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<section id='main' class="content mod">
<h1 class='fqn'><span class='in-band'>Crate <a class='mod' href=''>log</a></span><span class='out-of-band'><span id='render-detail'>
<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
[<span class='inner'>−</span>]
</a>
</span><a id='src-0' class='srclink' href='../src/log/lib.rs.html#11-773' title='goto source code'>[src]</a></span></h1>
<div class='docblock'><p>A lightweight logging facade.</p>
<p>A logging facade provides a single logging API that abstracts over the
actual logging implementation. Libraries can use the logging API provided
by this crate, and the consumer of those libraries can choose the logging
framework that is most suitable for its use case.</p>
<p>If no logging implementation is selected, the facade falls back to a "noop"
implementation that ignores all log messages. The overhead in this case
is very small - just an integer load, comparison and jump.</p>
<p>A log request consists of a target, a level, and a body. A target is a
string which defaults to the module path of the location of the log
request, though that default may be overridden. Logger implementations
typically use the target to filter requests based on some user
configuration.</p>
<h1 id="use" class='section-header'><a
href="#use">Use</a></h1>
<h2 id="in-libraries" class='section-header'><a
href="#in-libraries">In libraries</a></h2>
<p>Libraries should link only to the <code>log</code> crate, and use the provided
macros to log whatever information will be useful to downstream consumers.</p>
<h3 id="examples" class='section-header'><a
href="#examples">Examples</a></h3><pre class='rust rust-example-rendered'>
<span class='attribute'>#[<span class='ident'>macro_use</span>]</span>
<span class='kw'>extern</span> <span class='kw'>crate</span> <span class='ident'>log</span>;
<span class='kw'>pub</span> <span class='kw'>fn</span> <span class='ident'>shave_the_yak</span>(<span class='ident'>yak</span>: <span class='kw-2'>&</span><span class='ident'>Yak</span>) {
<span class='macro'>info</span><span class='macro'>!</span>(<span class='ident'>target</span>: <span class='string'>"yak_events"</span>, <span class='string'>"Commencing yak shaving for {:?}"</span>, <span class='ident'>yak</span>);
<span class='kw'>loop</span> {
<span class='kw'>match</span> <span class='ident'>find_a_razor</span>() {
<span class='prelude-val'>Ok</span>(<span class='ident'>razor</span>) <span class='op'>=></span> {
<span class='macro'>info</span><span class='macro'>!</span>(<span class='string'>"Razor located: {}"</span>, <span class='ident'>razor</span>);
<span class='ident'>yak</span>.<span class='ident'>shave</span>(<span class='ident'>razor</span>);
<span class='kw'>break</span>;
}
<span class='prelude-val'>Err</span>(<span class='ident'>err</span>) <span class='op'>=></span> {
<span class='macro'>warn</span><span class='macro'>!</span>(<span class='string'>"Unable to locate a razor: {}, retrying"</span>, <span class='ident'>err</span>);
}
}
}
}
</pre>
<h2 id="in-executables" class='section-header'><a
href="#in-executables">In executables</a></h2>
<p>Executables should chose a logging framework and initialize it early in the
runtime of the program. Logging frameworks will typically include a
function to do this. Any log messages generated before the framework is
initialized will be ignored.</p>
<p>The executable itself may use the <code>log</code> crate to log as well.</p>
<h3 id="warning" class='section-header'><a
href="#warning">Warning</a></h3>
<p>The logging system may only be initialized once.</p>
<h3 id="examples-1" class='section-header'><a
href="#examples-1">Examples</a></h3><pre class='rust rust-example-rendered'>
<span class='attribute'>#[<span class='ident'>macro_use</span>]</span>
<span class='kw'>extern</span> <span class='kw'>crate</span> <span class='ident'>log</span>;
<span class='kw'>extern</span> <span class='kw'>crate</span> <span class='ident'>my_logger</span>;
<span class='kw'>fn</span> <span class='ident'>main</span>() {
<span class='ident'>my_logger</span>::<span class='ident'>init</span>();
<span class='macro'>info</span><span class='macro'>!</span>(<span class='string'>"starting up"</span>);
<span class='comment'>// ...</span>
}
</pre>
<h1 id="logger-implementations" class='section-header'><a
href="#logger-implementations">Logger implementations</a></h1>
<p>Loggers implement the <code>Log</code> trait. Here's a very basic example that simply
logs all messages at the <code>Error</code>, <code>Warn</code> or <code>Info</code> levels to stdout:</p>
<pre class='rust rust-example-rendered'>
<span class='kw'>extern</span> <span class='kw'>crate</span> <span class='ident'>log</span>;
<span class='kw'>use</span> <span class='ident'>log</span>::{<span class='ident'>LogRecord</span>, <span class='ident'>LogLevel</span>, <span class='ident'>LogMetadata</span>};
<span class='kw'>struct</span> <span class='ident'>SimpleLogger</span>;
<span class='kw'>impl</span> <span class='ident'>log</span>::<span class='ident'>Log</span> <span class='kw'>for</span> <span class='ident'>SimpleLogger</span> {
<span class='kw'>fn</span> <span class='ident'>enabled</span>(<span class='kw-2'>&</span><span class='self'>self</span>, <span class='ident'>metadata</span>: <span class='kw-2'>&</span><span class='ident'>LogMetadata</span>) <span class='op'>-></span> <span class='ident'>bool</span> {
<span class='ident'>metadata</span>.<span class='ident'>level</span>() <span class='op'><=</span> <span class='ident'>LogLevel</span>::<span class='ident'>Info</span>
}
<span class='kw'>fn</span> <span class='ident'>log</span>(<span class='kw-2'>&</span><span class='self'>self</span>, <span class='ident'>record</span>: <span class='kw-2'>&</span><span class='ident'>LogRecord</span>) {
