如何将C数组的指针转换为python数组 [英] How to convert pointer to c array to python array
问题描述
我有一个C ++回调函数,可使用ctypes调用Python。此函数的参数是一个指向double和元素数量数组的指针。
I have a C++ callback function that calls into Python using ctypes. This function's parameters are a pointer to an array of double and the number of elements.
元素很多,大约2,000,000。我需要将其发送到scipy函数中。
There are a lot of elements, approximately 2,000,000. I need to send this into scipy functions.
C ++原型为:
bool (*ptsetDataSource)(double*, long long);
这是以下python代码:
which is the following python code:
CPF_setDataSource = CFUNCTYPE(c_bool, POINTER(c_double),c_longlong)
CPF_setSelection= CFUNCTYPE(c_bool,c_char_p, c_longlong,c_longlong)
CPF_ResetSequence = CFUNCTYPE(c_bool)
def setDataSource(Data, DataLength):
Datalist=[0.0]*100
for i in range(0,100):
Datalist[i]=Data[i]
print Datalist
return True
问题是打印数据列表返回:
The problem is that print datalist returns:
[0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
这是不正确的(在c ++端进行检查时,数据填充了很多其他数字。
which is not correct(data is filled with a lot of other numbers when checked on the c++ side.
此外,如果我使用此代码将数据转换为python列表,则会在分配步骤锁定计算机。
Also, if I use this code to convert the data to a python list, it locks up the computer at the allocate step.
是否有从C ++数组加载数据,然后将其转换为适合scipy的数组?
Is there anyway to load the data from the C++ array and then convert it to an array fit for scipy?
推荐答案
如果 Data
是(c_double * DataLength.value)
数组,则您可以:
If Data
were (c_double*DataLength.value)
array then you could:
a = np.frombuffer(Data) # no copy. Changes in `a` are reflected in `Data`
如果 Data
是 POINTER(c_double)
,您可以使用 numpy.fromiter()
。与您的问题相同,但循环更快:
If Data
is a POINTER(c_double)
you could get numpy array using numpy.fromiter()
. It is the same loop as in your question but faster:
a = np.fromiter(Data, dtype=np.float, count=DataLength.value) # copy
要从 POINTER创建一个numpy数组(c_double)
实例而不进行复制,则可以使用 .from_address ()
方法:
To create a numpy array from POINTER(c_double)
instance without copying you could use .from_address()
method:
ArrayType = ctypes.c_double*DataLength.value
addr = ctypes.addressof(Data.contents)
a = np.frombuffer(ArrayType.from_address(addr))
或
array_pointer = ctypes.cast(Data, ctypes.POINTER(ArrayType))
a = np.frombuffer(array_pointer.contents)
两种方法都转换 POINTER( c_double)
实例传递到(c_double * DataLength)
实例,然后再将其传递给 numpy.frombuffer()
。
Both methods convert POINTER(c_double)
instance to (c_double*DataLength)
before passing it to numpy.frombuffer()
.
是否仍然可以从C ++数组加载数据,然后将其转换为适合scipy的数组?
Is there anyway to load the data from the C++ array and then convert it to an array fit for scipy?
以下是Python的C扩展模块(用Cython编写),提供了C API转换功能:
Here's C extension module for Python (written in Cython) that provide as C API the conversion function:
cimport numpy as np
np.import_array() # initialize C API to call PyArray_SimpleNewFromData
cdef public api tonumpyarray(double* data, long long size) with gil:
if not (data and size >= 0): raise ValueError
cdef np.npy_intp dims = size
#NOTE: it doesn't take ownership of `data`. You must free `data` yourself
return np.PyArray_SimpleNewFromData(1, &dims, np.NPY_DOUBLE, <void*>data)
它可以与 ctypes
一起使用,如下所示:
It could be used with ctypes
as follows:
from ctypes import (PYFUNCTYPE, py_object, POINTER, c_double, c_longlong,
pydll, CFUNCTYPE, c_bool, cdll)
import pointer2ndarray
tonumpyarray = PYFUNCTYPE(py_object, POINTER(c_double), c_longlong)(
("tonumpyarray", pydll.LoadLibrary(pointer2ndarray.__file__)))
@CFUNCTYPE(c_bool, POINTER(c_double), c_longlong)
def callback(data, size):
a = tonumpyarray(data, size)
# call scipy functions on the `a` array here
return True
cpplib = cdll.LoadLibrary("call_callback.so") # your C++ lib goes here
cpplib.call_callback(callback)
其中 call_callback
是: void call_callback(bool(*)(double *,long long))
。
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