1log
2===
3
4A Rust library providing a lightweight logging *facade*.
5
6[![Build status](https://img.shields.io/github/actions/workflow/status/rust-lang/log/main.yml?branch=master)](https://github.com/rust-lang/log/actions)
7[![Latest version](https://img.shields.io/crates/v/log.svg)](https://crates.io/crates/log)
8[![Documentation](https://docs.rs/log/badge.svg)](https://docs.rs/log)
9![License](https://img.shields.io/crates/l/log.svg)
10
11* [`log` documentation](https://docs.rs/log)
12
13A logging facade provides a single logging API that abstracts over the actual
14logging implementation. Libraries can use the logging API provided by this
15crate, and the consumer of those libraries can choose the logging
16implementation that is most suitable for its use case.
17
18
19## Minimum supported `rustc`
20
21`1.60.0+`
22
23This version is explicitly tested in CI and may be bumped in any release as needed. Maintaining compatibility with older compilers is a priority though, so the bar for bumping the minimum supported version is set very high. Any changes to the supported minimum version will be called out in the release notes.
24
25## Usage
26
27### In libraries
28
29Libraries should link only to the `log` crate, and use the provided macros to
30log whatever information will be useful to downstream consumers:
31
32```toml
33[dependencies]
34log = "0.4"
35```
36
37```rust
38use log::{info, trace, warn};
39
40pub fn shave_the_yak(yak: &mut Yak) {
41    trace!("Commencing yak shaving");
42
43    loop {
44        match find_a_razor() {
45            Ok(razor) => {
46                info!("Razor located: {razor}");
47                yak.shave(razor);
48                break;
49            }
50            Err(err) => {
51                warn!("Unable to locate a razor: {err}, retrying");
52            }
53        }
54    }
55}
56```
57
58### In executables
59
60In order to produce log output, executables have to use a logger implementation compatible with the facade.
61There are many available implementations to choose from, here are some options:
62
63* Simple minimal loggers:
64    * [`env_logger`](https://docs.rs/env_logger/*/env_logger/)
65    * [`colog`](https://docs.rs/colog/*/colog/)
66    * [`simple_logger`](https://docs.rs/simple_logger/*/simple_logger/)
67    * [`simplelog`](https://docs.rs/simplelog/*/simplelog/)
68    * [`pretty_env_logger`](https://docs.rs/pretty_env_logger/*/pretty_env_logger/)
69    * [`stderrlog`](https://docs.rs/stderrlog/*/stderrlog/)
70    * [`flexi_logger`](https://docs.rs/flexi_logger/*/flexi_logger/)
71    * [`call_logger`](https://docs.rs/call_logger/*/call_logger/)
72    * [`std-logger`](https://docs.rs/std-logger/*/std_logger/)
73    * [`structured-logger`](https://docs.rs/structured-logger/latest/structured_logger/)
74* Complex configurable frameworks:
75    * [`log4rs`](https://docs.rs/log4rs/*/log4rs/)
76    * [`fern`](https://docs.rs/fern/*/fern/)
77* Adaptors for other facilities:
78    * [`syslog`](https://docs.rs/syslog/*/syslog/)
79    * [`systemd-journal-logger`](https://docs.rs/systemd-journal-logger/*/systemd_journal_logger/)
80    * [`slog-stdlog`](https://docs.rs/slog-stdlog/*/slog_stdlog/)
81    * [`android_log`](https://docs.rs/android_log/*/android_log/)
82    * [`win_dbg_logger`](https://docs.rs/win_dbg_logger/*/win_dbg_logger/)
83    * [`db_logger`](https://docs.rs/db_logger/*/db_logger/)
84    * [`log-to-defmt`](https://docs.rs/log-to-defmt/*/log_to_defmt/)
85    * [`logcontrol-log`](https://docs.rs/logcontrol-log/*/logcontrol_log/)
86* For WebAssembly binaries:
87    * [`console_log`](https://docs.rs/console_log/*/console_log/)
88* For dynamic libraries:
89    * You may need to construct [an FFI-safe wrapper over `log`](https://github.com/rust-lang/log/issues/421) to initialize in your libraries.
90* Utilities:
91    * [`log_err`](https://docs.rs/log_err/*/log_err/)
92    * [`log-reload`](https://docs.rs/log-reload/*/log_reload/)
93    * [`alterable_logger`](https://docs.rs/alterable_logger/*/alterable_logger)
94
95Executables should choose a logger implementation and initialize it early in the
96runtime of the program. Logger implementations will typically include a
97function to do this. Any log messages generated before the logger is
98initialized will be ignored.
99
100The executable itself may use the `log` crate to log as well.
101
102## Structured logging
103
104If you enable the `kv` feature, you can associate structured data with your log records:
105
106```rust
107use log::{info, trace, warn};
108
109pub fn shave_the_yak(yak: &mut Yak) {
110    // `yak:serde` will capture `yak` using its `serde::Serialize` impl
111    //
112    // You could also use `:?` for `Debug`, or `:%` for `Display`. For a
113    // full list, see the `log` crate documentation
114    trace!(target = "yak_events", yak:serde; "Commencing yak shaving");
115
116    loop {
117        match find_a_razor() {
118            Ok(razor) => {
119                info!(razor; "Razor located");
120                yak.shave(razor);
121                break;
122            }
123            Err(e) => {
124                // `e:err` will capture `e` using its `std::error::Error` impl
125                warn!(e:err; "Unable to locate a razor, retrying");
126            }
127        }
128    }
129}
130```
131