[racket-dev] ffi vectors

When you run this program on a 32-bit machine:

 #include <stdio.h>

 void go(float a[4])
 {
   printf("in go: %d\n", sizeof(a));
 }

 int main() {
   float a[4];
   printf("in main: %d\n", sizeof(a));
   go(a);
 }

you'll see "in main: 16" and "in go: 4".

As far as I know, the "4" in " void go(float a[4])" is ignored. It's
really the same as "void go(float a[])" or "void go(float *a)". Unless
the declaration of an array variable is allocating the variable, the
variable is really a pointer, and sizeof() reflects that.

Along the same lines, a `_cvector' in the FFI always has a pointer
size, because it's always like a pointer.

A `(_cvector o _float 4)' or `(_f32vector o 4)' is probably
what you want, if the function you'll calling fills in the vector. A
`_float4-pointer' (not `_float4'!) if you allocate it yourself or
`(_pointer o _float4)' is also fine.


At Thu, 30 Sep 2010 16:41:29 -0600, Jay McCarthy wrote:
> I'd like to be able to define ctypes like I would make a typedef in C like
> 
> typedef float           float4[4];
> 
> But it doesn't seem like this works in the FFI. See the program below
> with its awkward work-around:
> 
> #lang racket
> (require ffi/unsafe
>          ffi/unsafe/cvector
>          ffi/vector
>          tests/eli-tester)
> 
> (test
>  (ctype-sizeof _float) => 4
> 
>  (ctype-sizeof _cvector) => 4
>  (ctype-sizeof (_cvector i _float)) => 4
>  (ctype-sizeof (_cvector o _float 4)) => (* 4 4)
>  (ctype-sizeof (_cvector io _float 4)) => (* 4 4)
> 
>  (ctype-sizeof _f32vector) => 4
>  (ctype-sizeof (_f32vector i)) => 4
>  (ctype-sizeof (_f32vector o 4)) => (* 4 4)
>  (ctype-sizeof (_f32vector io 4)) => (* 4 4)
> 
>  (local [(define-cstruct _float4
>            ([f0 _float]
>             [f1 _float]
>             [f2 _float]
>             [f3 _float]))]
>    (test
>     (ctype-sizeof _float4) => 16)))
> 
> Output is:
> 
> test: 5/11 test failures:
> unsaved-editor4119:11:1: test failure in (ctype-sizeof (_cvector i _float))
>   expected: 4
>        got: error: expand: unbound identifier in module
> unsaved-editor4119:12:1: test failure in (ctype-sizeof (_cvector o _float 4))
>   expected: 16
>        got: 4
> unsaved-editor4119:13:1: test failure in (ctype-sizeof (_cvector io _float 4))
>   expected: 16
>        got: error: expand: unbound identifier in module
> unsaved-editor4119:17:1: test failure in (ctype-sizeof (_f32vector o 4))
>   expected: 16
>        got: 4
> unsaved-editor4119:18:1: test failure in (ctype-sizeof (_f32vector io 4))
>   expected: 16
>        got: error: expand: unbound identifier in module
> 
> Normally I would just go do this, but I don't really understand the
> FFI. If someone can point me appropriately, I'll go do it.
> 
> Jay
> 
> -- 
> Jay McCarthy <jay at cs.byu.edu>
> Assistant Professor / Brigham Young University
> http://teammccarthy.org/jay
> 
> "The glory of God is Intelligence" - D&C 93
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