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Esperanto, when OCaml meets Cosmopolitan

An open question in the Caml community is how to produce a native binary that will work everywhere. Indeed, OCaml has the advantage of producing bytecode like Java where the minimal requirement to run the program is to have the OCaml distribution on your machine.

However, it is often desired to distribute a native program instead of bytecode — especially for performance reasons. Portability versus speed.

The issue of distribution (and portability) of a native executable is partially resolved. Indeed, the first barrier to distribution is the static link that must operate when the executable is created. MirageOS has a long experience of this goal because the very principle of creating an operating system requires that there is systematically a static link resolution (and yes, an OS - without a file system - certainly can't "load" third party *.so libraries at boot time, everything must be statically linked).

The MirageOS shism

There are different views on how to solve this problem:

This is the big difference between MirageOS 3 and MirageOS 4. The former has the advantage of taking an ecosystem as it is and only dealing with the link. The issue of compiling artefacts is left to the maintainers of the libraries (so they can use b0 for example without there being any incompatibilities). The disadvantage is that cross-compilation becomes difficult or impossible. Indeed, leaving the problem of compilation to third parties (maintainers) makes it difficult to cross-compile their artefacts. Especially when a library imports object files that are not OCaml (where only *.o is available without its *.cmo equivalent).

This is why some libraries (like digestif) distribute a "cross-compiled"1 version of these objects along with a default version. MirageOS 3, using a hack with ocamlfind and META files can then statically link these object files with the operating system correctly.

1: When I talk about cross-compiling here, it is not necessarily producing a different assembler than the host (even if this is part of the objectives). The definition is indeed somewhat broader, it is sometimes about producing the same assembler as the host but under different constraints (with different options) than the usual compilation. For example, we would not want to include the standard C library in the object needed for our operating system BUT we would still want to include it in the object that would be used in the production of a normal native executable.

This is particularly the case with Solo5 where the objective is not to compile to another assembler but to compile with other options (like -nostdlib).

The second solution has the advantage of leaving us in control of the compilation. Indeed, vendoring the dependencies (and thus getting the source directly) and then controlling how we should compile the source files gives us the freedom to produce what we want and in what context we want to produce the final artifact. The biggest disadvantage is that all dependencies must use the same build system in order to orchestrate the compilation of what may be ~11000 artefacts (like pasteur) with the same tool.

With this solution, it becomes "easy" to ask to statically link all the artefacts or more generally, to use another more specific and appropriate compilation toolchain to produce our operating system. Of course, it underlies a constraint to homogenise an ecosystem that belongs to no one to a central tool, in this case: dune. A monopoly situation which, according to my political views, is problematic.

However, experience speaks for itself and it turns out that MirageOS 4 and especially Solo5 (and Martin's wonderful work) makes way for a particularly interesting design in terms of producing our programs. Indeed, in the second solution, it is a question of a toolchain which remains to be defined but finally structures all the compilation of all that is necessary to the production of our "binary" (a simple executable or an operating system) as well as the link, final stage but nevertheless very important, especially in the production of an operating system. To recall, MirageOS 3 was essentially about linking.


Cosmopolitan is a great project that proposes a C library that would be "portable" on all systems. The difficulty with portability in C is the ABI. Indeed, a C library proposes a whole bunch of functions which only communicate with the kernel in order to obtain and manipulate the resources of your computer. Unfortunately, depending on the kernel (Windows, Linux, Mac, *BSD), the ceremony to manipulate resources are not totally the same - even if they are similar.

But cosmopolitan is not only a simple C library (like musl), the project proposes a linking method to create an artefact able to run on all machines. Indeed, beyond the question of the ABI proposed by the kernels which differ, the format of the executable expected by the systems is also different. Cosmpolitan allows to link the necessary object files in such a way that the produced program can be launched on any system!

Of course, at this stage, there is some black magic involved, akin to summoning the god Emacs so that it can change some bits of your program in place to make it run on your machine...

But in the end, what Cosmopolitan proposes is basically a toolchain in the same way that MirageOS 4 can be expected to create an operating system:

The question now remains: how to integrate Cosmopolitan into an OCaml project to build-once executable that can run-anywhere?


For this, we will mainly use the same design as Solo5 (also applied to Gilbraltar to make a MirageOS on Raspberry Pi 4) but apply it with Cosmopolitan. Essentially, we need to propose a new toolchain that will consist of a new C compiler, a new linker, a *.a that will correspond to our standard C library, some *.o that we will have to link well at the end as well as some *.h to give the right definitions to the caml runtime (which remains only C code after all).

