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Cosmopolitan Honeybadger



Cosmopolitan Libc makes C a build-once run-anywhere language, like Java, except it doesn't need an interpreter or virtual machine. Instead, it reconfigures stock GCC and Clang to output a POSIX-approved polyglot format that runs natively on Linux + Mac + Windows + FreeBSD + OpenBSD + NetBSD + BIOS with the best possible performance and the tiniest footprint imaginable.


For an introduction to this project, please read the αcτµαlly pδrταblε εxεcµταblε blog post and cosmopolitan libc website. We also have API documentation.

Getting Started

If you're doing your development work on Linux or BSD then you need just five files to get started. Here's what you do on Linux:

printf 'main() { printf("hello world\\n"); }\n' >hello.c
gcc -g -Os -static -nostdlib -nostdinc -fno-pie -no-pie -mno-red-zone \
  -fno-omit-frame-pointer -pg -mnop-mcount \
  -o hello.c -fuse-ld=bfd -Wl,-T, \
  -include cosmopolitan.h crt.o ape-no-modify-self.o cosmopolitan.a
objcopy -S -O binary

You now have a portable program.

bash -c './'  # zsh/fish workaround (we patched them in 2021)

Since we used the ape-no-modify-self.o bootloader (rather than ape.o) your executable will not modify itself when it's run. What it'll instead do, is extract a 4kb program to ${TMPDIR:-${HOME:-.}} that maps your program into memory without needing to copy it. It's possible to install the APE loader systemwide as follows.

# (1) linux systems that want binfmt_misc

# (2) for linux/freebsd/netbsd/openbsd systems
cp build/bootstrap/ape.elf /usr/bin/ape

# (3) for mac os x systems
cp build/bootstrap/ape.macho /usr/bin/ape

If you followed steps (2) and (3) then there's going to be a slight constant-time startup latency each time you run an APE binary. Your system might also prevent your APE program from being installed to a system directory as a setuid binary or a script interpreter. To solve that, you can use the following flag to turn your binary into the platform local format (ELF or Mach-O):

$ file DOS/MBR boot sector
./ --assimilate
$ file ELF 64-bit LSB executable

There's also some other useful flags that get baked into your binary by default:

./ --strace   # log system calls to stderr
./ --ftrace   # log function calls to stderr

If you want your program to be much tinier, more on the order of 16kb rather than 60kb, then all you have to do is use instead. See


If you're developing on MacOS you can install the GNU compiler collection for x86_64-elf via homebrew:

brew install x86_64-elf-gcc

Then in the above scripts just replace gcc and objcopy with x86_64-elf-gcc and x86_64-elf-objcopy to compile your APE binary.


If you're developing on Windows then you need to download an x86_64-pc-linux-gnu toolchain beforehand. See the Compiling on Windows tutorial. It's needed because the ELF object format is what makes universal binaries possible.

Cosmopolitan officially only builds on Linux. However, one highly experimental (and currently broken) thing you could try, is building the entire cosmo repository from source using the cross9 toolchain.

mkdir -p o/third_party
rm -rf o/third_party/gcc
mv cross9 o/third_party/gcc

Source Builds

Cosmopolitan can be compiled from source on any Linux distro. First, you need to download or clone the repository.

tar xf cosmopolitan.tar.gz  # see releases page
cd cosmopolitan

This will build the entire repository and run all the tests:

build/bootstrap/ -j16
find o -name \*.com | xargs ls -rShal | less

If you get an error running then it's probably because you have WINE installed to binfmt_misc. You can fix that by installing the the APE loader as an interpreter. It'll improve build performance too!


Since the Cosmopolitan repository is very large, you might only want to build a particular thing. Cosmopolitan's build config does a good job at having minimal deterministic builds. For example, if you wanted to build only then you could do that as follows:

build/bootstrap/ -j16 o//examples/

Sometimes it's desirable to build a subset of targets, without having to list out each individual one. You can do that by asking make to build a directory name. For example, if you wanted to build only the targets and subtargets of the chibicc package including its tests, you would say:

build/bootstrap/ -j16 o//third_party/chibicc
o//third_party/chibicc/ --help

Cosmopolitan provides a variety of build modes. For example, if you want really tiny binaries (as small as 12kb in size) then you'd say:

build/bootstrap/ -j16 MODE=tiny

Here's some other build modes you can try:

build/bootstrap/ -j16 MODE=dbg       # asan + ubsan + debug
build/bootstrap/ -j16 MODE=asan      # production memory safety
build/bootstrap/ -j16 MODE=opt       # -march=native optimizations
build/bootstrap/ -j16 MODE=rel       # traditional release binaries
build/bootstrap/ -j16 MODE=optlinux  # optimal linux-only performance
build/bootstrap/ -j16 MODE=tinylinux # tiniest linux-only 4kb binaries

For further details, see //build/


Here's the recommended ~/.gdbinit config:

set host-charset UTF-8
set target-charset UTF-8
set target-wide-charset UTF-8
set osabi none
set complaints 0
set confirm off
set history save on
set history filename ~/.gdb_history
define asm
  layout asm
  layout reg
define src
  layout src
  layout reg

You normally run the .com.dbg file under gdb. If you need to debug the .com file itself, then you can load the debug symbols independently as

gdb -ex 'add-symbol-file 0x401000'

Support Vector

Platform Min Version Circa
AMD K8 Venus 2005
Intel Core 2006
New Technology Vista 2006
GNU/Systemd 2.6.18 2007
XNU's Not UNIX! 15.6 2018
FreeBSD 13 2020
OpenBSD 6.4 2018
NetBSD 9.2 2021

Special Thanks

Funding for this project is crowdsourced using GitHub Sponsors and Patreon. Your support is what makes this project possible. Thank you! We'd also like to give special thanks to the following individuals:

For publicly sponsoring our work at the highest tier.

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