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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="A lightweight logging facade."><title>log - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-dd39b87e5fcfba68.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="log" data-themes="" data-resource-suffix="" data-rustdoc-version="1.80.0 (051478957 2024-07-21)" data-channel="1.80.0" data-search-js="search-d52510db62a78183.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-118b08c4c78b968e.js"></script><script defer src="../crates.js"></script><script defer src="../static.files/main-20a3ad099b048cf2.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-df360f571f6edeae.css"></noscript><link rel="icon" href="https://www.rust-lang.org/favicon.ico"></head><body class="rustdoc mod crate"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle" title="show sidebar"></button><a class="logo-container" href="../log/index.html"><img src="https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png" alt=""></a></nav><nav class="sidebar"><div class="sidebar-crate"><a class="logo-container" href="../log/index.html"><img src="https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png" alt="logo"></a><h2><a href="../log/index.html">log</a><span class="version">0.4.22</span></h2></div><div class="sidebar-elems"><ul class="block"><li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#macros">Macros</a></li><li><a href="#structs">Structs</a></li><li><a href="#enums">Enums</a></li><li><a href="#constants">Constants</a></li><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</a></li></ul></section></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><h1>Crate <a class="mod" href="#">log</a><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../src/log/lib.rs.html#11-1878">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>A lightweight logging facade.</p>
<p>The <code>log</code> crate 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
implementation 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 <em>target</em>, a <em>level</em>, and a <em>body</em>. 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>
<h2 id="usage"><a class="doc-anchor" href="#usage">§</a>Usage</h2>
<p>The basic use of the log crate is through the five logging macros: <a href="./macro.error.html"><code>error!</code></a>,
<a href="./macro.warn.html"><code>warn!</code></a>, <a href="./macro.info.html"><code>info!</code></a>, <a href="./macro.debug.html"><code>debug!</code></a> and <a href="./macro.trace.html"><code>trace!</code></a>
where <code>error!</code> represents the highest-priority log messages
and <code>trace!</code> the lowest. The log messages are filtered by configuring
the log level to exclude messages with a lower priority.
Each of these macros accept format strings similarly to <a href="https://doc.rust-lang.org/stable/std/macro.println.html"><code>println!</code></a>.</p>
<p>Avoid writing expressions with side-effects in log statements. They may not be evaluated.</p>
<h3 id="in-libraries"><a class="doc-anchor" href="#in-libraries">§</a>In libraries</h3>
<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>
<h4 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h4>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>log::{info, warn};
<span class="kw">pub fn </span>shave_the_yak(yak: <span class="kw-2">&amp;mut </span>Yak) {
<span class="macro">info!</span>(target: <span class="string">"yak_events"</span>, <span class="string">"Commencing yak shaving for {yak:?}"</span>);
<span class="kw">loop </span>{
<span class="kw">match </span>find_a_razor() {
<span class="prelude-val">Ok</span>(razor) =&gt; {
<span class="macro">info!</span>(<span class="string">"Razor located: {razor}"</span>);
yak.shave(razor);
<span class="kw">break</span>;
}
<span class="prelude-val">Err</span>(err) =&gt; {
<span class="macro">warn!</span>(<span class="string">"Unable to locate a razor: {err}, retrying"</span>);
}
}
}
}</code></pre></div>
<h3 id="in-executables"><a class="doc-anchor" href="#in-executables">§</a>In executables</h3>
<p>Executables should choose a logging implementation and initialize it early in the
runtime of the program. Logging implementations will typically include a
function to do this. Any log messages generated before
the implementation is initialized will be ignored.</p>
<p>The executable itself may use the <code>log</code> crate to log as well.</p>
<h4 id="warning"><a class="doc-anchor" href="#warning">§</a>Warning</h4>
<p>The logging system may only be initialized once.</p>
<h3 id="structured-logging"><a class="doc-anchor" href="#structured-logging">§</a>Structured logging</h3>
<p>If you enable the <code>kv</code> feature you can associate structured values
with your log records. If we take the example from before, we can include
some additional context besides whats in the formatted message:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>log::{info, warn};
<span class="kw">pub fn </span>shave_the_yak(yak: <span class="kw-2">&amp;mut </span>Yak) {
<span class="macro">info!</span>(target: <span class="string">"yak_events"</span>, yak:serde; <span class="string">"Commencing yak shaving"</span>);
<span class="kw">loop </span>{
<span class="kw">match </span>find_a_razor() {
<span class="prelude-val">Ok</span>(razor) =&gt; {
<span class="macro">info!</span>(razor; <span class="string">"Razor located"</span>);
yak.shave(razor);
<span class="kw">break</span>;
}
<span class="prelude-val">Err</span>(e) =&gt; {
<span class="macro">warn!</span>(e:err; <span class="string">"Unable to locate a razor, retrying"</span>);
}
}
}
}</code></pre></div>
<p>See the [<code>kv</code>] module documentation for more details.</p>
<h2 id="available-logging-implementations"><a class="doc-anchor" href="#available-logging-implementations">§</a>Available logging implementations</h2>
<p>In order to produce log output executables have to use
a logger implementation compatible with the facade.