Our real goal in this story is to produce a libasmrun.a from the OCaml compiler source to have a caml runtime compiled with cosmopolitan. Indeed, in the generation of a native caml program, an important step is to link your OCaml code (which has become assembly) with libasmrun.a so that there is the implementation of the Garbage Collector as well as the necessary functions needed for the standard OCaml library (stdlib.cmxa).

Unfortunately, one cannot simply take libasmrun.a from the OCaml compiler of your host system since it explicitly uses the standard C library of your host system (instead of our Cosmopolitan library) - this library is probably glibc for Linux systems. The latter, by the way, expects there to be dynamic loading of third-party libraries (*.so) which is in contradiction with our first point: getting a program whose link to third-party libraries is static.

Of course, the problem is more difficult than simply recompiling libasmrun.a with another standard C library. The OCaml compiler has this ability to compile C files to object files (ocamlopt -c strlen.c -o strlen.o). In truth, it makes sense to let ocamlopt compile the C files needed for your project into object files to ensure certain #include and options - and toolchain consistency by ultimately using the same C compiler that compiled libasmrun.a to compile our *.c.

In other words, we don't just need to compile libasmrun.a, we also need to compile another OCaml compiler...



If you are observant, Cosmopolitan only requires a few more options to be given to our C compiler (like GCC or Clang). We could very well compile the OCaml distribution with these options and have our libasmrun.a (compiled with the expected options) and our ocamlopt (using the expected options) without much difficulty, but that's without the linkage and order.

If you have ever played with ld, you know that the order of the arguments is very important. It turns out that even though we end up being able to compile OCaml with Cosmopolitan, our new ocamlopt doesn't necessarily emit the options for the link in the right order... Which makes it unusable...

In this respect, it is a common point with ld & ocamlopt! Indeed, we still have to topologically sort the dependencies to make sure that ld binds the objects in the same way that ocamlopt binds the *.cmx{,a}.

This mostly means that we need to hack the C toolchain used to compile OCaml and thus create our own C compiler and C linker which will take care of reordering the arguments so that an invocation by the OCaml distribution's Makefile (to build our libasmrun.a) or an invocation by ocamlopt (to compile the C stubs and link objects) works!

Let's not go any further into the arcane mythology of the creation of the OCaml god by the OCaml titan with incantations to autotools and Makefile, we basically just need to make 3 shell scripts that will replace cc, ld and objcopy:

prog="$(basename $0)"
I="$(dirname $0)/../include/x86_64-esperanto-none-static"
[ ! -d "${I}" ] && echo "$prog: Could not determine include path" 1>&2 && exit 1
L="$(dirname $0)/../lib/x86_64-esperanto-none-static"
[ ! -d "${L}" ] && echo "$prog: Could not determine library path" 1>&2 && exit 1
[ "$#" -lt 1 ] && \
    echo "$prog: No input files. Compilation terminated." 1>&1 && exit 1
[ "$#" -eq 1 -a "$1" = "-v" ] && exec cc "$@"
for arg do
    if [ -n "${Z}" ]; then
        # handle -z linker-arg
        case "$arg" in
                set -- "$@" "-z" "$arg"

    case "$arg" in
            if [ -z "${Z}" ]; then
    set -- "$@" "$arg"
case ${M} in
        [ -n "${B}" ] && B="-DSUPPORT_VECTOR=${B}"
        [ -n "${__V}" ] && set -x
        exec cc \
            -D__ESPERANTO__ -mstack-protector-guard=global \
	    -g -Os -nostdlib -nostdinc \
	    -fno-pie -no-pie -mno-red-zone -fno-omit-frame-pointer -pg -mnop-mcount \
	    -I ${I} \
	    ${B} \
	    "$@" \
        [ -n "${B}" ] && B="-DSUPPORT_VECTOR=${B}"
        [ -n "${S}" ] && S="${L}/startup.o"
        [ -n "${__V}" ] && set -x
        exec cc \
            -D__ESPERANTO__ -mstack-protector-guard=global \
	    -g -Os -static -nostdlib -nostdinc \
	    -fno-pie -no-pie -mno-red-zone -fno-omit-frame-pointer -pg -mnop-mcount \
            -Wl,--build-id=none ${S} ${B} \
            "$@" \
            -fuse-ld=bfd -Wl,-T,${L}/ \
	    -I ${I} \
	    ${L}/crt.o ${L}/ape.o ${L}/cosmopolitan.a

This shell script will be our C compiler. As you can see, it introspects the options the caller gives (again, the OCaml Makefile or ocamlopt) and infers whether the caller is looking to compile or link. In this way, 2 options are really manipulated:

Of course, we can see, as far as compilation is concerned, the necessary options such as -nostdlib and -nostdinc (a small -D__ESPERANTO__ allowing third party libraries to know that we are creating an object file for Cosmopolitan).

