如何将float*数组从内存分配发生在C层的python脚本传递到C方法

Question

我正试图从Python脚本中调用C方法，C方法调用inturn C++方法。我正在使用malloc((在getResults((方法内部分配数组。现在的问题是如何将参数传递给python脚本中的float*oresults，该脚本的内存分配发生在C层中。这是io.c

int getResults(char* iFilename, char* iStagename, int iStateidCnt, 
int* Stateids, int iEntityIdCount, int* iEntityids, char* iEntityType,
char* iVariablegroup, char* ivariable, int *oRescount,
float* oResults)
{
int Status, i;
EString etype(iEntityType), stagename(iStagename);
EString vargroup(iVariablegroup);
std::vector<ERF_INT> entity_ids;
std::vector<ERF_INT> stateids;
std::vector<ERF_FLOAT> results;
_CopyIntArrayIntoVector(iStateidCnt, Stateids, stateids);
_CopyIntArrayIntoVector(iEntityIdCount, iEntityids, entity_ids);
CreateIoInstance(iFilename, iStagename);
ioData pIodata = CreateIoDataInstance();
if (iEntityIdCount <= 0)
pIodata.setWholeSection(true);
else
{
pIodata.setWholeSection(false);
pIodata.setEntityList(entity_ids);
}

pIodata.setStateList(stateids);
pIodata.setType(etype);
pIodata.setVariableGroup(iVariablegroup);
pIodata.setVariable(ivariable);
//This is C++ method
Status = pIo->get_results(pIodata, results);
*oRescount = results.size();
//allocation for oresults whose size > 2
oResults = (float*)malloc(results.size() * sizeof(float));
_CopyVectorIntoDoubleArray(results, oResults);
return Status;
}

测试.py

from ctypes import *
import os, sys
dll = CDLL('D:\erf_utils_python\erf_utils_io.dll')
dll.getresults.argtypes = (c_char_p,c_char_p,c_int,POINTER(c_int),c_int,POINTER(c_int),c_char_p,
c_char_p,c_char_p,POINTER(c_int),POINTER(c_float))
dll.getresults.restype = c_int

def make_array(ctype,arr):
return len(arr),(ctype * len(arr))(*arr)
def getresults(filename,stagename,sids,eids,entitytype,groups,variables):
if(len(sids)>0):
stateidcount,stateids = make_array(c_int,sids)
if(len(eids)>0):
entityidcount,entityid = make_array(c_int,eids)
oresultlen = c_int()
float_values = POINTER(c_float)
err = dll.getresults(filename,stagename,stateidcount,stateids,entityidcount,entityid,
entitytype,groups,variables,byref(oresultlen), byref(float_values))
return err,oresultlen.value, float_values
filename = b'D:\inputfile.h5'
stagename = b"post"
stateids = [2]
stateidcount = 1
entityidcount = 1
entityid = [1]
entitytype = b"test"
variablecount = 1
variablegroup = b"testdata"
variable = b"next"
err,oreslen,ores = getresults(filename,stagename,stateids,entityid,entitytype,variablegroup,variable)

TypeError:byref((参数必须是ctypes实例，而不是"_ctypes.PyCPointerType"这是我运行剧本我对如何发送float的参数有点困惑*脚本中的结果。

Answer 1

在C++代码中，签名int getResults(..., float* oResults)无法将分配的指针传回调用者。线路

oResults = (float*)malloc(results.size() * sizeof(float));

在getResults中本地设置oResults指针，而不影响调用方。为了输出指针，必须将其return或使用指向指针参数的指针：int getResults(..., float** oResults)。

在Python代码中，我不熟悉ctypes，但看起来float_values = POINTER(c_float)是个问题。POINTER(c_float)为要浮动的指针创建一个Python类型。您可能希望POINTER(c_float)()生成这样一个指针(最初为null(的实例。

指针上的ctypes文档：https://docs.python.org/3/library/ctypes.html#pointers

Answer 2

float* oResults参数是通过值传递的，因此不可能在该参数中返回已分配的指针。相反，请使用float** oResults。

此外，float_values = POINTER(c_float)是类型，而不是类型的实例。因此CCD_ 11等价于无效的C CCD_。相反，您需要一个指针POINTER(c_float)()的实例(注意括号(，并通过引用传递，类似于Cfloat *p; func(&p)。这将按地址将指针传递给C函数，然后C函数可以将其修改为输出参数。

这里有一个简化的例子，只关注int *oRescount和float** oResults参数。还需要一个释放分配的功能：

test.cpp

#include <vector>
#define API __declspec(dllexport)
extern "C" {
API int getResults(size_t *oRescount, float** oResults) {
std::vector<float> results {1.25,2.5,3.75,5.0}; // Simulated results
*oRescount = results.size(); // Return size of results
auto tmp = new float[results.size()]; // allocate
for(size_t i = 0; i < results.size(); ++i) // copy vector to allocation
tmp[i] = results[i];
*oResults = tmp; // return allocation
return 0;
}
API void freeResults(float* oResults) {
delete [] oResults;
}
}

测试.py

from ctypes import *
dll = CDLL('./test')
dll.getResults.argtypes = POINTER(c_size_t),POINTER(POINTER(c_float))
dll.getResults.restype = c_int
def getresults():
oRescount = c_size_t()         # instance to hold the returned size
oResults = POINTER(c_float)()  # instance of a float* to hold the returned allocation.
err = dll.getResults(byref(oRescount), byref(oResults))
# oResults is a float* and it is possible to index past the end.
# Make a copy into a Python list slicing to the correct size,
# then free it so there is no memory leak.
results = oResults[:oRescount.value]
dll.freeResults(oResults)
return err,results
err,ores = getresults()
print(err,ores)

输出：

0 [1.25, 2.5, 3.75, 5.0]