There are many available implementations to choose from,
here are some of the most popular ones:</p>
<ul>
<li>Simple minimal loggers:
<ul>
<li><a href="https://docs.rs/env_logger/*/env_logger/">env_logger</a></li>
<li><a href="https://docs.rs/colog/*/colog/">colog</a></li>
<li><a href="https://github.com/borntyping/rust-simple_logger">simple_logger</a></li>
<li><a href="https://github.com/drakulix/simplelog.rs">simplelog</a></li>
<li><a href="https://docs.rs/pretty_env_logger/*/pretty_env_logger/">pretty_env_logger</a></li>
<li><a href="https://docs.rs/stderrlog/*/stderrlog/">stderrlog</a></li>
<li><a href="https://docs.rs/flexi_logger/*/flexi_logger/">flexi_logger</a></li>
<li><a href="https://docs.rs/call_logger/*/call_logger/">call_logger</a></li>
<li><a href="https://docs.rs/structured-logger/latest/structured_logger/">structured-logger</a></li>
</ul>
</li>
<li>Complex configurable frameworks:
<ul>
<li><a href="https://docs.rs/log4rs/*/log4rs/">log4rs</a></li>
<li><a href="https://docs.rs/fern/*/fern/">fern</a></li>
</ul>
</li>
<li>Adaptors for other facilities:
<ul>
<li><a href="https://docs.rs/syslog/*/syslog/">syslog</a></li>
<li><a href="https://docs.rs/slog-stdlog/*/slog_stdlog/">slog-stdlog</a></li>
<li><a href="https://docs.rs/systemd-journal-logger/*/systemd_journal_logger/">systemd-journal-logger</a></li>
<li><a href="https://docs.rs/android_log/*/android_log/">android_log</a></li>
<li><a href="https://docs.rs/win_dbg_logger/*/win_dbg_logger/">win_dbg_logger</a></li>
<li><a href="https://docs.rs/db_logger/*/db_logger/">db_logger</a></li>
<li><a href="https://docs.rs/log-to-defmt/*/log_to_defmt/">log-to-defmt</a></li>
<li><a href="https://docs.rs/logcontrol-log/*/logcontrol_log/">logcontrol-log</a></li>
</ul>
</li>
<li>For WebAssembly binaries:
<ul>
<li><a href="https://docs.rs/console_log/*/console_log/">console_log</a></li>
</ul>
</li>
<li>For dynamic libraries:
<ul>
<li>You may need to construct an FFI-safe wrapper over <code>log</code> to initialize in your libraries</li>
</ul>
</li>
<li>Utilities:
<ul>
<li><a href="https://docs.rs/log_err/*/log_err/">log_err</a></li>
<li><a href="https://docs.rs/log-reload/*/log_reload/">log-reload</a></li>
</ul>
</li>
</ul>
<h2 id="implementing-a-logger"><a class="doc-anchor" href="#implementing-a-logger">§</a>Implementing a Logger</h2>
<p>Loggers implement the <a href="trait.Log.html"><code>Log</code></a> trait. Heres a very basic example that simply
logs all messages at the <a href="enum.Level.html"><code>Error</code></a>, <a href="enum.Level.html"><code>Warn</code></a> or
<a href="enum.Level.html"><code>Info</code></a> levels to stdout:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>log::{Record, Level, Metadata};
<span class="kw">struct </span>SimpleLogger;
<span class="kw">impl </span>log::Log <span class="kw">for </span>SimpleLogger {
<span class="kw">fn </span>enabled(<span class="kw-2">&amp;</span><span class="self">self</span>, metadata: <span class="kw-2">&amp;</span>Metadata) -&gt; bool {
metadata.level() &lt;= Level::Info
}
<span class="kw">fn </span>log(<span class="kw-2">&amp;</span><span class="self">self</span>, record: <span class="kw-2">&amp;</span>Record) {
<span class="kw">if </span><span class="self">self</span>.enabled(record.metadata()) {
<span class="macro">println!</span>(<span class="string">"{} - {}"</span>, record.level(), record.args());
}
}
<span class="kw">fn </span>flush(<span class="kw-2">&amp;</span><span class="self">self</span>) {}
}
</code></pre></div>
<p>Loggers are installed by calling the <a href="fn.set_logger.html"><code>set_logger</code></a> function. The maximum