And finally, for the linker, we ask to use the file which describes how to link our objects with cosmopolitan.a (the famous portable C library).

The script will be x86_64-esperanto-none-static-cc which follows the "Target Triplet" rule (like GCC). The linker offers more or less the same thing and objcopy just ensures consistency in our C toolchain.

The cosmopolitan distribution in OPAM

Finally, it is a question of proposing an OPAM package allowing, like the Solo5 package, to install and make available this C toolchain. However, before attacking the big part, i.e. the tuned OCaml compiler, we are going to spend a little time on the distribution of this toolchain, the genesis of which is essential in a simple containing our archive, our objects and our header (which you can download and use yourself).

As you may know, I like to redo everything in OCaml. So I made a little tool that extracts from the given the files needed for our toolchain. The tool goes a bit further than just unzipping a file, of course. It will also check the contents and ensure the integrity of the files by their hashes (SHA256).

For such a tool, we essentially need 2 libraries:

For the first one, as you know, one of my oldest libraries is digestif which allows to calculate several hashes including SHA256. Let's take it for our tool.

The second is a bit more problematic. I did indeed make a library that implements the zlib format but zip, beyond using zlib (or more precisely RFC1951/DEFLATE), also describes a structure (folders & files). So I mixed what can be found in our good old camlzip and I replaced the Zlib part by decompress!

Finally, our tool is:

module Digest = Digestif.SHA256

let ( / ) = Filename.concat

let expected =
  [ "dst/cosmopolitan.h",
    (Digest.of_hex "bc5c3d52214eb744d0dbdba95c628b8c53a53efacc558cd36f60c1d585e60e36")
  ; "dst/",
    (Digest.of_hex "bcd87c737408ab18a6d4ffce2848a6bbf3cf2e8df904fa3aac475d11a1f938f3")
  ; "dst/crt.o",
    (Digest.of_hex "d4874eaed8fe78ea8745cd3acc66e8eb7ccf2ba2d296aed64f878528b45e2d9b")
  ; "dst/ape.o",
    (Digest.of_hex "7e8d302fd5654235ebebc5a94ab1a22437b90164032d4304ac55729755bc7f5d")
  ; "dst/ape-no-modify-self.o",
    (Digest.of_hex "1ed036a5d9d7d56d473874bcde93901717a71e2f4502f605b1e70913023fadd8")
  ; "dst/cosmopolitan.a",
    (Digest.of_hex "50ae3c76998007ba0a6626dc33f33d387cf12da6261dafb7e05d287bcc656ca4") ]

let digest filename =
  let tmp = Bytes.create 0x1000 in
  let rec digest ic acc = match input ic tmp 0 (Bytes.length tmp) with
    | 0 -> Digest.get acc
    | exception End_of_file -> Digest.get acc
    | len ->
      digest ic (Digest.feed_bytes acc ~off:0 ~len tmp) in
  let ic = open_in filename in
  let hash = digest ic Digest.empty in
  close_in ic ; hash

let extract filename dst =
  let ic = Zip.open_in filename in
  let entries = Zip.entries ic in
  let rec go acc = function
    | [] -> List.rev acc
    | entry :: rest ->
      let filename = dst / entry.Zip.filename in
      Zip.copy_entry_to_file ic entry filename ;
      go (filename :: acc) rest in
  let files = go [] entries in
  Zip.close_in ic ;

let verify bindings =
  let a = List.sort (fun (k0, _) (k1, _) -> k0 k1) bindings in
  let b = List.sort (fun (k0, _) (k1, _) -> k0 k1) expected in
  a = b 

let () = match Sys.argv with
  | [| _; "list" |] ->
    List.iter (fun (filename, _) -> "%s\n%!" filename) expected
  | [| _; zip; dst; |] when Sys.file_exists zip ->
    if Sys.file_exists dst && not (Sys.is_directory dst)
    then Fmt.failwith "%s already exists and it is not a directory" dst ;
    if not (Sys.file_exists dst) then Sys.mkdir dst 0o700 ;
    let files = extract zip dst in
    let hashes = digest files in
    let bindings = List.combine files hashes in
    if verify bindings
    then exit 0 else exit 1
  | _ -> Fmt.epr "%s [ <directory>|list]\n%!" Sys.argv.(0)

