(Used in FD-SoulEngine)

A very lightweight single-file .ini-style config parser for C++17 / C11 / Python. (.sein extension, but works with any text file)

• Sections [SectionName]
• Key = value pairs
• Comments via #
• Multi-line values with \ at the end of a line
• Raw multi-line strings "R( ... )R"
• @include "other.sein" directive (recursive, with depth limit)
• @set VAR = value global variables
• Referencing values via ${Section.key} or ${VAR}
• System env variables via ${SYSENV:VAR} - always reads from the OS, never from @set
• Section inheritance [Child : [Parent]]
• Async parsing - usage_async flag in parse_sein / create_new_config
• Subkey entries key[subkey] = value with dedicated getters
• Fully typed values - parsed once as Int, Float, or String; all getters work on the stored type directly
• Automatic whitespace trimming
• Getters: string, int, float, bool, arrays, subkeys
• Writer API for creating and editing configs programmatically

• No external dependencies
• mmap-based file reading (POSIX & Windows) - zero extra copy into kernel cache
• Lock-free hot paths in C (_Atomic counts, acquire/release ordering)
• std::from_chars in C++ - no allocations, no exceptions for number parsing
• Background thread parsing (pthreads / std::thread / Python threading)
• Case-insensitive bool recognition (true/yes/1, false/no/0)
• Safe default values on all getters

# colors.sein

[Colors]
red   = 225; 0; 0; 1
green = 0; 225; 0; 1

# example.sein

@include "colors.sein"

@set PROJECT_NAME = "My Awesome App"
@set VERSION      = 1.4.2

[App]
name      = ${PROJECT_NAME}
full_name = ${PROJECT_NAME} v${VERSION}
author    = John Doe
debug     = true

# ${SYSENV:HOME} always reads from the OS environment - never from @set
home_dir  = ${SYSENV:HOME}

color_name = ${Colors.red}

[Server]
port          = 9000
timeout       = 30.5
backoff_times = 0.5; 1.0; 2.0; 4.0; 8.0

# Section inheritance
[Character]
name   = none
health = 100
speed  = 5.0
level  = 1

# Player inherits health/level from Character; overrides name and speed
[Player : [Character]]
name  = Hero
speed = 8.5

# Enemy also inherits from Character
[Enemy : [Character]]
name   = Goblin
health = 40

[Animation]
animation["idle"]  = 0; 64; 4; 16; 16; true
animation["run"]   = 0; 16; 6; 16; 16; true

Include: #include "sein.hpp"
Compile: -std=c++17

Parse

Getters (cfg = result.data)

Values are stored as typed SeinValue (Int, Float, String, Array). All getters work directly on the stored type — no intermediate string conversion. Array getters handle typed single values and multi-element strings equally, so size = 32; 32 and port = 9000 both work correctly through get_int_array.

Writer (all prefixed with doc_)

Include: #define SEIN_IMPLEMENTATION then #include "sein.h"
Compile: -lpthread

Lifecycle

Getters

Writer

Import: import sein

Parse

Getters (cfg = result.data)

Values are stored as typed SeinValue objects (SeinType.Int, SeinType.Float, SeinType.String, SeinType.Array). Each value exposes .as_int(), .as_float(), .as_bool(), .as_str() methods. All getters work on the stored type directly.

Writer

• Thanks to the use of mmap (zero-copy read), std::from_chars and minimal allocations, SEIN shows very good parsing speed, especially in C/C++ versions.
• On small configs, it is comparable to classic INI parsers. On large files with many @include, mmap offers a noticeable advantage over approaches that read the entire file into memory.

• When minimum size and no dependencies are important
• In game engines and tools (C++ core + Python scripts)
• Split config into multiple files if necessary (@include)
• When you need presets through section inheritance
• If you want a single config format across multiple languages

• No built-in support for datetime
• Limited support for deep nesting (dot sections are better)
• No built-in schema validation

VSCode Extension

ext install Fepsid.sein-language-support