log level also needs to be adjusted via the <a href="fn.set_max_level.html"><code>set_max_level</code></a> function. The
logging facade uses this as an optimization to improve performance of log
messages at levels that are disabled. Its important to set it, as it
defaults to <a href="enum.LevelFilter.html"><code>Off</code></a>, so no log messages will ever be captured!
In the case of our example logger, well want to set the maximum log level
to <a href="enum.LevelFilter.html"><code>Info</code></a>, since we ignore any <a href="enum.Level.html"><code>Debug</code></a> or
<a href="enum.Level.html"><code>Trace</code></a> level log messages. A logging implementation should
provide a function that wraps a call to <a href="fn.set_logger.html"><code>set_logger</code></a> and
<a href="fn.set_max_level.html"><code>set_max_level</code></a>, handling initialization of the logger:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>log::{SetLoggerError, LevelFilter};
<span class="kw">static </span>LOGGER: SimpleLogger = SimpleLogger;
<span class="kw">pub fn </span>init() -&gt; <span class="prelude-ty">Result</span>&lt;(), SetLoggerError&gt; {
log::set_logger(<span class="kw-2">&amp;</span>LOGGER)
.map(|()| log::set_max_level(LevelFilter::Info))
}</code></pre></div>
<p>Implementations that adjust their configurations at runtime should take care
to adjust the maximum log level as well.</p>
<h2 id="use-with-std"><a class="doc-anchor" href="#use-with-std">§</a>Use with <code>std</code></h2>
<p><code>set_logger</code> requires you to provide a <code>&amp;'static Log</code>, which can be hard to
obtain if your logger depends on some runtime configuration. The
<code>set_boxed_logger</code> function is available with the <code>std</code> Cargo feature. It is
identical to <code>set_logger</code> except that it takes a <code>Box&lt;Log&gt;</code> rather than a
<code>&amp;'static Log</code>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">pub fn </span>init() -&gt; <span class="prelude-ty">Result</span>&lt;(), SetLoggerError&gt; {
log::set_boxed_logger(Box::new(SimpleLogger))
.map(|()| log::set_max_level(LevelFilter::Info))
}</code></pre></div>
<h2 id="compile-time-filters"><a class="doc-anchor" href="#compile-time-filters">§</a>Compile time filters</h2>
<p>Log levels can be statically disabled at compile time by enabling one of these Cargo features:</p>
<ul>
<li><code>max_level_off</code></li>
<li><code>max_level_error</code></li>
<li><code>max_level_warn</code></li>
<li><code>max_level_info</code></li>
<li><code>max_level_debug</code></li>
<li><code>max_level_trace</code></li>
</ul>
<p>Log invocations at disabled levels will be skipped and will not even be present in the
resulting binary. These features control the value of the <code>STATIC_MAX_LEVEL</code> constant. The
logging macros check this value before logging a message. By default, no levels are disabled.</p>
<p>It is possible to override this level for release builds only with the following features:</p>
<ul>
<li><code>release_max_level_off</code></li>
<li><code>release_max_level_error</code></li>
<li><code>release_max_level_warn</code></li>
<li><code>release_max_level_info</code></li>
<li><code>release_max_level_debug</code></li>
<li><code>release_max_level_trace</code></li>
</ul>
<p>Libraries should avoid using the max level features because theyre global and cant be changed
once theyre set.</p>
<p>For example, a crate can disable trace level logs in debug builds and trace, debug, and info
level logs in release builds with the following configuration:</p>
<div class="example-wrap"><pre class="language-toml"><code>[dependencies]
log = { version = &quot;0.4&quot;, features = [&quot;max_level_debug&quot;, &quot;release_max_level_warn&quot;] }
