Re: [Caml-list] Re: OCaml binary formats -- how are they linked?

[Philippe Wang <mail@philippewang.info>]

On Fri, Jun 14, 2013 at 8:17 PM, Ömer Sinan Ağacan <omeragacan@gmail.com> wrote:
> Dawid, thanks for your answer. It really helped.

>
>> Bytecode is compiled to a files with #! prefix.
>
> Interesting, I just tried reading an OCaml executable created with
> ocamlc, and it had a ELF header. Am I compiling to my program to
> native by mistake? I'm not calling ocamlopt, only ocamlc.
>

If you're using ocamlc and have ELF header, it probably means that you
used the option -custom. In that case, instead of a
#!/path/to/ocamlrun, you get ocamlrun (the binary) as the first part
of the binary.
Actually, when the OCaml VM runs a bytecode executable, it reads from
the end of file (where there are information on, e.g., where the
bytecode is), hence the fact that virtually anything can be put before
the actual bytecode.

$ echo 'print_endline "hello";;' > p.ml
$ ocamlc p.ml -o p
$ ( head -n 1 p ; cat p.ml p ) > q
$ chmod a+x q
$ ./q
hello

Oh, a binary that contains its source code! :-)
(It's not that easy if it's compiled using -custom, because then you
have to find out when the ELF binary ends in order to insert whatever
you might want to insert).

--
Philippe Wang
   mail@philippewang.info