslog.</li>
    </ul>
    <p>The library configuration of logging is though 3 environment variables
    allowing to control the logging behaviour:</p>
    <ul>
      <li>LIBVIRT_DEBUG: it can take the four following values:
      <ul>
        <li>1 or "debug": asking the library to log every message emitted,
            though the filters can be used to avoid filling up the output</li>
        <li>2 or "info": log all non-debugging informations</li>
        <li>3 or "warn": log warnings and errors, that's the default value</li>
        <li>4 or "error": log only error messages</li>
      </ul></li>
      <li>LIBVIRT_LOG_FILTERS: allow to define logging filters</li>
      <li>LIBVIRT_LOG_OUTPUTS: allow to define logging outputs</li>
    </ul>
    <p>Similary the daemon logging behaviour can be tuned using 3 config
    variables, stored in the configuration file:
    <ul>
      <li>log_level: accepts the following values:
      <ul>
        <li>0: no logging at all</li>
        <li>4: only errors</li>
        <li>3: warnings and errors</li>
        <li>2: informations, warnings and errors</li>
        <li>1: debug and everything</li>
      </ul></li>
      <li>log_filters: allow to define logging filters</li>
      <li>log_outputs: allow to define logging outputs</li>
    </ul>
    <p>In both case the syntax for filters and outputs is similar.</p>
    <p>The format for a filter is:</p>
    <pre>x:name</pre>
    <p>where <code>name</code> is a match string e.g. <code>remote</code> or
    <code>qemu</code> and the x is the minimal level where matching messages
    should be logged:</p>
    <ul>
      <li>1: DEBUG</li>
      <li>2: INFO</li>
      <li>3: WARNING</li>
      <li>4: ERROR</li>
    </ul>
    <p>Multiple filter can be defined in a single string, they just need to be
    separated by spaces, e.g: <code>"3:remote 4:event"</code> to only get
    warning or errors from the remote layer and only errors from the event
    layer.<p>
    <p>The format for an output can be one of those 3 forms:</p>
    <ul>
      <li><code>x:stderr</code> output goes to stderr</li>
      <li><code>x:syslog:name</code> use syslog for the output and use the
      given <code>name</code> as the ident</li>
      <li><code>x:file:file_path</code>output to a file, with the given
      filepath</li>
    </ul>
    <p>In all cases the x prefix is the minimal level, acting as a filter:</p>
    <ul>
      <li>0: everything</li>
      <li>1: DEBUG</li>
      <li>2: INFO</li>
      <li>3: WARNING</li>
      <li>4: ERROR</li>
    </ul>
    <p>Multiple output can be defined , they just need to be separated by
       spaces, e.g.: <code>"3:syslog:libvirtd 0:file:/tmp/libvirt.log"</code>
       will log all warnings and errors to syslog under the libvirtd ident
       but also log everything debugging and informations included in the
       file <code>/tmp/libvirt.log</code></p>
    <p>For example setting up the following:</p>
    <pre>export LIBVIRT_DEBUG=1
export LIBVIRT_LOG_OUTPUTS="0:file:virsh.log"</pre>
    <p>and then running virsh will accumulate the logs in the
    <code>virsh.log</code> file in a way similar to:</p>
    <pre>14:29:04.771: debug : virInitialize:278 : register drivers
14:29:04.771: debug : virRegisterDriver:618 : registering Test as driver 0</pre>
    <p>the messages are timestamped, there is also the level recorded,
    if debug the name of the function is also printed and then the formatted
    message. This should be sufficient to at least get a precise idea of
    what is happening and where things are going wrong, allowing to then
    put the correct breakpoints when running under a debugger.</p>
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