# Cosmopolitan Libc — Build Once, Run Anywhere C/C++ Executables

> An open-source C library that produces Actually Portable Executables — single binaries that run natively on Linux, macOS, Windows, FreeBSD, OpenBSD, and NetBSD without recompilation.

## Install

Save as a script file and run:

# Cosmopolitan Libc — Build Once, Run Anywhere C/C++ Executables

## Quick Use
```bash
# Download the toolchain
mkdir -p cosmocc && cd cosmocc
curl -LO https://cosmo.zip/pub/cosmocc/cosmocc.zip
unzip cosmocc.zip
# Compile a portable binary
bin/cosmocc -o hello hello.c
./hello  # Runs on Linux, macOS, Windows, FreeBSD, OpenBSD, NetBSD
```

## Introduction
Cosmopolitan Libc is a C library that lets you compile C and C++ programs into Actually Portable Executables (APE). A single binary produced with Cosmopolitan runs on six operating systems and multiple architectures without any recompilation, emulation, or container layer. It was created by Justine Tunney to make cross-platform native code distribution trivially simple.

## What Cosmopolitan Does
- Produces single-file executables that run natively on Linux, macOS, Windows, FreeBSD, OpenBSD, and NetBSD
- Provides a POSIX-compatible C library with extensions for modern systems programming
- Bundles a toolchain (cosmocc) based on GCC that cross-compiles for all supported targets at once
- Supports both x86-64 and ARM64 architectures in the same binary via fat APE format
- Includes optimized implementations of libc functions tuned for each target OS at runtime

## Architecture Overview
Cosmopolitan works by embedding a polyglot binary header that is valid as a Unix shell script, a Windows PE executable, and a macOS Mach-O binary simultaneously. At load time, a tiny boot stub detects the host operating system and architecture, then jumps to the appropriate code path. The C library translates POSIX system calls to native OS calls on each platform, so application code uses a single standard API. The cosmocc toolchain wraps GCC to produce these APE binaries by linking against Cosmopolitan instead of the system libc.

## Self-Hosting & Configuration
- Download the cosmocc toolchain from cosmo.zip and extract it
- Use `cosmocc` as a drop-in replacement for `gcc` or `cc` in build scripts
- Set `CC=cosmocc` and `CXX=cosmoc++` to integrate with Makefiles and CMake projects
- Link with `-static` by default since Cosmopolitan produces fully static binaries
- Deploy the output binary directly to any supported OS with no runtime dependencies

## Key Features
- True write-once, run-anywhere native executables without a VM or interpreter
- Full POSIX threads, sockets, and file I/O support across all six operating systems
- Fat binary format supporting both x86-64 and ARM64 in a single file
- Small binary overhead — the runtime stub adds minimal size compared to the application code
- Powers the llamafile project, which distributes LLMs as single portable executables

## Comparison with Similar Tools
- **Go cross-compilation** — Go can target multiple OS/arch pairs but produces separate binaries per target; Cosmopolitan ships one binary for all platforms
- **Docker / containers** — containers isolate at the OS level and require a runtime; Cosmopolitan runs natively with zero dependencies
- **musl-libc static builds** — musl produces static Linux binaries; Cosmopolitan extends this to macOS, Windows, and BSDs in one file
- **GraalVM native-image** — compiles JVM apps to native binaries per platform; Cosmopolitan targets C/C++ and covers more OSes per binary
- **Zig cross-compilation** — Zig's toolchain cross-compiles C/C++ for many targets; Cosmopolitan uniquely bundles all targets into a single polyglot executable

## FAQ
**Q: Does the binary need to be renamed or given an extension on Windows?**
A: APE binaries can be run directly on modern Windows versions. Older systems may need the file renamed with a .exe extension or the APE loader installed.

**Q: Can I use third-party C libraries with Cosmopolitan?**
A: Yes, as long as they compile with standard C and link statically. Many popular libraries have been ported and tested.

**Q: How large is a typical Cosmopolitan binary?**
A: A minimal hello-world binary is around 100 KB. Real applications vary based on which libc features they pull in.

**Q: Is it production-ready?**
A: Yes. Cosmopolitan powers production tools like llamafile and has been tested across major operating systems and cloud providers.

## Sources
- https://github.com/jart/cosmopolitan
- https://cosmo.zip

---
Source: https://tokrepo.com/en/workflows/asset-798162f9
Author: Script Depot