README.md
1## Send Rust logs to Logcat
2
3[](https://crates.io/crates/android_logger)
4[](https://github.com/rust-mobile/android_logger-rs/actions/workflows/ci.yml/)
5
6
7This library is a drop-in replacement for `env_logger`. Instead, it outputs messages to
8android's logcat.
9
10This only works on Android and requires linking to `log` which
11is only available under android. With Cargo, it is possible to conditionally require
12this library:
13
14```toml
15[target.'cfg(target_os = "android")'.dependencies]
16android_logger = "0.15"
17```
18
19Example of initialization on activity creation, with log configuration:
20
21```rust
22#[macro_use] extern crate log;
23extern crate android_logger;
24
25use log::LevelFilter;
26use android_logger::{Config,FilterBuilder};
27
28fn native_activity_create() {
29 android_logger::init_once(
30 Config::default()
31 .with_max_level(LevelFilter::Trace) // limit log level
32 .with_tag("mytag") // logs will show under mytag tag
33 .with_filter( // configure messages for specific crate
34 FilterBuilder::new()
35 .parse("debug,hello::crate=error")
36 .build())
37 );
38
39 trace!("this is a verbose {}", "message");
40 error!("this is printed by default");
41}
42```
43
44To allow all logs, use the default configuration with min level Trace:
45
46```rust
47#[macro_use] extern crate log;
48extern crate android_logger;
49
50use log::LevelFilter;
51use android_logger::Config;
52
53fn native_activity_create() {
54 android_logger::init_once(
55 Config::default().with_max_level(LevelFilter::Trace),
56 );
57}
58```
59
60There is a caveat that this library can only be initialized once
61(hence the `init_once` function name). However, Android native activity can be
62re-created every time the screen is rotated, resulting in multiple initialization calls.
63Therefore this library will only log a warning for subsequent `init_once` calls.
64
65This library ensures that logged messages do not overflow Android log message limits
66by efficiently splitting messages into chunks.
67
68## Consistent log filtering in mixed Rust/C/C++ apps
69
70Android's C logging API determines the effective log level based on [a
71combination](https://cs.android.com/android/platform/superproject/main/+/main:system/logging/liblog/properties.cpp;l=243;drc=b74a506c1b69f5b295a8cdfd7e2da3b16db15934)
72of a process-wide global variable, [system-wide
73properties](https://cs.android.com/android/platform/superproject/main/+/main:system/logging/logd/README.property;l=45;drc=99c545d3098018a544cb292e1501daca694bee0f),
74and call-specific default. `log` + `android_logger` crates add another layer of
75log filtering on top of that, independent from the C API.
76
77```
78 .-----.
79 | app |
80 '-----' Rust
81C/C++ | '--------------.
82 | v
83 | .-----------. filter by log::STATIC_MAX_LEVEL +
84 | | log crate | - log::MAX_LOG_LEVEL_FILTER,
85 | '-----------' overrideable via log::set_max_level
86 | |
87 | v
88 | .----------------------.
89 | | android_logger crate | - filter by Config::max_level
90 | '----------------------'
91 | |
92 | .------------'
93 v v
94 .--------.
95 | liblog | - filter by global state or system-wide properties
96 '--------'
97```
98
99`liblog` APIs introduced in Android API 30 let `android_logger` delegate log
100filtering decision to `liblog`, making the log level consistent across C, C++
101and Rust calls.
102
103If you build `android_logger` with `android-api-30` feature enabled, the logger
104will consider the process-wide global state (set via
105[`__android_log_set_minimum_priority`](https://cs.android.com/android/platform/superproject/main/+/main:prebuilts/runtime/mainline/runtime/sdk/common_os/include/system/logging/liblog/include/android/log.h;l=364;drc=4cf460634134d51dba174f8af60dffb10f703f51))
106and Android system properties when deciding if a message should be logged or
107not. In this case, the effective log level is the _least verbose_ of the levels
108set between those and Rust log facilities.
109
110## License
111
112Licensed under either of
113
114 * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
115 * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
116
117at your option.
118
119### Contribution
120
121Unless you explicitly state otherwise, any contribution intentionally
122submitted for inclusion in the work by you, as defined in the Apache-2.0
123license, shall be dual licensed as above, without any additional terms or
124conditions.
125
){kind=link}
){kind=link}