如何在没有c2hs或其他工具的情况下为此结构创建可存储的实例？ [英] How do I create a storable instance for this structure without c2hs or other tools?

问题描述

$ b

  typedef struct Atom_ {
 float x; 
 float y; 
 float z; 
 int col; 
} Atom; 
  

对应于此类型：

  data Atom = Atom {pos :: V3 Float，col :: Int} 
  

如何为 Atom 创建一个可存储的实例，以便我可以将Haskell上的 Atom s发送给C函数，期望一个 Atom ？

解决方案

保证这将完全按照所显示的工作（我没有在这台计算机上设置环境），但它应该是朝着正确的方向迈出的第一步：

  import Foreign.Storable 
 import Foreign.Ptr 
 
 instance可存储的Atom，其中
  - 不让这个静态的，让编译器选择取决于
  - 在平台上
 sizeOf _ = 3 * sizeOf（0 :: Float）+ sizeOf（0 :: Int）
  - 您可能需要摆弄这个，如果你有这个能力，还有你的相应的C 
  -  code对齐可能会非常棘手，但
  - 在这种情况下，您可以非常安全地假定它将是
  - 对于'Float` 
 alignment _ = alignment（0 ::浮动）
  - 这些非常简单明了，显然订单在这里
  - 很重要。我清楚地没有把它做成最优化它
  - 可能是，我要清晰的代码而不是
 peek ptr = do 
 let floatSize = sizeOf（0 :: Float ）
x < -  peek $ ptr`plusPtr`（0 * floatSize）
y < -  peek $ ptr`plusPtr`（1 * floatSize）
z<  -  peek $ ptr`plusPtr` （2 * floatSize）
 col < -  peek $ ptr`plusPtr`（3 * floatSize）
 return $ Atom（V3 xyz）col 
 poke ptr（Atom（V3 xyz）col） = do 
 let floatSize = sizeOf（0 :: Float）
 poke（ptr`plusPtr`（0 * floatSize））x 
 poke（ptr`plusPtr`（1 * floatSize））y 
 poke（ptr`plusPtr`（2 * floatSize））z 
 poke（ptr`plusPtr`（3 * floatSize））col 
  

这应该是正常的！它可能高度依赖于你的C编译器和你的平台，不过，你需要做一些广泛的测试，以确保它已经正确布置。

This structure:

typedef struct Atom_ {
    float x;
    float y;
    float z;
    int col;
} Atom;

corresponds to this type:

data Atom = Atom { pos :: V3 Float, col :: Int }

How do I create a storable instance for Atom so that I can send an Atoms on Haskell to a C function that expects an Atom?

解决方案

I can't currently guarantee that this will work exactly as shown (I don't have the environment set up on this computer), but it should be a good first step towards getting it right:

import Foreign.Storable
import Foreign.Ptr

instance Storable Atom where
    -- Don't make this static, let the compiler choose depending
    -- on the platform
    sizeOf _ = 3 * sizeOf (0 :: Float) + sizeOf (0 :: Int)
    -- You may need to fiddle with this, and with your corresponding C
    -- code if you have the ability to, alignment can be tricky, but
    -- in this case you can pretty safely assume it'll be the alignment
    -- for `Float`
    alignment _ = alignment (0 :: Float)
    -- These are pretty straightforward, and obviously order matters here
    -- a great deal.  I clearly haven't made this the most optimized it
    -- could be, I'm going for clarity of code instead
    peek ptr = do
        let floatSize = sizeOf (0 :: Float)
        x   <- peek $ ptr `plusPtr` (0 * floatSize)
        y   <- peek $ ptr `plusPtr` (1 * floatSize)
        z   <- peek $ ptr `plusPtr` (2 * floatSize)
        col <- peek $ ptr `plusPtr` (3 * floatSize)
        return $ Atom (V3 x y z) col
    poke ptr (Atom (V3 x y z) col) = do
        let floatSize = sizeOf (0 :: Float)
        poke (ptr `plusPtr` (0 * floatSize)) x
        poke (ptr `plusPtr` (1 * floatSize)) y
        poke (ptr `plusPtr` (2 * floatSize)) z
        poke (ptr `plusPtr` (3 * floatSize)) col

And this should just work! It could be highly dependent on your C compiler and your platform, though, you'll want to do some extensive testing to make sure it's been laid out correctly.

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