The OCaml compiler and libasmrun.a

Again, we will mainly follow what ocaml-solo5 already does, namely, compile the OCaml compiler and libasmrun.a with our new C toolchain. However, even if imitation is the sincerest form of flattery (about Lucas's work), it is not sufficient for our purpose.

Indeed, OCaml expects basic #include such as stdlib.h or string.h. However, we only have cosmopolitan.h. The trick is to simply create these expected files and systematically #include "cosmopolitan.h" (which contains everything):

├── cosmopolitan.h
├── ctype.h
├── dirent.h
├── endian.h
├── errno.h
├── esperanto.h
├── fcntl.h
├── float.h
├── limits.h
├── math.h
├── setjmp.h
├── signal.h
├── stdarg.h
├── stddef.h
├── stdint.h
├── stdio.h
├── stdlib.h
├── string.h
├── strings.h
├── sys
│   ├── dir.h
│   ├── ioctl.h
│   ├── resource.h
│   ├── stat.h
│   ├── time.h
│   ├── times.h
│   ├── types.h
│   └── wait.h
└── time.h

Unfortunately, as far as OCaml 4.14 is concerned, in some of the C code in the caml runtime, there is the definition of a link structure and a write function. They conflict with the link(2) and write(2) functions that Cosmopolitan offers. Fortunately, these declarations remain internal, just rename them with sed2 and you're done!

typedef struct link {
  void *data;
  struct link *next;
} link;

Caml_inline void write(int c)
  if (extern_ptr >= extern_limit) grow_extern_output(1);
  *extern_ptr++ = c;
$ sed -i -e 's/\([^_]\)link/\1link_t/g' ocaml/runtime/roots_nat.c
$ sed -i -e 's/write(/_write(/' ocaml/runtime/extern.c

2: For this kind of problem, it would have been interesting to use coccinelle which allows to write semantic patches and to make modifications on C code which is a little more reliable than sed... But let's not use a gas factory for so little.

Constants in C

In the new iteration of compiling the OCaml compiler with my toolchain, I finally ran into a slightly thornier problem than usual. Indeed, there is a recurring design in OCaml which consists in "mapping" constants in C with ADT in OCaml.

static int sys_open_flags[] = {

CAMLexport int caml_convert_flag_list(value list, int *flags)
  int res;
  res = 0;
  while (list != Val_int(0)) {
    res |= flags[Int_val(Field(list, 0))];
    list = Field(list, 1);
  return res;

type open_flag =
    Open_rdonly | Open_wronly | Open_append
  | Open_creat | Open_trunc | Open_excl
  | Open_binary | Open_text | Open_nonblock

The trick here is that when compiling an ADT in OCaml, the first constructor will have the value 0, the second will have the value 1, and so on. Thus, a list of this ADT can easily turn into a bitset expected, in our example, by open(2). This "mapping" is necessary to be portable since the value of these constants is not the same for each system.

For example, O_CREAT is not the same on Linux as on FreeBSD.

But wait... if these values are not the same according to the system, how does Cosmopolitan manage these differences? For all values that do not have a consensus like O_CREAT, Cosmopolitan will simply create a global variable (static const unsigned O_CREAT;). Depending on the system from which you want to run a Cosmopolitan program, ape.o will simply modify itself these globals with their expected values (from

However, the error I have now concerns the initialization of this table:

error: initializer element is not constant

This is due to a known limitation of C which the specification explains:

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals.

You can't really be invasive in the code of the OCaml distribution and change that part so easily. So we need to find a way to fix the error while keeping the expected behaviour. One solution that has been described to me and that I've thought about quite a bit is to lie once when compiling the OCaml compiler and then to modify/reset this array at runtime with the real values that Cosmopolitan gives us only at runtime.

