Question

对ctypes不太了解，最近刚开始使用它。

我在C-like dll中有一个简单的函数，它返回一个指向动态生成的字符串的指针它工作正常，但是，因为我手动为字符串分配内存，我应该在使用后释放它。

我有这样的事情：

extern "C" char* DLL_EXPORT func(const char* str1, const char* str2)
{
    return getSomeString(str1, str2);
}

// Goal is to call this function correctly from Python.    
extern "C" void DLL_EXPORT freeMem(void *mem)
    {
        if(mem!=NULL)
            delete mem;
    }

但我不知道，我怎样才能将收到的指针传回Python中删除？

Answer 1

你走在正确的轨道上。

// TestDLL.cpp
#include <string.h> // strcpy

extern "C" __declspec(dllexport) char* stringdup(const char* str) {
    char* p = new char[strlen(str)+1];
    strcpy(p,str);
    return p;
}

// if you have no good reason to use void*, use the type
// you've allocated. while it usually works for built-in
// types, it wouldn't work for classes (it wouldn't call
// the destructor)
extern "C" __declspec(dllexport) void stringfree(char* ptr) {
    // you don't need to check for 0 before you delete it,
    // but if you allocate with new[], free with delete[] !
    delete [] ptr; 
}

在python中：

# Test.py
import ctypes

lib = ctypes.cdll.TestDLL

# this creates a c-style char pointer, initialized with a string whose
# memory is managed by PYTHON! do not attempt to free it through the DLL!
cstr = ctypes.c_char_p("hello ctypes")

# call the dll function that returns a char pointer 
# whose memory is managed by the DLL.
p = lib.stringdup(cstr)

# p is just an integer containing the memory address of the 
# char array. therefore, this just prints the address:
print p

# this prints the actual string
print ctypes.c_char_p(p).value

# free the memory through the DLL
lib.stringfree(p)

Answer 2

通常，您在ctypes中使用的每个函数都应该声明其参数和返回类型，以便Python可以检查正确的参数数量和类型，并将Python对象参数转换为正确的C数据对象。不幸的是，在这种情况下，func的正常返回值为c_char_p，但ctypes会尝试提供帮助并将c_char_p返回值转换为Python字符串，从而失去访问权限到原始C指针值。相反，您可以将返回类型声明为c_void_p并使用cast来检索字符串值，从而使返回值保留为c_char_p对象。

这是一个例子（Python 3）：

import ctypes

func = ctypes.cdll.TestDLL.func
func.argtypes = [ctypes.c_char_p,ctypes.c_char_p]
func.restype = ctypes.c_void_p

freeMem = ctypes.cdll.TestDLL.freeMem
freeMem.argtypes = [ctypes.c_void_p]
freeMem.restype = None

s = func(b'abcdef',b'ghijkl')
s = ctypes.cast(s,ctypes.c_char_p)
print(s.value)
freeMem(s)

Answer 3

对我来说

#cdll.execute.restype = ctypes.c_char_p               # failed to free
cdll.execute.restype = ctypes.POINTER(ctypes.c_char)  # worked

p = cdll.execute('...')
print(
    ctypes.cast(p, ctypes.c_char_p).value.decode('utf-8')
)

cdll.free_pointer(p)

如何在ctypes中传回指针？

3 个答案: