[Caml-list] Some questions regarding OCaml FFI

[Caml-list] Some questions regarding OCaml FFI

[Alexey Egorov]

Hello,

I have some questions about OCaml FFI:

1) Should I declare local variable with CAMLlocalN if its used "temporarily", for example:
CAMLreturn(Val_long(1));
vs
CAMLlocal1(longval);
longval = Val_long(1);
CAMLreturn(longval);

2) Should I call 'caml_acquire_runtime_system()' prior to calling OCaml functions from C main()? If so - should I call it before or after 'caml_startup()'?

Thanks!

Re: [Caml-list] Some questions regarding OCaml FFI

[Gerd Stolpmann]

On 03.04.17 00:58, Alexey Egorov wrote:

> Hello,
>
> I have some questions about OCaml FFI:
>
> 1) Should I declare local variable with CAMLlocalN if its used "temporarily", for example:
> CAMLreturn(Val_long(1));
> vs
> CAMLlocal1(longval);
> longval = Val_long(1);
> CAMLreturn(longval);
The specialty of a CAMLlocal variable is that the value is then a local 
root, and this has important effects on the garbage collector. In 
particular the value is considered as reachable (and thus not 
deallocated if it is the last reference to it), and second, if the block 
for this value is moved to a new memory address, the pointer is updated.

Of course, this all is only meaningful if the value is really a block 
(and not just an integer). So in your case this alone means that 
CAMLlocal is not needed. Also, you only need this if you call into the 
OCaml runtime for memory allocation (i.e. a GC is possible). You do not 
do this here (CAMLreturn is harmless in this respect - just deletes the 
local roots).

> 2) Should I call 'caml_acquire_runtime_system()' prior to calling OCaml functions from C main()? If so - should I call it before or after 'caml_startup()'?
Hah! The new naming is not clearer than the old... No, you don't need to.

Gerd