The trick is to define these values as a constant only during the first compilation of the caml runtime:


#undef O_CREAT
#define O_CREAT 0

#endif /* __OCAML_ESPERANTO__ */
#endif /* CAML_NAME_SPACE */

Luckily, when compiling the runtime, -DCAML_NAME_SPACE is passed all the time which allows us to define a *.h (which we will add to cosmopolitan.h) allowing us to lie only when compiling the runtime.

Now we just have to initialise this array before the caml program starts (as the values are false). The first step is to add a function to modify this array. The second step is to call this function before calling caml_startup.

$ cat esperanto_sys.c

void _esperanto_init_sys_open_flags(const int cosmopolitan_sys_open_flags[]) {
  memcpy(sys_open_flags, cosmopolitan_sys_open_flags,
$ cat esperanto_sys.c >> ocaml/runtime/sys.c

For our second step, if you remember, we had a -z caml-startup option that allowed us to add a startup.o file during our link. At the begining, it should look like this (a simple call to caml_startup):

int main(int argc, char *argv[]) {
  return (0);

Now it should look like this:

#include "cosmopolitan.h"

extern void _esperanto_init_sys_open_flags(const int[]);

int main(int argc, char *argv[]) {
  int open_flags[] = {
      0,        0,        O_NONBLOCK}; // Cosmopolitan does not provide O_BINARY
                                       // and O_TEXT



  return (0);

The most important thing to note is that our array is initialized in the main function (and not outside of it as was the case for sys_open_flags).

Gimme a try!

We finally have our compiler! The big drawback for now is that it doesn't yet handle unix.cmxa which is much harder to integrate but not impossible! However, we can already make a small OCaml program and produce a completely portable binary.

However, do you remember the constraint of the MirageOS projects? Never depend on unix.cmxa! And that means especially that digestif and decompress do NOT depend on unix.cmxa. Even the Zip version of camlzip depends very little on unix.cmxa (just to get the date). Let's start having fun!

So let's take our The goal here is mainly to 1) get the dependencies (in this case, digestif, decompress, eqaf, checkseum and optint) 2) create a dune's context using our new OCaml compiler 3) compile our with our new OCaml compiler!

$ cat extract.opam
opam-version: "2.0"
name:         "extract-cosmopolitan"
maintainer:   [ "Romain Calascibetta <>" ]
authors:      [ "Romain Calascibetta <>" ]
homepage:     ""
bug-reports:  ""
dev-repo:     "git+"
doc:          ""
license:      "MIT"
synopsis:     "The tool used by esperanto"
description:  "The tool used by esperanto compiled by esperanto..."

build: [
  [ "dune" "build" "-p" name "-j" jobs ]
install:  [
  [ "dune" "install" "-p" name ] {with-test}

depends: [
  "ocaml"           {>= "4.08.0"}
  "dune"            {>= "2.6.0"}
$ cat >dune-workspace<<EOF
(lang dune 2.0)

(context (default))

  (name esperanto)
  (toolchain esperanto)
  (host default)))
$ cat >dune<<EOF
 (name extract)
 (link_flags :standard -cclib "-z caml-startup")
 (libraries bigstringaf fmt digestif))

 (deps extract.exe)
 (mode promote)
 (action (run objcopy -S -O binary %{deps} %{target})))
$ opam monorepo lock
$ opam monorepo pull
$ dune build ./
$ wget
$ bash -c './ dst'
$ ls dst  ape-no-modify-self.o  ape.o  cosmopolitan.a  cosmopolitan.h  crt.o

This now mainly means that the tool to create our Cosmopolitan toolchain was created from our Cosmopolitan toolchain!


Of course, and this is the whole point of this article, can run on Windows, Mac or *BSD! At this point, one can easily imagine an integration into MirageOS in the same way as there is Solo5/ocaml-solo5. But to be much more ambitious, it might be more interesting to integrate unix.cmxa and thus have a complete OCaml distribution that generates portable binaries!


I think this article shows an aspect of the OCaml community that I love: doing something hacky & dirty! Most of what I have just done is clearly unconventional but it leads to a very interesting result.

Cosmopolitan is a beautiful, complex but very interesting project. A bit magical, but:

Any sufficiently advanced technology is indistinguishable from magic — Arthur C. Clarke

I really hope to have the time to continue this project and to go a bit further into the multitudes of tools that I could make in OCaml and that helped me in my development. The next goal would be to be able to compile hxd which is certainly small but uses unix.cmxa. From there, everything becomes possible!

Then it will be time for an official release for the happiness of all hackers like me!