Haskell FFI-您可以从Haskell数据结构中获取C指针吗? [英] Haskell FFI - can you obtain a C Pointer from a Haskell data structure?

问题描述

我有很多类似的C结构

typedef struct {
    unsigned int a;
    unsigned int b;
} StructA;

还有很多功能，例如

void doSomethingWith(StructA*,StructB*,StructC*);

有没有一种简单的方法可以使用Haskell FFI调用这些函数?例如，是否有类似&的行为C中的运算符? (我想没有，但是如果有的话我想知道).我必须使Haskell端的data实例可存储(这些结构没有任何构造函数).

Is there an easy way to call these functions with Haskell FFI? Like, is there something that behaves like the & operator in C? (I imagine there isn't, but if there was I'd like to know). Do I have to make the Haskell side data's instance of storeable (I don't have any constructor functions for these structs).

也:如果我必须传递一个结构而不是一个结构指针(这不是一个假设的问题，我有一些类似的功能-这不是我的代码，所以我对此无能为力)，我可以传递结构的组件呢?就像我想打电话

Also: If I have to pass a struct instead of a struct pointer (not a hypothetical question, I have a few functions like that - it's not my code so I can't do anything about it), can I just pass the components of the struct instead? Like if I want to call

void function(StructA);

我可以用

foreign import ccall "function" :: CUInt -> CUInt -> IO()

?

推荐答案

要将对Haskell数据的引用传递给C，该内存在Haskell堆中分配，并且C将直接对数据进行操作，您需要:

To pass a reference to Haskell data to C, where the memory is allocated in the Haskell heap, and C will operate on the data directly, you need to:

• 确保它在内存中具有正确的形状(通过 mallocForeignPtr

• ensure it has the correct shape in memory (via a Storable instance that maps A to the an identical byte structure as StructA).
• allocate and fill pinned memory on the Haskell heap, via mallocForeignPtr

这种方法有几个后果要考虑:

There are several consequences to consider in this approach:

• 一旦删除对ForeignPtr的所有引用，GHC就会取消分配该值-因此，您需要确保C端不会再碰到它
• 您要让C弄乱Haskell堆上的内容，因此请确保其正确

其他选项:

• 通过StablePtr
将不透明引用传递给C
• 在C端分配和存储，并使用终结器将其释放.

• pass opaque references to C via a StablePtr
• allocate and memory on the C side, and use a finalizer to free it.

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