我正在尝试设计一个可以在Python中轻松扩展的C接口(使用ctypes)。我在 C 中使用了自然习语:
struct format {
int (*can_open)(const char *filename);
struct format * (*open)(const char *filename);
void (*delete)(struct format *self);
int (*read)(struct format *self, char *buf, size_t len);
};
如果我想直接从 C 扩展此接口,它工作得很好:
struct derived /* concrete implementation */
{
struct format base;
};
但我真正想做的是使用 ctypes 从 Python 实现这个接口。这是我到目前为止所拥有的:
CANOPENFUNC = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_char_p)
#OPENFUNC = ctypes.CFUNCTYPE(ctypes.c_void_p, ctypes.c_char_p)
#OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER( python_format ), ctypes.c_char_p)
#DELETEFUNC = ctypes.CFUNCTYPE(None, ctypes.c_void_p)
#READFUNC = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p)
def py_canopen_func( string ):
print "py_canopen_func", string
return 1
canopen_func = CANOPENFUNC(py_canopen_func)
#open_func = OPENFUNC( py_open_func)
#delete_func = DELETEFUNC(py_canopen_func)
#read_func = READFUNC(py_canopen_func)
class python_format(ctypes.Structure):
_fields_ = (
('can_open', CANOPENFUNC),
('open', OPENFUNC),
('delete', DELETEFUNC),
('read', READFUNC),
)
def __init__(self):
self.can_open = canopen_func
OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER(python_format), ctypes.c_char_p)
def py_open_func2( string ):
print "py_open_func2", string
return ctypes.byref(self)
self.open = OPENFUNC( py_open_func2 )
#self.delete = delete_func
#self.read = read_func
真的,我正在努力定义OPENFUNC
的原型。从技术上讲,它应该是:
OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER(python_format), ctypes.c_char_p)
但是我需要先定义python_format
,这反过来又需要对OPENFUNC
进行定义。
奖励点:实际的功能实现是什么?例如:
def func( str ): return None
或
def func( str ): i = python_format(); return ctypes.pointer(i)
两者都给了我:
class python_format(ctypes.Structure):
pass
OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER( python_format ), ctypes.c_char_p)
OPENFUNC( func )
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: invalid result type for callback function
这是否与另一个问题有关?如果是这样,我应该更改我最初的 C 设计,因为我将无法从回调返回指向python_format
实例的指针?
在ctypes.Structure._fields_
文档中,它解释了如何执行此操作:
可以在类之后定义
_fields_
类变量 定义结构子类的语句,这允许创建 直接或间接引用自身的数据类型
这意味着您可以添加:
class python_format(ctypes.Structure): # forward declaration
pass
然后在定义OPENFUNC
(和其他函数类型)之后:
OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER( python_format ), ctypes.c_char_p)
DELETEFUNC = etc...
然后能够这样定义python_format._fields_
:
python_format._fields_ = (
('can_open', CANOPENFUNC),
('open', OPENFUNC),
('delete', DELETEFUNC),
('read', READFUNC),
)
下面是基于您的代码的更完整示例:
import ctypes
class python_format(ctypes.Structure): # forward declaration
pass
CANOPENFUNC = ctypes.PYFUNCTYPE(ctypes.c_int, ctypes.c_char_p)
OPENFUNC = ctypes.PYFUNCTYPE(ctypes.c_int,
ctypes.POINTER(python_format),
ctypes.c_char_p)
DELETEFUNC = ctypes.PYFUNCTYPE(None, ctypes.c_void_p)
READFUNC = ctypes.PYFUNCTYPE(ctypes.c_int, ctypes.c_void_p)
def py_canopen_func(self, string):
print "py_canopen_func", string
return 1
def py_open_func(self, string):
print "py_open_func2", string
# Return types from callbacks cannot be anything other than simple
# datatypes (c_int, c_float, ..., c_void_p). For other datatypes
# (STRUCTURE, POINTER, ...), ctypes returns the following error
# "Invalid result type for callback function"
# see http://bugs.python.org/issue5710
return 1 # can't return ctypes.byref(self)
canopen_func = CANOPENFUNC(py_canopen_func)
open_func = OPENFUNC(py_open_func)
#delete_func = DELETEFUNC(py_canopen_func)
#read_func = READFUNC(py_canopen_func)
class python_format(ctypes.Structure):
python_format._fields_ = (
('can_open', CANOPENFUNC),
('open', OPENFUNC),
('delete', DELETEFUNC),
('read', READFUNC),
)
def __init__(self):
self.can_open = canopen_func
self.open = open_func
#self.delete = delete_func
#self.read = read_func
pf = python_format()
为了得到一个规范的答案,我将回答我自己的问题,感谢@eryksun指导。
所以首先,虽然从文档中不清楚,但不能从回调函数返回复杂类型。因此,不能映射 C 函数指针:
struct format {
struct format * (*open)(const char *filename);
};
自
class python_format:
pass
OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER(python_format), ctypes.c_char_p)
def py_canopen_func( string ):
return None
open_func = OPENFUNC(py_open_func)
上面的代码将优雅地编译,但在运行时,得到:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: invalid result type for callback function
长答案是:
尝试将非简单类型用作 回调的结果没有多大帮助。回调的结果 类型必须在其 StgDictObject 中有一个 setfunc(一个 ctypes 扩展 常规的 PyDictObject)。此要求限制您使用 简单的类型,如c_void_p[...]
因此,截至今天,在问题 5710 得到解决之前,这里唯一的解决方案如下:
class python_format(ctypes.Structure):
__self_ref = []
def __init__(self):
self.open = self.get_open_func()
# technically should be a @classmethod but since we are self-referencing
# ourself, this is just a normal method:
def get_open_func(self):
def py_open_func( string ):
python_format.__self_ref.append( self )
return ctypes.addressof(self)
return OPENFUNC( py_open_func )
OPENFUNC = ctypes.CFUNCTYPE(ctypes.c_void_p, ctypes.c_char_p)
# delay init required because `read_info` requires a forward declaration:
python_format._fields_ = (
('open', OPENFUNC),
)