CFFI调用PointCloud.dll触发访问违例，Ctypes可正常运行求助

环境信息 #

• Windows 10（64位）
• Python版本：3.9（64位）、3.10（64位）

问题描述 #

实现了两种调用PointCloud.dll的方式，该DLL接收结构体缓冲区指针和缓冲区大小，返回点云数据。注：DLL写入点云缓冲区时仅参考传入的iBufSize参数，而非结构体中的nPoints变量。

CFFI实现运行时触发访问违例，退出码为-1073741819（0xC0000005），但相同逻辑的Ctypes实现可正常执行。两种实现在DLL旧版本下均能正常运行，DLL更新后对外接口未变但出现此问题。

相关代码片段 #

CFFI实现代码 #

from cffi import FFI

ffi = FFI()
lib = ffi.dlopen("PointCloud.dll")
ffi.cdef(
"""
typedef struct {
    double x;
    double y;
    double z;
} PointXYZ;

typedef struct {
    uint32_t nPoints;
    PointXYZ * pointcloud;
} PointCloud;

int GetPointCloud(void * ptrPointCloudBuf, int iBufSize);
"""
)

nPts = 5000000
buf_size = nPts * ffi.sizeof('PointXYZ')
pc_struct = ffi.new(
    "PointCloud *", {
        "nPoints": ffi.cast("uint32_t", 0),
        "pointcloud": ffi.new("PointXYZ []", nPts )
    }
)

retcode = lib.GetPointCloud(pc_struct, buf_size)

Ctypes实现代码 #

from ctypes import *

lib = cdll.LoadLibrary("PointCloud.dll")

class PointXYZ(Structure):
    _fields_ = [
        ('x', c_double),
        ('y', c_double),
        ('z', c_double)
    ]

class PointCloud(Structure):
    _fields_ = [
        ('nPoints', c_uint32),
        ('pointcloud', POINTER(PointXYZ)),
    ]

GetPointCloud = lib['GetPointCloud']
GetPointCloud.restype = c_int
GetPointCloud.argtypes = [POINTER(PointCloud), c_int]

nPts = 5000000
buf_size = nPts * sizeof(PointXYZ)
pc_struct = pointer(
            PointCloud(
                nPoints=c_uint32(0),
                pointcloud=cast((PointXYZ * nPts)(), POINTER(PointXYZ))
            )
        )

retcode = lib.GetPointCloud(pc_struct, buf_size)

DLL头文件相关内容 #

#ifndef POINTCLOUD_H
#define POINTCLOUD_H
#if defined(_MSC_VER)
    #define POINTCLOUD_DLL_EXPORT __declspec(dllexport)        
#else
    #define POINTCLOUD_DLL_EXPORT __attribute__ ((visibility ("default")))
    #define _stdcall
#endif

// Point structs
typedef struct {
    double x;
    double y;
    double z;
} PointXYZ;

struct PointCloud {
    uint32_t number_of_points;
    PointXYZ * pointcloud;
    PointCloud() {
        pointcloud= nullptr;
        number_of_points = 0;
    }
};

extern "C" POINTCLOUD_DLL_EXPORT int  GetPointCloud(void* buffer, int buffer_size);

#endif // POINTCLOUD_H

官方C/C++演示代码片段 #

struct PointXYZ
{
    double x;
    double y;
    double z;
};
struct PointCloud {
    uint32_t nPoints;
    PointXYZ * pointcloud;
    PointCloud() {
        pointcloud = nullptr;
        nPoints = 0;
    }
};

PointCloud pcloud;
int nPts = 5000000;

pcloud.pointcloud = new PointXYZ[nPts];
buf_size = sizeof(PointXYZ) * nPts;
int n = GetPointCloud(&pcloud, buf_size);   
if(n == 0)
{
    printf("Success, result = %d\n", n);    
}
else
{
    printf("Failure, result = %d\n", n);
}
/*
Do the rest with the point cloud.
*/

分析与修复方案 #

核心问题定位 #

对比两种实现与DLL定义，发现关键差异：

1. 结构体字段名不匹配：DLL头文件中PointCloud结构体的字段名为number_of_points，但CFFI的cdef中写的是nPoints，而Ctypes和官方演示代码用的是nPoints，这会导致结构体内存布局错位，DLL写入时访问错误内存地址。
2. 不排除DLL更新后调整了内存对齐规则，CFFI默认对齐方式与MSVC编译的DLL不兼容。

修复步骤 #

1. 修正CFFI结构体字段名
   对齐DLL头文件的定义，修改CFFI的cdef部分：

   typedef struct {
       uint32_t number_of_points;
       PointXYZ * pointcloud;
   } PointCloud;
   

   同时调整结构体初始化的字段名：

   pc_struct = ffi.new(
       "PointCloud *", {
           "number_of_points": ffi.cast("uint32_t", 0),
           "pointcloud": ffi.new("PointXYZ []", nPts )
       }
   )
   

2. 显式指定内存对齐（可选）
   若DLL采用MSVC默认对齐，可在CFFI结构体定义中添加对齐属性，确保内存布局一致：

   typedef struct __attribute__((aligned(8))) {
       uint32_t number_of_points;
       PointXYZ * pointcloud;
   } PointCloud;
   

3. 验证调用约定
   头文件中MSVC环境未定义_stdcall，函数默认使用cdecl调用约定，CFFI和Ctypes默认均为cdecl，无需额外调整。

最终修正后的CFFI代码 #

from cffi import FFI

ffi = FFI()
lib = ffi.dlopen("PointCloud.dll")
ffi.cdef(
"""
typedef struct {
    double x;
    double y;
    double z;
} PointXYZ;

typedef struct __attribute__((aligned(8))) {
    uint32_t number_of_points;
    PointXYZ * pointcloud;
} PointCloud;

int GetPointCloud(void * ptrPointCloudBuf, int iBufSize);
"""
)

nPts = 5000000
buf_size = nPts * ffi.sizeof('PointXYZ')
pc_struct = ffi.new(
    "PointCloud *", {
        "number_of_points": ffi.cast("uint32_t", 0),
        "pointcloud": ffi.new("PointXYZ []", nPts )
    }
)

retcode = lib.GetPointCloud(pc_struct, buf_size)

内容的提问来源于stack exchange，提问作者Bawoo