[cffi-devel] offer of code contribution - with-array-as-foreign-pointer

[JTK]

Hello,

I'm using cffi, and I need to perform the common task of passing Lisp arrays to foreign code.

This task comes up a lot in scientific code, matrix math, image processing, etc.

Normally, it involves a lot of boilerplate to allocate a pointer, copy data Lisp  to foreign
memory, copy the data back, and deallocate the memory.   Getting the copying to run
fast can be tricky.

Hence I wrote a package to automate it.    It maintains a set of specialized copying
functions to copy from array of Lisp type X to foreign memory of type Y.   On SBCL, copying
doesn't cons and is reasonably fast (~200 MB a second, there and back)

I've put the package here:

  http://www.megaupload.com/?d=CX1XFMU6

Sorry for the megaupload, and its imposed delay, but I didn't want to spam the list with a 100K file.

The package includes a MANUAL.txt and a test package.

I'd be happy to have this included in a user contrib section of CFFI, if the maintainers 
think it would be useful.




Here's an example the main macro does:


  (LET ((U (MAKE-ARRAY 100 :ELEMENT-TYPE 'SINGLE-FLOAT :INITIAL-ELEMENT 1.0)))
    ;;
    (WITH-ARRAY-AS-FOREIGN-POINTER
           (U PU :FLOAT              ;; LISP-VAR    POINTER-VAR    CFFI-FOREIGN-TYPE
           :LISP-TYPE SINGLE-FLOAT   ;; promise U is a single-float array
           :START 2                  ;; begin at index 2 of of Lisp array
           :END   7                  ;; last index used is 6
           :COPY-TO-FOREIGN   T      ;; put contents of Lisp into foreign memory
           :COPY-FROM-FOREIGN T)     ;; copy back from FFI space to lisp array
         ;;
         ;; at this point, PU is a foreign pointer containing data copied
         ;; from the array starting at element 1 and ending at 6, of type :FLOAT.

         (SOME-FOREIGN-FUNCTION PU))
       ;;
       ;; at end, all foreign memory is deallocated, and U has been copied
       ;; back from foreign space, but the 0th element of U is untouched
       ;; because START was 1, not 0
     )


-John