<span class='kw'>if</span> <span class='self'>self</span>.<span class='ident'>enabled</span>(<span class='ident'>record</span>.<span class='ident'>metadata</span>()) {
<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{} - {}"</span>, <span class='ident'>record</span>.<span class='ident'>level</span>(), <span class='ident'>record</span>.<span class='ident'>args</span>());
}
}
}
</pre>
<p>Loggers are installed by calling the <code>set_logger</code> function. It takes a
closure which is provided a <code>MaxLogLevel</code> token and returns a <code>Log</code> trait
object. The <code>MaxLogLevel</code> token controls the global maximum log level. The
logging facade uses this as an optimization to improve performance of log
messages at levels that are disabled. In the case of our example logger,
we'll want to set the maximum log level to <code>Info</code>, since we ignore any
<code>Debug</code> or <code>Trace</code> level log messages. A logging framework should provide a
function that wraps a call to <code>set_logger</code>, handling initialization of the
logger:</p>
<pre class='rust rust-example-rendered'>
<span class='kw'>pub</span> <span class='kw'>fn</span> <span class='ident'>init</span>() <span class='op'>-></span> <span class='prelude-ty'>Result</span><span class='op'><</span>(), <span class='ident'>SetLoggerError</span><span class='op'>></span> {
<span class='ident'>log</span>::<span class='ident'>set_logger</span>(<span class='op'>|</span><span class='ident'>max_log_level</span><span class='op'>|</span> {
<span class='ident'>max_log_level</span>.<span class='ident'>set</span>(<span class='ident'>LogLevelFilter</span>::<span class='ident'>Info</span>);
<span class='ident'>Box</span>::<span class='ident'>new</span>(<span class='ident'>SimpleLogger</span>)
})
}
</pre>
</div><h2 id='macros' class='section-header'><a href="#macros">Macros</a></h2>
<table>
<tr class=' module-item'>
<td><a class='macro' href='macro.debug!.html'
title='log::debug!'>debug!</a></td>
<td class='docblock short'>
<p>Logs a message at the debug level.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='macro' href='macro.error!.html'
title='log::error!'>error!</a></td>
<td class='docblock short'>
<p>Logs a message at the error level.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='macro' href='macro.info!.html'
title='log::info!'>info!</a></td>
<td class='docblock short'>
<p>Logs a message at the info level.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='macro' href='macro.log!.html'
title='log::log!'>log!</a></td>
<td class='docblock short'>
<p>The standard logging macro.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='macro' href='macro.log_enabled!.html'
title='log::log_enabled!'>log_enabled!</a></td>
<td class='docblock short'>
<p>Determines if a message logged at the specified level in that module will
be logged.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='macro' href='macro.trace!.html'
title='log::trace!'>trace!</a></td>
<td class='docblock short'>
<p>Logs a message at the trace level.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='macro' href='macro.warn!.html'
title='log::warn!'>warn!</a></td>
<td class='docblock short'>
<p>Logs a message at the warn level.</p>
</td>
</tr>
</table><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
<table>
<tr class=' module-item'>
<td><a class='struct' href='struct.LogLocation.html'
title='log::LogLocation'>LogLocation</a></td>
<td class='docblock short'>
<p>The location of a log message.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='struct' href='struct.LogMetadata.html'
title='log::LogMetadata'>LogMetadata</a></td>
<td class='docblock short'>
<p>Metadata about a log message.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='struct' href='struct.LogRecord.html'
title='log::LogRecord'>LogRecord</a></td>
<td class='docblock short'>
<p>The "payload" of a log message.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='struct' href='struct.MaxLogLevelFilter.html'
title='log::MaxLogLevelFilter'>MaxLogLevelFilter</a></td>
<td class='docblock short'>
<p>A token providing read and write access to the global maximum log level
filter.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='struct' href='struct.SetLoggerError.html'
title='log::SetLoggerError'>SetLoggerError</a></td>
<td class='docblock short'>
<p>The type returned by <code>set_logger</code> if <code>set_logger</code> has already been called.</p>
</td>
</tr>
</table><h2 id='enums' class='section-header'><a href="#enums">Enums</a></h2>
<table>
<tr class=' module-item'>
<td><a class='enum' href='enum.LogLevel.html'
title='log::LogLevel'>LogLevel</a></td>
<td class='docblock short'>
<p>An enum representing the available verbosity levels of the logging framework</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='enum' href='enum.LogLevelFilter.html'
title='log::LogLevelFilter'>LogLevelFilter</a></td>
<td class='docblock short'>
<p>An enum representing the available verbosity level filters of the logging
framework.</p>
</td>
</tr>
</table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
<table>
<tr class=' module-item'>
<td><a class='trait' href='trait.Log.html'
title='log::Log'>Log</a></td>
<td class='docblock short'>
<p>A trait encapsulating the operations required of a logger</p>
</td>
</tr>
</table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
<table>
<tr class=' module-item'>
<td><a class='fn' href='fn.max_log_level.html'
title='log::max_log_level'>max_log_level</a></td>
<td class='docblock short'>
<p>Returns the current maximum log level.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class='fn' href='fn.set_logger.html'
title='log::set_logger'>set_logger</a></td>
<td class='docblock short'>
<p>Sets the global logger.</p>
</td>
</tr>
</table></section>
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