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Flecs is a fast and lightweight Entity Component System that lets you build games and simulations with millions of entities (join the Discord!). Here are some of the framework's highlights:

  • Fast native C99 API that can be used with most game engines and scripting languages
  • Modern type-safe C++11 API that doesn't rely on STL types
  • Entire framework builds in less than 5 seconds.
  • Cache friendly archetype/SoA storage that can process millions of entities every frame
  • Automatic component registration that works out of the box across shared libraries/DLLs
  • Run games on multiple CPU cores with a fast lockless scheduler and command queue
  • First open source ECS with full support for entity relationships!
  • Compiles warning-free on 8 compilers on all major platforms, with CI running more than 3000 tests
  • No need to reinvent the wheel with fast native hierarchy, prefab and reflection implementations
  • A web-based dashboard (click to try!) for exploring entities, running queries & learning Flecs:
Screen Shot 2021-11-07 at 10 20 51 PM

You're looking at Flecs v3! While v3 stability is good, API changes can still happen.

Last stable v2 release: Flecs v2.4.8.

What is an Entity Component System?

ECS is a new way of organizing code and gameplay data that lets you build game worlds that are larger, more complex and are easier to extend. ECS is different in how it dynamically binds simulation logic ('systems') with game data ('components').

Something is typically called an ECS when it:

  • Has entities, that uniquely identify objects in a game
  • Has components, which are datatypes that can be added to entities
  • Has systems which are functions that run for all entities matching a component query

A simple example of this would be a game with two components, Position and Velocity, and a Move system that finds all entities with both components, and adds Velocity to Position.

For more info on ECS, check the ECS FAQ!

Getting Started

Here is some awesome content provided by the community :heart: :

Show me the code!

C99 example:

typedef struct {
  float x, y;
} Position, Velocity;

void Move(ecs_iter_t *it) {
  Position *p = ecs_term(it, Position, 1);
  Velocity *v = ecs_term(it, Velocity, 2);
  for (int i = 0; i < it->count; i ++) {
    p[i].x += v[i].x;
    p[i].y += v[i].y;

int main(int argc, char *argv[]) {
  ecs_world_t *ecs = ecs_init();

  ECS_COMPONENT(ecs, Position);
  ECS_COMPONENT(ecs, Velocity);

  ECS_SYSTEM(ecs, Move, EcsOnUpdate, Position, Velocity);

  ecs_entity_t e = ecs_new_id(ecs);
  ecs_set(ecs, e, Position, {10, 20});
  ecs_set(ecs, e, Velocity, {1, 2});

  while (ecs_progress(ecs, 0)) { }

Same example in C++11:

struct Position {
  float x, y;

struct Velocity {
  float x, y;

int main(int argc, char *argv[]) {
  flecs::world ecs;

  ecs.system<Position, const Velocity>()
    .each([](Position& p, const Velocity& v) {
      p.x += v.x;
      p.y += v.y;

  auto e = ecs.entity()
    .set([](Position& p, Velocity& v) {
      p = {10, 20};
      v = {1, 2};
  while (ecs.progress()) { }


Flecs has a modular architecture that makes it easy to only build the features you really need. By default all addons are built. To customize a build, first define FLECS_CUSTOM_BUILD, then add defines for the addons you need. For example:

#define FLECS_CUSTOM_BUILD  // Don't build all addons
#define FLECS_SYSTEM        // Build FLECS_SYSTEM

Here's an overview of all supported addons:

Addon Description Define
Module Organize game logic into reusable modules FLECS_MODULE
System Create & run systems FLECS_SYSTEM
Pipeline Automatically schedule & multithread systems FLECS_PIPELINE
Timer Run systems at time intervals or at a rate FLECS_TIMER
Meta Flecs reflection system FLECS_META
Meta_C (C) Utilities for auto-inserting reflection data FLECS_META_C
Expr String format optimized for ECS data FLECS_EXPR
Doc Add documentation to components, systems & more FLECS_DOC
Coredoc Documentation for builtin components & modules FLECS_COREDOC
Http Tiny HTTP server for processing simple requests FLECS_HTTP
Rest REST API for showing entities in the browser FLECS_REST
Parser Create entities & queries from strings FLECS_PARSER
Plecs Small utility language for asset/scene loading FLECS_PLECS
Rules Powerful prolog-like query language FLECS_RULES
Snapshot Take snapshots of the world & restore them FLECS_SNAPSHOT
Stats See what's happening in a world with statistics FLECS_STATS
Log Extended tracing and error logging FLECS_LOG
App Flecs application framework FLECS_APP
OS API Impl Default OS API implementation for Posix/Win32 FLECS_OS_API_IMPL

Flecs Hub

Flecs Hub is a handy collection of repositories built with Flecs that showcase basic ways of how to build engine features like input handling, transformations and rendering:

Module Description
flecs.components.cglm Component registration for cglm (math) types
flecs.components.input Components that describe keyboard and mouse input
flecs.components.transform Components that describe position, rotation and scale
flecs.components.physics Components that describe physics and movement
flecs.components.geometry Components that describe geometry
flecs.components.graphics Components used for computer graphics
flecs.components.gui Components used to describe GUI components
flecs.systems.transform Hierarchical transforms for scene graphs
flecs.systems.physics Systems for moving objects and collision detection
flecs.systems.sdl2 SDL window creation & input management
flecs.systems.sokol Sokol-based renderer

Language bindings

The following language bindings have been developed with Flecs! Note that these are projects built and maintained by helpful community members, and may not always be up to date with the latest commit from master!

Supporting Flecs ♥️

Supporting Flecs goes a long way towards keeping the project going and the community alive! If you like the project, consider: