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<h4 class="subsection">8.7.4 Calling Lisp From C</h4>
<p>Calling Lisp functions from C is sometimes possible, but is extremely
hackish and poorly supported as of SBCL 0.7.5. See <code>funcall0</code>
<small class="dots">...</small> <code>funcall3</code> in the runtime system. The arguments must be
valid SBCL object descriptors (so that e.g. fixnums must be
left-shifted by 2.) As of SBCL 0.7.5, the format of object descriptors
is documented only by the source code and, in parts, by the old CMUCL
<samp><span class="file">INTERNALS</span></samp> documentation.
<p>Note that the garbage collector moves objects, and won't be
able to fix up any references in C variables. There are three
mechanisms for coping with this:
<ol type=1 start=1>
<li>The <code>sb-ext:purify</code> moves all live Lisp
data into static or read-only areas such that it will never be moved
(or freed) again in the life of the Lisp session
<li><code>sb-sys:with-pinned-objects</code> is a macro which arranges for some
set of objects to be pinned in memory for the dynamic extent of its
body forms. On ports which use the generational garbage collector (as
of SBCL 0.8.3, only the x86) this has a page granularity - i.e. the
entire 4k page or pages containing the objects will be locked down. On
other ports it is implemented by turning off GC for the duration (so
could be said to have a whole-world granularity).
<li>Disable GC, using the <code>without-gcing</code> macro.
</ol>
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<!-- LaTeX \subsection{Accessing Lisp Arrays} -->
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<!-- LaTeX allocated.}. -->
<!-- Empirically determined to be considerably >8Mb on this x86 linux -->
<!-- machine, but I don't know what the actual values are - dan 2003.09.01 -->
<!-- Note that this technique is used in SB-GROVEL in the SBCL contrib -->
<!-- LaTeX -->
<!-- LaTeX To overcome this limitation, it is possible to access the content of -->
<!-- LaTeX Lisp arrays which are limited only by the amount of physical memory -->
<!-- LaTeX and swap space available. However, this technique is only useful if -->
<!-- LaTeX the foreign function takes pointers to memory instead of allocating -->
<!-- LaTeX memory for itself. In latter case, you will have to modify the -->
<!-- LaTeX foreign functions. -->
<!-- LaTeX -->
<!-- LaTeX This technique takes advantage of the fact that \cmucl{} has -->
<!-- LaTeX specialized array types (\pxlref{specialized-array-types}) that match -->
<!-- LaTeX a typical C array. For example, a \code{(simple-array double-float -->
<!-- LaTeX (100))} is stored in memory in essentially the same way as the C -->
<!-- LaTeX array \code{double x[100]} would be. The following function allows us -->
<!-- LaTeX to get the physical address of such a Lisp array: -->
<!-- LaTeX \begin{example} -->
<!-- LaTeX (defun array-data-address (array) -->
<!-- LaTeX "Return the physical address of where the actual data of an array is -->
<!-- LaTeX stored. -->
<!-- LaTeX -->
<!-- LaTeX ARRAY must be a specialized array type in CMU Lisp. This means ARRAY -->
<!-- LaTeX must be an array of one of the following types: -->
<!-- LaTeX -->
<!-- LaTeX double-float -->
<!-- LaTeX single-float -->
<!-- LaTeX (unsigned-byte 32) -->
<!-- LaTeX (unsigned-byte 16) -->
<!-- LaTeX (unsigned-byte 8) -->
<!-- LaTeX (signed-byte 32) -->
<!-- LaTeX (signed-byte 16) -->
<!-- LaTeX (signed-byte 8) -->
<!-- LaTeX " -->
<!-- LaTeX (declare (type (or #+signed-array (array (signed-byte 8)) -->
<!-- LaTeX #+signed-array (array (signed-byte 16)) -->
<!-- LaTeX #+signed-array (array (signed-byte 32)) -->
<!-- LaTeX (array (unsigned-byte 8)) -->
<!-- LaTeX (array (unsigned-byte 16)) -->
<!-- LaTeX (array (unsigned-byte 32)) -->
<!-- LaTeX (array single-float) -->
<!-- LaTeX (array double-float)) -->
<!-- LaTeX array) -->
<!-- LaTeX (optimize (speed 3) (safety 0)) -->
<!-- LaTeX (ext:optimize-interface (safety 3))) -->
<!-- LaTeX ;; with-array-data will get us to the actual data. However, because -->
<!-- LaTeX ;; the array could have been displaced, we need to know where the -->
<!-- LaTeX ;; data starts. -->
<!-- LaTeX (lisp::with-array-data ((data array) -->
<!-- LaTeX (start) -->
<!-- LaTeX (end)) -->
<!-- LaTeX (declare (ignore end)) -->
<!-- LaTeX ;; DATA is a specialized simple-array. Memory is laid out like this: -->
<!-- LaTeX ;; -->
<!-- LaTeX ;; byte offset Value -->
<!-- LaTeX ;; 0 type code (should be 70 for double-float vector) -->
<!-- LaTeX ;; 4 4 * number of elements in vector -->
<!-- LaTeX ;; 8 1st element of vector -->
<!-- LaTeX ;; ... ... -->
<!-- LaTeX ;; -->
<!-- LaTeX (let ((addr (+ 8 (logandc1 7 (kernel:get-lisp-obj-address data)))) -->
<!-- LaTeX (type-size (let ((type (array-element-type data))) -->
<!-- LaTeX (cond ((or (equal type '(signed-byte 8)) -->
<!-- LaTeX (equal type '(unsigned-byte 8))) -->
<!-- LaTeX 1) -->
<!-- LaTeX ((or (equal type '(signed-byte 16)) -->
<!-- LaTeX (equal type '(unsigned-byte 16))) -->
<!-- LaTeX 2) -->
<!-- LaTeX ((or (equal type '(signed-byte 32)) -->
<!-- LaTeX (equal type '(unsigned-byte 32))) -->
<!-- LaTeX 4) -->
<!-- LaTeX ((equal type 'single-float) -->
<!-- LaTeX 4) -->
<!-- LaTeX ((equal type 'double-float) -->
<!-- LaTeX 8) -->
<!-- LaTeX (t -->
<!-- LaTeX (error "Unknown specialized array element type")))))) -->
<!-- LaTeX (declare (type (unsigned-byte 32) addr) -->
<!-- LaTeX (optimize (speed 3) (safety 0) (ext:inhibit-warnings 3))) -->
<!-- LaTeX (system:int-sap (the (unsigned-byte 32) -->
<!-- LaTeX (+ addr (* type-size start))))))) -->
<!-- LaTeX \end{example} -->
<!-- LaTeX -->
<!-- LaTeX Assume we have the C function below that we wish to use: -->
<!-- LaTeX \begin{example} -->
<!-- LaTeX double dotprod(double* x, double* y, int n) -->
<!-- LaTeX \{ -->
<!-- LaTeX int k; -->
<!-- LaTeX double sum = 0; -->
<!-- LaTeX -->
<!-- LaTeX for (k = 0; k < n; ++k) \{ -->
<!-- LaTeX sum += x[k] * y[k]; -->
<!-- LaTeX \} -->
<!-- LaTeX \} -->
<!-- LaTeX \end{example} -->
<!-- LaTeX The following example generates two large arrays in Lisp, and calls the C -->
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<!-- LaTeX possible using \code{malloc} or \code{make-alien} since we need about -->
<!-- LaTeX 16 MB of memory to hold the two arrays. -->
<!-- LaTeX \begin{example} -->
<!-- LaTeX (define-alien-routine "dotprod" double -->
<!-- LaTeX (x (* double-float) :in) -->
<!-- LaTeX (y (* double-float) :in) -->
<!-- LaTeX (n int :in)) -->
<!-- LaTeX -->
<!-- LaTeX (let ((x (make-array 1000000 :element-type 'double-float)) -->
<!-- LaTeX (y (make-array 1000000 :element-type 'double-float))) -->
<!-- LaTeX ;; Initialize X and Y somehow -->
<!-- LaTeX (let ((x-addr (system:int-sap (array-data-address x))) -->
<!-- LaTeX (y-addr (system:int-sap (array-data-address y)))) -->
<!-- LaTeX (dotprod x-addr y-addr 1000000))) -->
<!-- LaTeX \end{example} -->
<!-- LaTeX In this example, it may be useful to wrap the inner \code{let} -->
<!-- LaTeX expression in an \code{unwind-protect} that first turns off garbage -->
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<!-- LaTeX prevent garbage collection from moving \code{x} and \code{y} after we -->
<!-- LaTeX have obtained the (now erroneous) addresses but before the call to -->
<!-- LaTeX \code{dotprod} is made. -->
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