Common Lisp的，CFFI，和实例C结构 [英] Common lisp, CFFI, and instantiating c structs

问题描述

我已经在谷歌一下，哦，寻找一个解决方案，以这3小时的问题。我试图找出如何使用LISP实例CFFI C结构。我有在C结构：

I've been on google for about, oh, 3 hours looking for a solution to this "problem." I'm trying to figure out how to instantiate a C structure in lisp using CFFI. I have a struct in c:

struct cpVect{cpFloat x,y;}

简单吧？我有自动生成CFFI绑定（痛饮，我认为）这个结构：

Simple right? I have auto-generated CFFI bindings (swig, I think) to this struct:

(cffi:defcstruct #.(chipmunk-lispify "cpVect" 'classname)
    (#.(chipmunk-lispify "x" 'slotname) :double)
    (#.(chipmunk-lispify "y" 'slotname) :double))

这会产生一个结构VECT与插槽：X和：Y，其中外商槽的名字证实（请注意，我既不产生的绑定或编程的C库（花栗鼠物理），但实际功能是从口齿不清就好）调用。

This generates a struct "VECT" with slots :X and :Y, which foreign-slot-names confirms (please note that I neither generated the bindings or programmed the C library (chipmunk physics), but the actual functions are being called from lisp just fine).

我搜索远播，也许我已经看到了100倍，轻轻带过，但我无法弄清楚如何在LISP等功能，使用创建cpVect的实例。

I've searched far and wide, and maybe I've seen it 100 times and glossed over it, but I cannot figure out how to create a instance of cpVect in lisp to use in other functions.

请注意功能：

cpShape *cpPolyShapeNew(cpBody *body, int numVerts, cpVect *verts, cpVect offset)

不仅需要一个cpVect，还指向了一套cpVects，这引出了我的第二个问题：我怎么创建一个指向一组结构的

Takes not only a cpVect, but also a pointer to a set of cpVects, which brings me to my second question: how do I create a pointer to a set of structs?

我去过 http://common-lisp.net /project/cffi/manual/html_node/defcstruct.html 并尝试了code，却得到错误：未绑定变​​量：PTR（我在Clozure CL），更何况，看起来只返回一个指针，而不是一个实例

I've been to http://common-lisp.net/project/cffi/manual/html_node/defcstruct.html and tried the code, but get "Error: Unbound variable: PTR" (I'm in Clozure CL), not to mention that looks to only return a pointer, not an instance.

我是新来口齿不清，已经持续pretty强，到目前为止，但是这是第一个真正的问题，我已经打了，我想不通。谢谢！

I'm new to lisp, been going pretty strong so far, but this is the first real problem I've hit that I can't figure out. Thanks!

推荐答案

大多数的Common Lisp实现不允许传递堆栈结构。有一个 fsbv库，使用libffi补充说，能力。如果您知道的结构布局可以手动分解它作为一系列的基本论点，但是这显然是易碎的。

Most Common Lisp implementations do not allow passing structures on stack. There is a fsbv library which uses libffi to add that capability. If you know the structure layout you can decompose it manually as a series of basic arguments, but that is obviously brittle.

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