python - 如何在 python 中处理输出参数,如 char** 数组?
问题描述
我正在尝试从 Python 脚本调用 C 方法,但是在调用将 char** 数组作为参数输出的方法时遇到了问题。C层的方法如下。helper.c
文件:
//This method takes filename as input and oNames as output
extern C int GetNames(char* iFilename, char** oNames)
{
int oNumNames, oStatus;
/*io* pIo = GetIoInstance();*/
std::vector<EString> names;
CreateIoInstance(iFilename);
oStatus = pIo->get_names(names);
oNumNames = (int)names.size();
for (int ii = 0; ii < oNumNames; ii++)
{
strcpy(oNames[ii], names[ii].c_str());
}
return 0;
}
请帮助我从 python 脚本中调用此方法。
from ctypes import *
dll = CDLL('D:\\python\\working.dll')
dll.GetNames = dll.GetNames
dll.GetNames.argtypes = (c_char_p, POINTER(c_char_p))
dll.GetStageNames.restype = c_int
filename = "in.h5"
def GetNames(filename):
ostagenames = POINTER(c_char_p)
err = dll.GetStageNames(filename, ostagenames)
return err, ostagenames.value
解决方案
我已经简化了函数以专注于char**
参数。在编写示例函数时,它假定内存是预先分配的,但是没有接口来指示用于输出的数组的大小或数组中的各个字符串。在这个例子中,内存是为一个 3 元素数组预先分配的,每个字符串最多 20 个字符,但实际上用户必须预先分配足够长的字符串来保存实际的返回值-生活状况。
测试.cpp:
#define API __declspec(dllexport)
#include <vector>
#include <string>
#include <cstring>
using namespace std;
//This method takes filename as input and oNames as output
extern "C" API void GetNames(char** oNames)
{
vector<string> names { "one", "two", "three" };
for (size_t i = 0; i < names.size(); ++i) {
strcpy(oNames[i], names[i].c_str());
}
}
测试.py:
from ctypes import *
dll = CDLL('./test')
dll.GetNames.argtypes = POINTER(c_char_p),
dll.GetNames.restype = None
def get_names():
ARR3 = c_char_p * 3 # equivalent to char*[3] type in C
# list of 3 mutable pointers to buffers
buffers = [cast(create_string_buffer(20),c_char_p) for _ in range(3)]
names = ARR3(*buffers) # array initalized with buffers
dll.GetNames(names)
return list(names)
print(get_names())
输出:
[b'one', b'two', b'three']
如果您可以随意更改 API,动态分配内存以便稍后释放它会使其更加灵活。这对于 char*** 来说相当难看,但它可以工作:
测试.cpp:
#define API __declspec(dllexport)
#include <vector>
#include <string>
#include <cstring>
using namespace std;
extern "C" {
API void GetNames(char*** oNames) {
vector<string> names { "one", "two", "three" };
auto arr = new char*[names.size() + 1];
for (size_t i = 0; i < names.size(); ++i) {
auto len = names[i].size() + 1;
arr[i] = new char[len];
strcpy_s(arr[i], len, names[i].c_str());
}
arr[names.size()] = nullptr;
*oNames = arr;
}
API void FreeNames(char** names) {
if(names) {
for(size_t i = 0; names[i]; ++i)
delete [] names[i];
delete [] names;
}
}
}
测试.py:
from ctypes import *
# ctypes.c_char_p has special handling for strings,
# but hides the pointer value. Deriving a type
# from c_char_p prevents this special handling
# and allows access to the pointer, so we can later
# free it.
class PCHAR(c_char_p):
pass
dll = CDLL('./test')
dll.GetNames.argtypes = POINTER(POINTER(PCHAR)),
dll.GetNames.restype = None
dll.FreeNames.argtypes = POINTER(PCHAR),
dll.FreeNames.restype = None
def get_names():
pnames = POINTER(PCHAR)() # allocate char**
dll.GetNames(byref(pnames)) # pass char***
i = 0
names = []
while pnames[i]: # returned char* array is terminated with null
names.append(pnames[i].value) # create and save the Python byte string
i += 1
dll.FreeNames(pnames)
return names
print(get_names())
输出:
[b'one', b'two', b'three']
推荐阅读
- python-3.x - 熊猫数据框根据条件更新值
- python - 在 LightGBM 中使用“predict_contrib”获取 SHAP 值
- flutter - 如何解决在 Flutter 上在 Iphone 11 上运行 pod install 的错误
- html - 使用 JQuery 在文本节点后添加 span/html 元素
- c# - 忽略(跳过)一个 NUnit 测试用例而不抛出异常
- excel - VBA 提取父文件夹名称
- sql-server - 联合子查询左连接 - 太慢了
- php - 如何在单引号和双引号中转义变量?PHP
- json - JSON - PL/SQL ORA-30625:不允许对 NULL SELF 参数进行方法调度
- python - 调试:二叉树的右视图