Re: [Chicken-users] C_alloc OK before C_invoke? (was Re: Constructing parameter lists in C)

[felix winkelmann]

On 7/22/05, Daniel B. Faken <address@hidden> wrote:
> 
> OK.
> From what I see in chicken.h, we have:
> # define C_alloca                   alloca
> #define C_alloc(n)                 ((C_word *)C_alloca((n) * sizeof(C_word)))
> 
> And as I undestand, alloca() is the same as allocating things on the stack
> directly, via C variable declarations.
> So does this mean instead of (for example):
>   C_word *data = C_alloc(N*C_SIZEOF_FLONUM);
>   C_word one = C_flonum(&data, 1.0);
>   C_word two = C_flonum(&data, 2.0);
>   C_word three = C_flonum(&data, 3.0);
>   etc.
> I could do
>   C_word realdata[N*C_SIZEOF_FLONUM];
>   C_word *data = realdata;
>   C_word one = C_flonum(&data, 1.0);
>   C_word two = C_flonum(&data, 2.0);
>   etc.
> ??

Yes, that's fine. CHICKEN_invoke takes the current stack-pointer
to set global stack-low- and high-marks, but from a quick skim
of the sources this shouldn't be a problem in this case (since your
allocated data is below the "low" mark).

> 
> That is to say:
> * is C_alloc() just a convenient way of avoiding getting the compiler to
>   understand &realdata (above) as a pointer-to-a-pointer?
>   (not sure if this is an ANSI-C issue - just know my compiler thought
>   &realdata was a pointer-to-a-C_word).

Yes.

> * are the issues of when to use C_alloc()-ated data identical to those
>   for the storing of data in the C stack?

Yes.

> Maybe this should be obvious.. but the documentation for C_pair(),
> C_list(), etc. saying "ptr should be the address of an allocation pointer
> created with C_alloc" made it seem like it was doing something special
> (beyond what just declaring the memory in a C local-variable would do).
>   If not, maybe this could be clarified in the documentation.
> 

Will do. Thanks for the tip!


cheers,
felix