Python ctypes 指向结构体的指针的指针

Question

我在使用指向指针的指针来处理一个结构体时遇到了问题。我的代码出现了一个错误，提示“ArgumentError: argument 1: : 需要一个 LP_LP_List 实例，而不是指向 LP_LP_List 的指针”。

class List(Structure):
    _fields_ = (
        ('head', POINTER(Node)),
        ('tail', POINTER(Node)),
        ('current', POINTER(Node)),
        ('saved', POINTER(Node)),
        ('infosize', c_int),
        ('listsize', c_ulong),
        ('current_index', c_ulong),
        ('save_index', c_ulong),
        ('modified', c_bool),
        ('search_origin', c_int),
        ('search_dir', c_int),
        )

list_p = POINTER(List)

create = lib.DLL_CreateList
create.argtypes = [POINTER(POINTER(List)),]
create.restype = POINTER(List)
mem = POINTER(list_p)()
retval = create(byref(mem))

这看起来是按照推荐的方法来做的，但就是不管用。

谢谢大家的帮助。

对于那些不想阅读下面所有细节来找到解决方案的人，最后的部分应该是这样的：

#list_p = POINTER(List)  # Not needed

create = lib.DLL_CreateList
create.argtypes = [POINTER(POINTER(List)),]
create.restype = POINTER(List)
control = POINTER(List)()
list_p = create(byref(control))

Answer 1

这个API听起来像是传入的指针会被create函数修改，所以必须通过引用传递，同时返回值是新创建的列表。

如果用"C"语言来写，我猜代码可能是这样的：

List *create(List **control);
List *control;
List *plist;
plist = create(&control);

在ctypes/Python中，这样的代码可以映射为：

create.argtypes = [POINTER(POINTER(List)),]
create.restype = POINTER(List)
control = POINTER(List)()
newlist = create(byref(control))

这样做是否更好呢？

编辑：因为create函数既修改了传入的参数，又返回了创建的列表，所以我建议不考虑返回值，直接保留参数：

create.argtypes = [POINTER(POINTER(List)),]
control = Pointer(List)()
create(byref(control))

为了让你的Python API更完整，你可以考虑写一个上下文管理器，这样可以在上下文管理器的__enter__方法被with语句调用时初始化列表控制块，并在with语句结束时自动调用Destroy。这样你的Python代码可能看起来像：

with listManager as ListManager():
    # do stuff with list managed by list manager

# end of with statement, listManager.__exit__ is automatically called, so that Destroy always
# gets called, even if an exception is raised - this is easier for API users than expecting them
# to write their own try-finally code, and put Destroy in the finally block

编辑：- 我在Ubuntu上构建了你的DLL，并使用了上面的Python代码，结果似乎确实创建了一个列表。

~/dev/python$ python
Python 2.6.6 (r266:84292, Sep 15 2010, 16:22:56) 
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from ctypes import *
>>> 
>>> class List(Structure): pass
... 
>>> lib = CDLL('./libdll.so')
>>> create = lib.DLL_CreateList
>>> create.argtypes = [POINTER(POINTER(List)),] 
>>> create.restype = POINTER(List) 
>>> 
>>> control = POINTER(List)() 
>>> create(byref(control))
<__main__.LP_List object at 0x7fdc0c607e60>

编辑 - 我写了一个pyparsing工具来读取C头文件，并输出占位符结构子类和函数参数类型及返回类型的定义。这是我为你的库得到的结果 - 虽然没有经过测试，但可能会帮助你在API测试中起步：

from ctypes import *
ll = CDLL("./libdll.so")

# user defined types
class ll_Info(Structure): pass
class ll_DLL_Boolean(Structure): pass
class ll_DLL_SrchOrigin(Structure): pass
class sys_select_fd_set(Structure): pass
class ll_List(Structure): pass
class sys_time_timeval(Structure): pass
class ll_DLL_SrchDir(Structure): pass
class ll_DLL_Return(Structure): pass
class ll_DLL_SearchModes(Structure): pass
class ll_DLL_InsertDir(Structure): pass

