c – 使用来自C的内存地址的Julia读/写结构数据

2019年7月28日 146次阅读

我将我的应用程序嵌入到Julia中,我需要一种从Julia和C读取/写入相同结构的好方法.

在Python中我可以简单地做到：

ffi.cdef("""
    struct keyboard_s {
        int forward;
        int backward;
        int left;
        int right;
        int jump;
    }

    struct keyboard_s *app_get_keyboard();
"""

app = ffi.dlopen("app.dll")

thekeyboard = app.app_get_keyboard();

thekeyboard.forward = 1; # this would immediatly change the memory in C

但是,我在Julia和Julia中尝试这样的事情总是只是复制数据并且无法从C更改外部存储器地址：

type keyboard_s
    forward::Int32
    backward::Int32
    left::Int32
    right::Int32
    jump::Int32
end

# lets imply this would return the memory struct just like app_get_keyboard()
# I just use malloc(sizeof(keyboard_s)) so everybody here can test for themselves...
address = ccall(:malloc, (Int64), (Int64, ), sizeof(keyboard_s))

# address is now a valid Int64 address, so lets map it as pointer of type keyboard_s
ptr = Ptr{keyboard_s}(address)

# thekeyboard contains now the random data from the c static memory
thekeyboard = unsafe_load(ptr)

# this will change only the value of "thekeyboard",
# it doesn't touch C the Int64 address memory pointer...
thekeyboard.forward = 123 # this has no effect on the real memory address :(

# lets load the keyboard again from same address
thekeyboard = unsafe_load(ptr)

thekeyboard.forward == 123 # this is false! no effect whatsoever in C memory from Julia

我怎么能在Julia中与C共享结构的内存地址？

最佳答案 好吧,我想出了一个很好的出路,这并不觉得太尴尬.我基本上重载[]运算符来通过符号访问struct字段,如下所示：

thekeyboard[:forward] = 123

代码：

function offsetof(type_, member::Symbol)
  for (i, item) in enumerate(fieldnames(type_))
    if item == member
      return fieldoffset(type_, i)
    end
    #print(typeof(i))
  end
  # what to do when symbol not in type_?
  throw("$type_ has no member named $member")
end

function GetStructType(type_, member::Symbol)
  for (i, item) in enumerate(fieldnames(type_))
    if item == member
      return fieldtype(type_, i)
    end
    #print(typeof(i))
  end
  # what to do when symbol not in type_?
  throw("$type_ has no member named $member")
end

function Base.getindex(ptr::Ptr{T}, s::Symbol) where {T}
  address = UInt(ptr)
  if address == 0
    throw("Base.getindex(Ptr::{$T}) would dereference a NULL pointer")
  end
  offset = offsetof(T, s)
  fieldtype = GetStructType(T, s)
  fieldptr = Ptr{fieldtype}(address + offset)
  #log("Symbol $s $ptrtype address=$address offset=$offset fieldtype=$fieldtype ptr=$ptr fieldptr=$fieldptr\n")
  #return 123
  return unsafe_load(fieldptr)
end

function Base.setindex!(ptr::Ptr{T}, value, s::Symbol) where {T}
  address = UInt(ptr)
  if address == 0
    throw("Base.setindex!(Ptr) would write to a NULL pointer")
  end
  offset = offsetof(T, s)
  fieldtype = GetStructType(T, s)
  fieldptr = Ptr{fieldtype}(address + offset)
  #log("Symbol $s $ptrtype address=$address offset=$offset fieldtype=$fieldtype ptr=$ptr fieldptr=$fieldptr\n")
  unsafe_store!(fieldptr, value)
  return value
end
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