63 lines
1.8 KiB
ReStructuredText
63 lines
1.8 KiB
ReStructuredText
How to Override the Default C Entrypoint With C Stubs
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-----------------------------------------------------
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In some cases, it may be necessary to override the default C entry point of an
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OCaml program. For example, this is the case if you want to let your program
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handle argument wildcards expansion on Windows.
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Let's consider a trivial "Hello world" program contained in a ``hello.ml``
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file:
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.. code:: ocaml
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let () = print_endline "Hello, world!"
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The default C entry point is a ``main`` function, originally defined in
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`runtime/main.c <https://github.com/ocaml/ocaml/blob/trunk/runtime/main.c>`_. It
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can be overridden by defining a ``main`` function that will at some point call
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the OCaml runtime. Let's write such a minimal example in a ``main.c`` file:
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.. code:: C
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#include <stdio.h>
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#define CAML_INTERNALS
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#include "caml/misc.h"
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#include "caml/mlvalues.h"
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#include "caml/sys.h"
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#include "caml/callback.h"
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/* This is the new entry point */
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int main(int argc, char_os **argv)
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{
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/* Here, we just print a statement */
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printf("Doing stuff before calling the OCaml runtime\n");
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/* Before calling the OCaml runtime */
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caml_main(argv);
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caml_do_exit(0);
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return 0;
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}
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The :doc:`foreign_stubs </reference/foreign-stubs>` stanza can be leveraged to
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compile and link our OCaml program with the new C entry point defined in
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``main.c``:
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.. code:: dune
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(executable
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(name hello)
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(foreign_stubs
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(language c)
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(names main)))
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With this ``dune`` file, the whole program can be compiled by merely calling
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``dune build``. When run, the output shows that it calls the custom entry point
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we defined:
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.. code:: shell-session
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$ dune build
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$ _build/default/hello.exe
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Doing stuff before calling the OCaml runtime
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Hello, world!
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