[cffi-devel] Foreign struct

[Liam Healy]

I'm having a problem defining/using a foreign struct for my package
GSLL (http://common-lisp.net/project/gsll), an interface to the GNU
Scientific Library (http://www.gnu.org/software/gsl/), written in C.
I am trying to handle complex numbers; GSL has a struct defined to
represent complex numbers in /usr/include/gsl/gsl_complex.h:
typedef struct
  {
    double dat[2];
  }
gsl_complex;
So I define
(cffi:defcstruct complex-double-c
  (dat :double :count 2))
I set this value with a function
(defun complex-to-gsl-df (number)
  (let* ((gsl (cffi:foreign-alloc 'complex-double-c))
	 (datslot
	  (cffi:foreign-slot-pointer gsl 'complex-double-c 'dat)))
    (setf (cffi:mem-aref datslot :double 0) (realpart number)
	  (cffi:mem-aref datslot :double 1) (imagpart number))
    gsl))
When I pass a complex number generated with this function to a GSL
function, I always get the following result: the real part is what I
specified for the imaginary part, and the imaginary part is the number
5.2635442471208903e-315, which I discovered is the single-float number
-1.0f0 interpreted as a double float (I don't know if that's
significant).  So for example the function #'test-set-all should set
every element of the complex vector to the same number 3+4i, but I get
4+5.2635442471208903e-315i:
(letm ((intvec (vector-complex-double-float 2)))
   (test-set-all intvec #c(3.0d0 4.0d0))
   (maref intvec 1))
#C(4.0 5.2635442471208903e-315)

There is a GSL function that sets the real and imaginary parts of the
struct (in C of course).  When I use this, I get the answer I expect.
Unfortunately, it's more restricted than setting the structure in CL,
so I'd like to solve the structure problem.

My platform is SBCL 0.9.16, amd64 Debian (stable). I have a feeling I
have misinterpreted the defcstruct usage.  Any ideas?

Thanks,
Liam