使用ctypes访问变量数组的内容 [英] Accessing the content of a variable array with ctypes

问题描述

我使用ctypes访问python中读取C函数的文件.由于读取的数据巨大且大小未知，因此我在C中使用**float.

I use ctypes to access a file reading C function in python. As the read data is huge and unknown in size I use **float in C .

int read_file(const char *file,int *n_,int *m_,float **data_) {...}

函数mallocs具有适当大小(这里为n和m)的2d数组(称为data)，并将值复制到引用的值中.请参见以下代码段:

The functions mallocs an 2d array, called data, of the appropriate size, here n and m, and copies the values to the referenced ones. See following snippet:

*data_ = data;
*n_ = n;
*m_ = m;

我使用以下python代码访问此功能:

I access this function with the following python code:

p_data=POINTER(c_float)
n=c_int(0)
m=c_int(0)
filename='datasets/usps'
read_file(filename,byref(n),byref(m),byref(p_data))

然后，我尝试使用contents访问p_data，但是我只得到一个浮点值.

Afterwards I try to acces p_data using contents, but I get only a single float value.

p_data.contents
c_float(-1.0)

我的问题是:如何在python中访问data?

My question is: How can I access data in python?

您推荐什么?如果我有不清楚的地方，请不要犹豫指出！

What do you recommended? Please don't hesitate to point out if I left something unclear!

推荐答案

使用struct库在python中完成整个操作可能会更简单.但是如果您以ctypes出售(我不怪你，那很酷):

might be simpler to do the whole thing in python with the struct library. but if you're sold on ctypes (and I don't blame you, it's pretty cool):

#include <malloc.h>
void floatarr(int* n, float** f)
{
    int i;
    float* f2 = malloc(sizeof(float)*10);
    n[0] = 10;
    for (i=0;i<10;i++)
    { f2[i] = (i+1)/2.0; }
    f[0] = f2;
}

然后在python中

from ctypes import *

fd = cdll.LoadLibrary('float.dll')
fd.floatarr.argtypes = [POINTER(c_int),POINTER(POINTER(c_float))]

fpp = POINTER(c_float)()
ip = c_int(0)
fd.floatarr(pointer(ip),pointer(fpp))
print ip
print fpp[0]
print fpp[1]

诀窍是大写字母POINTER创建类型，小写字母pointer创建指向现有存储的指针.您可以使用byref而不是pointer，他们声称这样做更快.我更喜欢pointer，因为这样可以更清楚地了解发生了什么.

the trick is that capitals POINTER makes a type and lowercase pointer makes a pointer to existing storage. you can use byref instead of pointer, they claim it's faster. I like pointer better because it's clearer what's happening.

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