为读取系统调用写入 Haskell FFI

Question

read函数定义如下：

#include <unistd.h>

ssize_t read(int fd, void *buf, size_t len)

我想为此编写一个 Haskell FFI。这是我的尝试（无法编译）：

foreign import ccall unsafe "read" c_read :: CInt -> Ptr () -> CSize -> IO CSsize

read :: Int -- ^ file descriptor
     -> Int64 -- ^ Bytes to allocate
     -> Word64 -- ^ Read upto this many bytes
     -> IO (String, Int64)
read fd bytes len = do
  allocaBytes (fromIntegral bytes) (\ptr -> do
                       size <- c_read (fromIntegral fd) ptr (fromIntegral len)
                       dat <- peekByteOff (castPtr ptr) (fromIntegral size) :: IO String
                       return (dat, (fromIntegral size))
                    )

它无法编译，因为没有 String 的 Storable 实例。关于如何为此编写 FFI 的任何提示？

Answer 1

Foreign.C.String 是这里的传统解决方案。

Answer 2

根据 Michael Snoyman 和 Reid Barton 的建议，我最终想出了这个解决方案作为开始：

foreign import ccall safe "read" c_read :: CInt -> Ptr () -> CSize -> IO CSsize

read :: Int -- ^ file descriptor
     -> Int64 -- ^ Bytes to allocate
     -> Word64 -- ^ Read upto this many bytes
     -> IO (ByteString, Int64)
read fd bytes len = do
  (ptr :: Ptr ()) <- mallocBytes (fromIntegral bytes)
  size <- c_read (fromIntegral fd) ptr (fromIntegral len)
  bstring <- packCStringLen (castPtr ptr, fromIntegral size) 
  free ptr
  return (bstring, (fromIntegral size))

为读取系统调用写入 Haskell FFI

Writing Haskell FFI for read system call

linux

haskell

ffi

system-calls