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Python调用C/C++动态链接库的方法详解

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Jun 16, 2016 am 08:43 AM

c c++ python dynamic link library transfer

本文以实例讲解了Python调用C/C++ DLL动态链接库的方法，具体示例如下：

示例一：

首先，在创建一个DLL工程(本例创建环境为VS 2005)，头文件：

//hello.h
#ifdef EXPORT_HELLO_DLL
#define HELLO_API __declspec(dllexport)
#else
#define HELLO_API __declspec(dllimport)
#endif
extern "C"
{
 HELLO_API int IntAdd(int , int);
}

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CPP文件：

//hello.cpp
#define EXPORT_HELLO_DLL
#include "hello.h"
HELLO_API int IntAdd(int a, int b)
{
 return a + b;
}

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这里有两个注意点：

（1）弄清楚编译的时候函数的调用约定采用的__cdecl还是__stdcall，因为根据DLL中函数调用约定方式，Python将使用相应的函数加载DLL。

（2）如果采用C++的工程，那么导出的接口需要extern "C"，这样python中才能识别导出的函数。

我的工程中采用__cdecl函数调用约定方式进行编译链接产生hello.dll，然后Python中采用ctypes库对hello.dll进行加载和函数调用：

from ctypes import *
dll = cdll.LoadLibrary('hello.dll');
ret = dll.IntAdd(2, 4);
print ret;

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至此，第一个小例子已经完成了，读者可以自己动手尝试一下运行效果。

示例二：

示例一只是一个"hello world"级别的程序，实际运用中更多的需要传递数据结构、字符串等，才能满足我们的需求。那么本示例将展示，如何传递数据结构参数，以及如何通过数据结构获取返回值。

首先编写DLL工程中的头文件：

//hello.h
#ifdef EXPORT_HELLO_DLL
#define HELLO_API __declspec(dllexport)
#else
#define HELLO_API __declspec(dllimport)
#endif
 
#define ARRAY_NUMBER 20
#define STR_LEN 20
 
struct StructTest
{
 int number;
 char* pChar;
 char str[STR_LEN];
 int iArray[ARRAY_NUMBER];
};
 
extern "C"
{
 //HELLO_API int IntAdd(int , int);
 HELLO_API char* GetStructInfo(struct StructTest* pStruct);
}

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CPP文件如下：

//hello.cpp
#include <string.h>
#define EXPORT_HELLO_DLL
#include "hello.h"
 
HELLO_API char* GetStructInfo(struct StructTest* pStruct)
{
 for (int i = 0; i < ARRAY_NUMBER; i++)
 pStruct->iArray[i] = i;
 pStruct->pChar = "hello python!";
 strcpy (pStruct->str, "hello world!");
 pStruct->number = 100;
 return "just OK";
}

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GetStructInfo这个函数通过传递一个StructTest类型的指针，然后对对象中的属性进行赋值，最后返回"just OK".

编写Python调用代码如下，首先在Python中继承Structure构造一个和C DLL中一致的数据结构StructTest，然后设置函数GetStructInfo的参数类型和返回值类型，最后创建一个StructTest对象，并将其转化为指针作为参数，调用函数GetStrcutInfo，最后通过输出数据结构的值来检查是否调用成功：

from ctypes import *
ARRAY_NUMBER = 20;
STR_LEN = 20;
#define type
INTARRAY20 = c_int * ARRAY_NUMBER;
CHARARRAY20 = c_char * STR_LEN;
#define struct
class StructTest(Structure):
  _fields_ = [
    ("number", c_int),
    ("pChar", c_char_p),
    ("str", CHARARRAY20),
    ("iArray", INTARRAY20)
        ]
#load dll and get the function object
dll = cdll.LoadLibrary('hello.dll');
GetStructInfo = dll.GetStructInfo;
#set the return type
GetStructInfo.restype = c_char_p;
#set the argtypes
GetStructInfo.argtypes = [POINTER(StructTest)];
objectStruct = StructTest();
#invoke api GetStructInfo
retStr = GetStructInfo(byref(objectStruct));
#check result
print "number: ", objectStruct.number;
print "pChar: ", objectStruct.pChar;
print "str: ", objectStruct.str;
for i,val in enumerate(objectStruct.iArray):
  print 'Array[i]: ', val;
print retStr;

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总结:

1. 用64位的Python去加载32位的DLL会出错
2. 以上只是些测试程序，在编写Python过程中尽可能的使用"try Except"来处理异常
3. 注意在Python与C DLL交互的时候字节对齐问题
4. ctypes库的功能还有待继续探索

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