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Re: How to pass the address of an pointer object to c?

From: Neil Jerram
Subject: Re: How to pass the address of an pointer object to c?
Date: Sun, 01 Apr 2018 13:13:57 +0100

Fis Trivial <address@hidden> writes:

> Hi, all.
> I'am trying a wrap a math library written in c with guile scheme. Here
> is an example declaration of c type and function in that math library:
> typedef void* array;
> typedef int err;
> err randu(array* result, int ndims, long long *dims, dtype type);
> The problem is I want to create a void* (aka array), and pass its
> address (aka array*) to the underlying c function for modification. Here
> the /array/ type acts as a handle.
> I tried the following scheme code:
> (let* ([val (make-pointer 0)]                   ; void* val = 0
>        [&val (make-pointer (object-address val))] ; a pointer points to val?
>        [randu (pointer->procedure int
>                                    ; a function generating random matrix.
>                                 (dynamic-func "randu" backend)
>                                    ; result, ndims, dims, type
>                                 (list '* uint32 '* int))]
>        ; dims is an byte-vector representing c array, 0 represents an
>        ; enum value in c.
>        (randu &val 4 dims 0)))
> But when I use gdb to watch the values in c code of randu with:
> print *result
> and gdb displayed:  0x1f,
> while it should be 0 as defined in scheme code.
> Is there anything I did wrong? Or there are other ways around? I want to
> do it in scheme, otherwise I have to wrap every c functions.
> Thanks. :)

Would it work for you to use `%null-pointer' ?

>From `(guile) Foreign Variables':

 -- Scheme Variable: %null-pointer
     A foreign pointer whose value is 0.

Best wishes,

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