</code></pre></div><h2 id="crate-feature-flags"><a class="doc-anchor" href="#crate-feature-flags">§</a>Crate Feature Flags</h2>
<p>The following crate feature flags are available in addition to the filters. They are
configured in your <code>Cargo.toml</code>.</p>
<ul>
<li><code>std</code> allows use of <code>std</code> crate instead of the default <code>core</code>. Enables using <code>std::error</code> and
<code>set_boxed_logger</code> functionality.</li>
<li><code>serde</code> enables support for serialization and deserialization of <code>Level</code> and <code>LevelFilter</code>.</li>
</ul>
<div class="example-wrap"><pre class="language-toml"><code>[dependencies]
log = { version = &quot;0.4&quot;, features = [&quot;std&quot;, &quot;serde&quot;] }
</code></pre></div><h2 id="version-compatibility"><a class="doc-anchor" href="#version-compatibility">§</a>Version compatibility</h2>
<p>The 0.3 and 0.4 versions of the <code>log</code> crate are almost entirely compatible. Log messages
made using <code>log</code> 0.3 will forward transparently to a logger implementation using <code>log</code> 0.4. Log
messages made using <code>log</code> 0.4 will forward to a logger implementation using <code>log</code> 0.3, but the
module path and file name information associated with the message will unfortunately be lost.</p>
</div></details><h2 id="macros" class="section-header">Macros<a href="#macros" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="macro" href="macro.debug.html" title="macro log::debug">debug</a></div><div class="desc docblock-short">Logs a message at the debug level.</div></li><li><div class="item-name"><a class="macro" href="macro.error.html" title="macro log::error">error</a></div><div class="desc docblock-short">Logs a message at the error level.</div></li><li><div class="item-name"><a class="macro" href="macro.info.html" title="macro log::info">info</a></div><div class="desc docblock-short">Logs a message at the info level.</div></li><li><div class="item-name"><a class="macro" href="macro.log.html" title="macro log::log">log</a></div><div class="desc docblock-short">The standard logging macro.</div></li><li><div class="item-name"><a class="macro" href="macro.log_enabled.html" title="macro log::log_enabled">log_enabled</a></div><div class="desc docblock-short">Determines if a message logged at the specified level in that module will
be logged.</div></li><li><div class="item-name"><a class="macro" href="macro.trace.html" title="macro log::trace">trace</a></div><div class="desc docblock-short">Logs a message at the trace level.</div></li><li><div class="item-name"><a class="macro" href="macro.warn.html" title="macro log::warn">warn</a></div><div class="desc docblock-short">Logs a message at the warn level.</div></li></ul><h2 id="structs" class="section-header">Structs<a href="#structs" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.Metadata.html" title="struct log::Metadata">Metadata</a></div><div class="desc docblock-short">Metadata about a log message.</div></li><li><div class="item-name"><a class="struct" href="struct.MetadataBuilder.html" title="struct log::MetadataBuilder">MetadataBuilder</a></div><div class="desc docblock-short">Builder for <a href="struct.Metadata.html"><code>Metadata</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.ParseLevelError.html" title="struct log::ParseLevelError">ParseLevelError</a></div><div class="desc docblock-short">The type returned by <a href="https://doc.rust-lang.org/std/str/trait.FromStr.html#tymethod.from_str"><code>from_str</code></a> when the string doesnt match any of the log levels.