log
===

A Rust library providing a lightweight logging *facade*.

[![Build Status](https://travis-ci.org/rust-lang-nursery/log.svg?branch=master)](https://travis-ci.org/rust-lang-nursery/log)
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* [`log` documentation](https://docs.rs/log)

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
implementation that is most suitable for its use case.

## Usage

## In libraries

Libraries should link only to the `log` crate, and use the provided macros to
log whatever information will be useful to downstream consumers:

```toml
[dependencies]
log = "0.4"
```

```rust
#[macro_use]
extern crate log;

pub fn shave_the_yak(yak: &mut Yak) {
    trace!("Commencing yak shaving");

    loop {
        match find_a_razor() {
            Ok(razor) => {
                info!("Razor located: {}", razor);
                yak.shave(razor);
                break;
            }
            Err(err) => {
                warn!("Unable to locate a razor: {}, retrying", err);
            }
        }
    }
}
```

If you use Rust 2018, you can use instead the following code to import the crate macros:

```rust
use log::{info, trace, warn};

pub fn shave_the_yak(yak: &mut Yak) {
    // …
}
```

## In executables

In order to produce log output executables have to use a logger implementation compatible with the facade.
There are many available implementations to chose from, here are some of the most popular ones:

* Simple minimal loggers:
    * [`env_logger`](https://docs.rs/env_logger/*/env_logger/)
    * [`simple_logger`](https://github.com/borntyping/rust-simple_logger)
    * [`simplelog`](https://github.com/drakulix/simplelog.rs)
    * [`pretty_env_logger`](https://docs.rs/pretty_env_logger/*/pretty_env_logger/)
    * [`stderrlog`](https://docs.rs/stderrlog/*/stderrlog/)
    * [`flexi_logger`](https://docs.rs/flexi_logger/*/flexi_logger/)
* Complex configurable frameworks:
    * [`log4rs`](https://docs.rs/log4rs/*/log4rs/)
    * [`fern`](https://docs.rs/fern/*/fern/)
* Adaptors for other facilities:
    * [`syslog`](https://docs.rs/syslog/*/syslog/)
    * [`slog-stdlog`](https://docs.rs/slog-stdlog/*/slog_stdlog/)
    * [`android_log`](https://docs.rs/android_log/*/android_log/)

Executables should choose a logger implementation and initialize it early in the
runtime of the program. Logger implementations will typically include a
function to do this. Any log messages generated before the logger is
initialized will be ignored.

The executable itself may use the `log` crate to log as well.