# functions
ll.DLL_CreateList.restype = POINTER(ll_List)
ll.DLL_CreateList.argtypes = (POINTER(POINTER(ll_List)),)
ll.DLL_DestroyList.restype = None
ll.DLL_DestroyList.argtypes = (POINTER(POINTER(ll_List)),)
ll.DLL_Version.restype = c_char_p
ll.DLL_Version.argtypes = ()
ll.DLL_IsListEmpty.restype = ll_DLL_Boolean
ll.DLL_IsListEmpty.argtypes = (POINTER(ll_List),)
ll.DLL_IsListFull.restype = ll_DLL_Boolean
ll.DLL_IsListFull.argtypes = (POINTER(ll_List),)
ll.DLL_CurrentPointerToHead.restype = ll_DLL_Return
ll.DLL_CurrentPointerToHead.argtypes = (POINTER(ll_List),)
ll.DLL_CurrentPointerToTail.restype = ll_DLL_Return
ll.DLL_CurrentPointerToTail.argtypes = (POINTER(ll_List),)
ll.DLL_DecrementCurrentPointer.restype = ll_DLL_Return
ll.DLL_DecrementCurrentPointer.argtypes = (POINTER(ll_List),)
ll.DLL_DeleteCurrentRecord.restype = ll_DLL_Return
ll.DLL_DeleteCurrentRecord.argtypes = (POINTER(ll_List),)
ll.DLL_DeleteEntireList.restype = ll_DLL_Return
ll.DLL_DeleteEntireList.argtypes = (POINTER(ll_List),)
ll.DLL_FindNthRecord.restype = ll_DLL_Return
ll.DLL_FindNthRecord.argtypes = (POINTER(ll_List),POINTER(ll_Info),c_ulong,)
ll.DLL_GetCurrentRecord.restype = ll_DLL_Return
ll.DLL_GetCurrentRecord.argtypes = (POINTER(ll_List),POINTER(ll_Info),)
ll.DLL_GetNextRecord.restype = ll_DLL_Return
ll.DLL_GetNextRecord.argtypes = (POINTER(ll_List),POINTER(ll_Info),)
ll.DLL_GetPriorRecord.restype = ll_DLL_Return
ll.DLL_GetPriorRecord.argtypes = (POINTER(ll_List),POINTER(ll_Info),)
ll.DLL_InitializeList.restype = ll_DLL_Return
ll.DLL_InitializeList.argtypes = (POINTER(ll_List),c_size_t,)
ll.DLL_IncrementCurrentPointer.restype = ll_DLL_Return
ll.DLL_IncrementCurrentPointer.argtypes = (POINTER(ll_List),)
ll.DLL_InsertRecord.restype = ll_DLL_Return
ll.DLL_InsertRecord.argtypes = (POINTER(ll_List),POINTER(ll_Info),ll_DLL_InsertDir,)
ll.DLL_RestoreCurrentPointer.restype = ll_DLL_Return
ll.DLL_RestoreCurrentPointer.argtypes = (POINTER(ll_List),)
ll.DLL_SaveList.restype = ll_DLL_Return
ll.DLL_SaveList.argtypes = (POINTER(ll_List),c_char_p,)
ll.DLL_SetSearchModes.restype = ll_DLL_Return
ll.DLL_SetSearchModes.argtypes = (POINTER(ll_List),ll_DLL_SrchOrigin,ll_DLL_SrchDir,)
ll.DLL_StoreCurrentPointer.restype = ll_DLL_Return
ll.DLL_StoreCurrentPointer.argtypes = (POINTER(ll_List),)
ll.DLL_SwapRecord.restype = ll_DLL_Return
ll.DLL_SwapRecord.argtypes = (POINTER(ll_List),ll_DLL_InsertDir,)
ll.DLL_UpdateCurrentRecord.restype = ll_DLL_Return
ll.DLL_UpdateCurrentRecord.argtypes = (POINTER(ll_List),POINTER(ll_Info),)
ll.DLL_GetSearchModes.restype = POINTER(ll_DLL_SearchModes)
ll.DLL_GetSearchModes.argtypes = (POINTER(ll_List),POINTER(ll_DLL_SearchModes),)
ll.DLL_GetCurrentIndex.restype = c_ulong
ll.DLL_GetCurrentIndex.argtypes = (POINTER(ll_List),)
ll.DLL_GetNumberOfRecords.restype = c_ulong
ll.DLL_GetNumberOfRecords.argtypes = (POINTER(ll_List),)