</div></li><li><div class="item-name"><a class="struct" href="struct.Record.html" title="struct log::Record">Record</a></div><div class="desc docblock-short">The “payload” of a log message.</div></li><li><div class="item-name"><a class="struct" href="struct.RecordBuilder.html" title="struct log::RecordBuilder">RecordBuilder</a></div><div class="desc docblock-short">Builder for <a href="struct.Record.html"><code>Record</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.SetLoggerError.html" title="struct log::SetLoggerError">SetLoggerError</a></div><div class="desc docblock-short">The type returned by <a href="fn.set_logger.html"><code>set_logger</code></a> if <a href="fn.set_logger.html"><code>set_logger</code></a> has already been called.</div></li></ul><h2 id="enums" class="section-header">Enums<a href="#enums" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="enum" href="enum.Level.html" title="enum log::Level">Level</a></div><div class="desc docblock-short">An enum representing the available verbosity levels of the logger.</div></li><li><div class="item-name"><a class="enum" href="enum.LevelFilter.html" title="enum log::LevelFilter">LevelFilter</a></div><div class="desc docblock-short">An enum representing the available verbosity level filters of the logger.</div></li></ul><h2 id="constants" class="section-header">Constants<a href="#constants" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="constant" href="constant.STATIC_MAX_LEVEL.html" title="constant log::STATIC_MAX_LEVEL">STATIC_MAX_LEVEL</a></div><div class="desc docblock-short">The statically resolved maximum log level.</div></li></ul><h2 id="traits" class="section-header">Traits<a href="#traits" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="trait" href="trait.Log.html" title="trait log::Log">Log</a></div><div class="desc docblock-short">A trait encapsulating the operations required of a logger.</div></li></ul><h2 id="functions" class="section-header">Functions<a href="#functions" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="fn" href="fn.logger.html" title="fn log::logger">logger</a></div><div class="desc docblock-short">Returns a reference to the logger.</div></li><li><div class="item-name"><a class="fn" href="fn.max_level.html" title="fn log::max_level">max_level</a></div><div class="desc docblock-short">Returns the current maximum log level.</div></li><li><div class="item-name"><a class="fn" href="fn.set_boxed_logger.html" title="fn log::set_boxed_logger">set_boxed_logger</a></div><div class="desc docblock-short">Sets the global logger to a <code>Box&lt;Log&gt;</code>.</div></li><li><div class="item-name"><a class="fn" href="fn.set_logger.html" title="fn log::set_logger">set_logger</a></div><div class="desc docblock-short">Sets the global logger to a <code>&amp;'static Log</code>.</div></li><li><div class="item-name"><a class="fn" href="fn.set_logger_racy.html" title="fn log::set_logger_racy">set_logger_racy</a><sup title="unsafe function"></sup></div><div class="desc docblock-short">A thread-unsafe version of <a href="fn.set_logger.html"><code>set_logger</code></a>.</div></li><li><div class="item-name"><a class="fn" href="fn.set_max_level.html" title="fn log::set_max_level">set_max_level</a></div><div class="desc docblock-short">Sets the global maximum log level.</div></li><li><div class="item-name"><a class="fn" href="fn.set_max_level_racy.html" title="fn log::set_max_level_racy">set_max_level_racy</a><sup title="unsafe function"></sup></div><div class="desc docblock-short">A thread-unsafe version of <a href="fn.set_max_level.html"><code>set_max_level</code></a>.</div></li></ul></section></div></main></body></html>