Struct fern::Output

pub struct Output(_);

Configuration for a logger output.

Implementations

impl Output

pub fn file<T: Into<Cow<'static, str>>>(file: File, line_sep: T) -> Self

Returns a file logger using a custom separator.

If the default separator of \n is acceptable, an fs::File instance can be passed into Dispatch::chain directly.

fern::Dispatch::new().chain(std::fs::File::create("log")?)

fern::Dispatch::new().chain(fern::log_file("log")?)

Example usage (using fern::log_file):

fern::Dispatch::new().chain(fern::Output::file(fern::log_file("log")?, "\r\n"))

pub fn writer<T: Into<Cow<'static, str>>>(
    writer: Box<dyn Write + Send>,
    line_sep: T
) -> Self

Returns a logger using arbitrary write object and custom separator.

If the default separator of \n is acceptable, an Box<Write + Send> instance can be passed into Dispatch::chain directly.

// Anything implementing 'Write' works.
let mut writer = std::io::Cursor::new(Vec::<u8>::new());

fern::Dispatch::new()
    // as long as we explicitly cast into a type-erased Box
    .chain(Box::new(writer) as Box<std::io::Write + Send>)

Example usage:

let writer = Box::new(std::io::Cursor::new(Vec::<u8>::new()));

fern::Dispatch::new().chain(fern::Output::writer(writer, "\r\n"))

pub fn stdout<T: Into<Cow<'static, str>>>(line_sep: T) -> Self

Returns an stdout logger using a custom separator.

If the default separator of \n is acceptable, an io::Stdout instance can be passed into Dispatch::chain() directly.

fern::Dispatch::new().chain(std::io::stdout())

Example usage:

fern::Dispatch::new()
    // some unix tools use null bytes as message terminators so
    // newlines in messages can be treated differently.
    .chain(fern::Output::stdout("\0"))

pub fn stderr<T: Into<Cow<'static, str>>>(line_sep: T) -> Self

Returns an stderr logger using a custom separator.

If the default separator of \n is acceptable, an io::Stderr instance can be passed into Dispatch::chain() directly.

fern::Dispatch::new().chain(std::io::stderr())

Example usage:

fern::Dispatch::new().chain(fern::Output::stderr("\n\n\n"))

pub fn sender<T: Into<Cow<'static, str>>>(
    sender: Sender<String>,
    line_sep: T
) -> Self

Returns a mpsc::Sender logger using a custom separator.

If the default separator of \n is acceptable, an mpsc::Sender<String> instance can be passed into Dispatch:: chain() directly.

Each log message will be suffixed with the separator, then sent as a single String to the given sender.

use std::sync::mpsc::channel;

let (tx, rx) = channel();
fern::Dispatch::new().chain(tx)

pub fn call<F>(func: F) -> Self where
    F: Fn(&Record<'_>) + Sync + Send + 'static, 

Returns a logger which simply calls the given function with each message.

The function will be called inline in the thread the log occurs on.

Example usage:

fern::Dispatch::new().chain(fern::Output::call(|record| {
    // this is mundane, but you can do anything here.
    println!("{}", record.args());
}))

Trait Implementations

impl Debug for Output

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl From<&'static (dyn Log + 'static)> for Output

fn from(log: &'static dyn Log) -> Self

Creates an output logger forwarding all messages to the custom logger.

impl From<Box<dyn Log + 'static>> for Output

fn from(log: Box<dyn Log>) -> Self

Creates an output logger forwarding all messages to the custom logger.

impl From<Box<dyn Write + 'static + Send>> for Output

fn from(writer: Box<dyn Write + Send>) -> Self

Creates an output logger which writes all messages to the writer with \n as the separator.

This does no buffering and it is up to the writer to do buffering as needed (eg. wrap it in BufWriter). However, flush is called after each log record.

impl From<Dispatch> for Output

fn from(log: Dispatch) -> Self

Creates an output logger forwarding all messages to the dispatch.

impl From<File> for Output

fn from(file: File) -> Self

Creates an output logger which writes all messages to the file with \n as the separator.

File writes are buffered and flushed once per log record.

impl From<Panic> for Output

fn from(_: Panic) -> Self

Creates an output logger which will panic with message text for all messages.

impl From<Sender<String>> for Output

fn from(stream: Sender<String>) -> Self

Creates an output logger which writes all messages to the given mpsc::Sender with '\n' as the separator.

All messages sent to the mpsc channel are suffixed with '\n'.

impl From<Stderr> for Output

fn from(stream: Stderr) -> Self

Creates an output logger which writes all messages to stderr with the given handle and \n as the separator.

impl From<Stdout> for Output

fn from(stream: Stdout) -> Self

Creates an output logger which writes all messages to stdout with the given handle and \n as the separator.