Python和Ctypes:将结构作为指针传递给函数以获取数据

Question

我已经看过其他答案,但似乎无法让这个工作.我正在尝试调用DLL中的函数来与SMBus设备进行通信.该函数接受一个结构的指针,该结构有一个数组作为其中一个字段.所以...

在C:

typedef struct _SMB_REQUEST
{
    unsigned char Address;
    unsigned char Command;
    unsigned char BlockLength;
    unsigned char Data[SMB_MAX_DATA_SIZE];
} SMB_REQUEST;

我想我必须在DLL填充数据数组时设置Address,Command和BlockLength的值.需要此结构的函数将其作为指针

SMBUS_API int SmBusReadByte( SMBUS_HANDLE handle, SMB_REQUEST *request );

所以我在Python中设置了这样的结构:

class SMB_REQUEST(ctypes.Structure):
    _fields_ = [("Address", c_char),
            ("Command", c_char),
            ("BlockLength", c_char),
            ("Data", type(create_string_buffer(SMB_MAX_DATA_SIZE))]

*注意:我也尝试过数据类型的ctypes.c_char*SMB_MAX_DATA_SIZE*

要将指向此类型结构的指针传递给函数,我首先尝试将其初始化,如下所示:

data = create_string_buffer(SMB_MAX_DATA_SIZE)
smb_request = SMB_REQUEST('\x53', \x00', 1, data)

这回应:

TypeError: expected string or Unicode object, c_char_Array_32 found

如果我尝试省略数据数组,如下所示:

smb_request = SMB_REQUEST('\x53', \x00', 1)

不,错误.但是,当我尝试将此传递给函数时:

int_response =  smbus_read_byte(smbus_handle, smb_request))

我明白了:

ArgumentError: argument 2: <type 'exceptions.TypeError'>: expected LP_SMB_REQUES
T instance instead of SMB_REQUEST

我已经尝试将它作为指针传递:

int_response =  smbus_read_byte(smbus_handle, ctypes.POINTER(smb_request))

我得到:

----> 1
      2
      3
      4
      5

TypeError: must be a ctypes type

这是我如何设置艺术类型:

smbus_read_byte.argtypes = (ctypes.c_void_p, ctypes.POINTER(SMB_REQUEST))

我已经尝试过铸造,但仍然没有去.任何人都可以为我阐明这一点吗？

更新:

如果我首先初始化结构如下:

smb_request = SMB_REQUEST('\xA6', '\x00', chr(1), 'a test string')

然后通过参考低音:

int_response =  smbus_receive_byte(smbus_handle, ctypes.byref(smb_request))

我没有错.但是,当函数返回"0"表示成功而非零表示失败时,函数返回-1.检查smb_request.Data的值会返回'测试字符串',因此不会有任何更改.任何有关可能会发生什么的建议都将不胜感激.

谢谢

更新:

由于我已经得到了一些关于我的手柄是否正确的询问,这就是我如何使用它.DLL的头文件声明如下:

typedef void *SMBUS_HANDLE;

//
// This function call initializes the SMBus, opens the driver and 
// allocates the resources associated with the SMBus.
// All SMBus API calls are valid 
// after making this call except to re-open the SMBus.
//
SMBUS_API SMBUS_HANDLE OpenSmbus(void);

所以这就是我在python中这样做的方式:

smbus_handle = c_void_p() # NOTE: I have also tried it without this line but same result

open_smbus = CDLL('smbus.dll').OpenSmbus
smbus_handle =  open_smbus()
print 'SMBUS_API SMBUS_HANDLE OpenSmbus(void): ' + str(smbus_handle)

我在调用smbus_read_byte()之前调用它.我试图设置,open_smbus.restype = c_void_p()但我收到一个错误:TypeError: restype must be a type, a callable, or None

Answer 1

这是一个有效的例子.看起来你将错误的类型传递给函数.

测试DLL代码(Windows上的"cl/W4/LD xc")

#include <stdio.h>

#define SMBUS_API __declspec(dllexport)
#define SMB_MAX_DATA_SIZE 5

typedef void* SMBUS_HANDLE;

typedef struct _SMB_REQUEST
{
    unsigned char Address;
    unsigned char Command;
    unsigned char BlockLength;
    unsigned char Data[SMB_MAX_DATA_SIZE];
} SMB_REQUEST;

SMBUS_API int SmBusReadByte(SMBUS_HANDLE handle,SMB_REQUEST *request)
{
    unsigned char i;
    for(i = 0; i < request->BlockLength; i++)
        request->Data[i] = i;
    return request->BlockLength;
}

SMBUS_API SMBUS_HANDLE OpenSmbus(void)
{
    return (void*)0x12345678;
}

Python代码

from ctypes import *
SMB_MAX_DATA_SIZE = 5
ARRAY5 = c_ubyte * SMB_MAX_DATA_SIZE

class SMB_REQUEST(Structure):
    _fields_ = [
        ("Address", c_ubyte),
        ("Command", c_ubyte),
        ("BlockLength", c_ubyte),
        ("Data", ARRAY5)]

smbus_read_byte = CDLL('x').SmBusReadByte
smbus_read_byte.argtypes = [c_void_p,POINTER(SMB_REQUEST)]
smbus_read_byte.restype = c_int
open_smbus = CDLL('x').OpenSmbus
open_smbus.argtypes = []
open_smbus.restype = c_void_p

handle = open_smbus()
print 'handle = %08Xh' % handle

smb_request = SMB_REQUEST(1,2,5)

print 'returned =',smbus_read_byte(handle,byref(smb_request))
print 'Address =',smb_request.Address
print 'Command =',smb_request.Command
print 'BlockLength =',smb_request.BlockLength
for i,b in enumerate(smb_request.Data):
    print 'Data[%d] = %02Xh' % (i,b)

产量

handle = 12345678h
returned = 5
Address = 1
Command = 2
BlockLength = 5
Data[0] = 00h
Data[1] = 01h
Data[2] = 02h
Data[3] = 03h
Data[4] = 04h

Answer 2

你快到了.您应该使用c_char * SMB_MAX_DATA_SIZE作为定义的类型Data.这适用于Mac OS X:

共享库:

$ cat test.c
#include <stdio.h>

#define SMB_MAX_DATA_SIZE 16

typedef struct _SMB_REQUEST
{
  unsigned char Address;
  unsigned char Command;
  unsigned char BlockLength;
  unsigned char Data[SMB_MAX_DATA_SIZE];
} SMB_REQUEST;

int SmBusReadByte(void *handle, SMB_REQUEST *request)
{
  printf("SmBusReadByte: handle=%p request=[%d %d %d %s]\n", handle, 
      request->Address, request->Command, request->BlockLength, request->Data);
  return 13;
}

$ gcc test.c -fPIC -shared -o libtest.dylib

Python驱动程序:

$ cat test.py
import ctypes

SMB_MAX_DATA_SIZE = 16

class SMB_REQUEST(ctypes.Structure):
    _fields_ = [("Address", ctypes.c_ubyte),
                ("Command", ctypes.c_ubyte),
                ("BlockLength", ctypes.c_ubyte),
                ("Data", ctypes.c_char * SMB_MAX_DATA_SIZE)]

libtest = ctypes.cdll.LoadLibrary('libtest.dylib')

req = SMB_REQUEST(1, 2, 3, 'test')

result = libtest.SmBusReadByte(ctypes.c_voidp(0x12345678), ctypes.byref(req))

print 'result: %d' % result

$ python test.py
SmBusReadByte: handle=0x12345678 request=[1 2 3 test]
result: 13

UPDATE

您遇到问题是因为您需要将结果类型设置open_smbus为void*.默认情况下,ctypes假定函数返回ints.你需要这样说:

open_smbus.restype = ctypes.c_void_p

您正在使用时收到错误c_void_p()(请注意额外的括号).c_void_p和之间有一个重要的区别c_void_p().前者是一种类型,后者是一种类型的实例. c_void_p表示C类型void*,而c_void_p()表示实际的指针实例(默认值为0).