# crun — Lightweight OCI Container Runtime Written in C

> A fast and low-memory OCI container runtime written in C that serves as a drop-in replacement for runc, supporting cgroups v2, rootless containers, and WebAssembly workloads.

## Install

Save as a script file and run:

# crun — Lightweight OCI Container Runtime Written in C

## Quick Use
```bash
# Install on Fedora/RHEL
sudo dnf install crun
# Or build from source
git clone https://github.com/containers/crun.git
cd crun && ./autogen.sh && ./configure && make
# Use with Podman (default on Fedora)
podman --runtime crun run --rm alpine echo "Hello from crun"
```

## Introduction
crun is an OCI-compliant container runtime written in C, developed as part of the Red Hat containers ecosystem. It is designed to be a faster and more memory-efficient alternative to runc (which is written in Go), while maintaining full compatibility with the OCI runtime specification used by Podman, Docker, and Kubernetes.

## What crun Does
- Executes OCI-compliant containers as a low-level runtime for container engines
- Starts containers faster and uses less memory than runc due to its C implementation
- Supports cgroups v2 natively including systemd cgroup driver integration
- Enables rootless container execution without requiring root privileges
- Runs WebAssembly workloads inside containers via WasmEdge and Wasmtime integration

## Architecture Overview
crun implements the OCI runtime specification as a standalone C binary with minimal dependencies. When a container engine (Podman, CRI-O, containerd) invokes crun, it reads the OCI bundle's `config.json`, sets up Linux namespaces, cgroups, seccomp filters, and the root filesystem, then execs the container process. The C implementation avoids the Go runtime overhead (goroutine scheduler, garbage collector), resulting in a smaller binary and lower startup latency.

## Self-Hosting & Configuration
- Install from package managers on Fedora, RHEL, Debian, Ubuntu, and Arch Linux
- Configure as the default runtime in Podman via `containers.conf` under `[engine]`
- Set as the CRI-O runtime by updating the `crio.conf` runtime handler configuration
- Use with containerd by adding a crun runtime entry in the containerd config
- Build from source with optional support for WasmEdge, Wasmtime, or libkrun for VM-isolated containers

## Key Features
- Written in C with no Go runtime overhead, resulting in a ~1 MB binary
- Container startup is measurably faster than runc in benchmarks
- Native cgroups v2 support with proper systemd integration for resource management
- Experimental krun mode for running containers inside lightweight micro-VMs
- WebAssembly runtime support for running Wasm modules as OCI containers

## Comparison with Similar Tools
- **runc** — Reference OCI runtime in Go; crun is faster and lighter as a C implementation
- **youki** — OCI runtime in Rust; crun has broader adoption and longer production track record
- **gVisor (runsc)** — Application kernel for sandboxing; crun uses standard Linux namespaces
- **Kata Containers** — VM-based isolation for security; crun can optionally use libkrun for similar isolation
- **WasmEdge** — WebAssembly runtime; crun can embed it as an alternative container execution backend

## FAQ
**Q: Is crun a drop-in replacement for runc?**
A: Yes. crun implements the same OCI runtime spec and command-line interface. You can switch by changing the runtime path in your container engine configuration.

**Q: Is it used in production?**
A: Yes. crun is the default container runtime on Fedora and RHEL-based systems using Podman and CRI-O.

**Q: How much faster is it than runc?**
A: Container creation is typically 40-50% faster and memory usage is significantly lower due to the absence of the Go runtime. Exact numbers vary by workload.

**Q: Does it work with Kubernetes?**
A: Yes. crun works as a runtime for CRI-O and containerd, which are the standard container runtimes for Kubernetes.

## Sources
- https://github.com/containers/crun
- https://crun.io/

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Source: https://tokrepo.com/en/workflows/asset-f4882dca
Author: Script Depot