Step 2

Creating a custom formatter will require some investigation of other crates. Crates that are used by FLogging:

• crono - Used for the date/time functionality, with its specifiers, in
• the dt_fmt string of each formatter.
• std::fmt - The format options used in the fmt_string string of each formatter.
• strfmt - Used to do the actual formatting.

Now you should check out the built-in formatters to find one that is the closest to what you are after.

For our example, none of the built-in formatters are really closer than the other, so we will choose the UnixTimestampFormatter, simply because I am running Linux as my O/S.

To find them, use either the API documentation, or the Github repository: flogging.

As with the Custom Handlers, to obtain the code from the API documentation, at the top navigation bar, click “flogging-X.X.X”, where “X.X.X” is the version number, then under “LINKS” click “Source”. This will bring up the “Source” tab. Now we need to navigate to the required file.

• src
  • handlers
    • formatter
      • unixtimestamp_formatter.rs

Now select ALL of the code, from the top down, then ‘copy [ctrl/c]’. You need to include the file header (Copyright).

In your project src directory somewhere, create your new formatter file, and paste this code into it.

For this example, (we’ll name it my_project) we’ll have the following basic layout:

• src/
  lib.rs
  main.rs
  • handlers/
    confile_handler.rs
    mod.rs
    • formatters/
      csv_formatter.rs
• test_logs/

Our file will be called: csv_formatter.rs, with the module: CsvFormatter.

First things first. We now need to do some changes:

• unixtimestamp_formatter.rs to csv_formatter.rs
• UnixTimestampFormatter to CsvFormatter
• use crate::FormatTrait; to use flogging::*;
• &crate::LogEntry to &LogEntry

I have used a form of ‘diff’ to represent the changes:

• ’- ’ old line code
• ’+ ’ new line code

//
- // File Name:    unixtimestamp_formatter.rs
+ // File Name:    csv_formatter.rs
// Directory:    src/handlers/formatters
- // Project Name: flogging
+ // Project Name: my_project
//
// Copyright (C) 2025 Bradley Willcott
//
// SPDX-License-Identifier: GPL-3.0-or-later
//
// This library (crate) is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This library (crate) is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this library (crate).  If not, see <https://www.gnu.org/licenses/>.
//

//!
- //! # UnixTimeStamp Formatter
+ //! # CSV Formatter
//!

use std::fmt;
- use crate::FormatTrait;
+ use flogging::*;

#[derive(Debug, Clone, Hash, PartialEq, Eq)]

///
- /// Unix Timestamp format.
+ /// CSV format.
///
/// The first part (before the decimal point) is
/// the number of seconds since 1970-01-01 00:00 UTC.
///
/// The second part is the number of nanoseconds since
/// the last whole second.
///
/// Example:
/// ```text
/// 1752817859.157970496
/// ```
/// Template:
/// - `dt` in the template would be the datetime string, similar to the above.
/// - `mod_path`, `fn_name`, `level`, and `message` all come out of the `LogEntry`
- ///   provided to the [`format()`][UnixTimestampFormatter::format] method.
+ ///   provided to the [`format()`][CsvFormatter::format] method.
///
/// ```ignore
/// format!("{dt} {mod_path}->{fn_name} [{level:7}] {message}");
/// ```
/// Sample output:
/// ```text
/// 1752818461.051538870 flogging->main [SEVERE ] Hurricanes are windy!
/// ```
///
- pub struct UnixTimestampFormatter {
+ pub struct CsvFormatter {
    dt_fmt: String,
    fmt_string: String,
}

- impl UnixTimestampFormatter {
+ impl CsvFormatter {
    ///
-    /// Creates a new instance of `UnixTimestampFormatter`.
+    /// Creates a new instance of `CsvFormatter`.
    ///
    pub fn new() -> Self {
        Self {
            dt_fmt: "%s.%f".to_string(),
            fmt_string: "{dt} {mod_path}->{fn_name} [{level:7}] {message}".to_string(),
        }
    }

    ///
    /// Returns the date/time format string.
    ///
    pub fn dt_fmt(&self) -> String {
        self.dt_fmt.clone()
    }

    ///
    /// Returns the primary format string.
    ///
    pub fn fmt_string(&self) -> String {
        self.fmt_string.clone()
    }
}

- impl Default for UnixTimestampFormatter {
+ impl Default for CsvFormatter {
    fn default() -> Self {
        Self::new()
    }
}

- impl fmt::Display for UnixTimestampFormatter {
+ impl fmt::Display for CsvFormatter {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "dt_fmt: \"{}\" - fmt_string: \"{}\"",
            self.dt_fmt, self.fmt_string
        )
    }
}

- impl FormatTrait for UnixTimestampFormatter {
+ impl FormatTrait for CsvFormatter {
-     fn format(&self, log_entry: &crate::LogEntry) -> String {
+     fn format(&self, log_entry: &LogEntry) -> String {
        self.ft_fmt(self.dt_fmt(), self.fmt_string(), log_entry)
    }
}