[Caml-list] void/unit in C calls

[Caml-list] void/unit in C calls

[Michael Furr]

I realize this is somewhat of a pedantic question but, what is the proper
prototype for a C function which has a parameter of type unit?
ie:
 external foo : unit -> unit = "foo_c"

should foo_c be declared
 void foo_c(void);
or
 value foo_c(value);

Would it ever cause a problem on any architecture if one used the wrong
form?

thanks,
-Mike

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Re: [Caml-list] void/unit in C calls

[Shawn Wagner]

On Fri, Feb 20, 2004 at 02:26:05PM -0500, Michael Furr wrote:
> 
> I realize this is somewhat of a pedantic question but, what is the proper
> prototype for a C function which has a parameter of type unit?
> ie:
>  external foo : unit -> unit = "foo_c"
> 
> should foo_c be declared
>  void foo_c(void);
> or
>  value foo_c(value);

This one.

> 
> Would it ever cause a problem on any architecture if one used the wrong
> form?

Yup. I've had problems with the bytecode runtime not liking C functions that
don't return anything when they should be returning a Val_unit.

-- 
Shawn Wagner
shawnw@speakeasy.org

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Re: [Caml-list] void/unit in C calls

[Markus Mottl]

On Fri, 20 Feb 2004, Michael Furr wrote:
> I realize this is somewhat of a pedantic question but, what is the proper
> prototype for a C function which has a parameter of type unit?
> ie:
>  external foo : unit -> unit = "foo_c"
> 
> should foo_c be declared
>  void foo_c(void);
> or
>  value foo_c(value);
> 
> Would it ever cause a problem on any architecture if one used the wrong
> form?

It makes a difference: you should always return Val_unit from
unit-functions implemented in C. Otherwise you might find obscure
things like:

  foo () <> ()

Or worse... - somebody reported to me that he sometimes saw programs
crash, though it isn't certain that this was the cause.

It shouldn't matter whether you pass void or value to C, I think, unless
you want to use the value - which is quite unlikely.

Regards,
Markus

-- 
Markus Mottl          http://www.oefai.at/~markus          markus@oefai.at

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Re: [Caml-list] void/unit in C calls

[Lars Nilsson]

>It makes a difference: you should always return Val_unit from
>unit-functions implemented in C. Otherwise you might find obscure
>things like:
>
>   foo () <> ()
>
>Or worse... - somebody reported to me that he sometimes saw programs
>crash, though it isn't certain that this was the cause.

Does this indicate an error in the O'Caml manual, section 18.5.1, where

  void foo(value v1, value v2, value v3)
  {
    CAMLparam3(v1, v2, v3);

    CAMLreturn0;
  }

is used, in addition to a paragraph preceeding it in the same section saying 
"If your C function is a procedure you must insert CAMLreturn0 at the end"? 
In fact, CAMLreturn0 existing at all seems to imply the fact that 
unit-returning functions (from O'Caml's point of view) can be void in the C 
implementation.

Lars

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