Python - ctypes - 如何调用函数和访问struct字段?

时间:2014-04-29 10:38:57

标签: python c++ c ctypes

我有一个C库:

smart_string.h

typedef struct SmartString {
    unsigned string_len;
    unsigned alloc_len;
    char *str;
    char *str_terminator;

} SmartString;

SmartString *SmartString_new(char *str);
... definitions of more functions ...

该实现位于名为 smart_string.c 的文件中。

我需要一个运行SmartString_new()函数并访问返回的struct-pointer字段的指南。

有人可以告诉我该怎么做吗?

谢谢!

1 个答案:

答案 0 :(得分:8)

回答自己并与您分享知识:

首先,需要从C文件创建一个共享库:

gcc -shared -fpic smart_string.c -o SmartString.so

然后,使用以下Python代码(有关每个已完成操作的说明,请参阅注释):

注意char*,如上面API中显示的是C可编辑字符串,而const char*是只读字符串。由于C API需要char*而不是const char*,因此我们必须向其传递可变字符串,因此可以通过C代码进行编辑。默认情况下,Python字符串是不可变的。因此,我们使用create_string_buffer()函数

<强> python_smart_string.py

import ctypes
from ctypes import *

# Defining the python type that represents the C SmartString
# It must extend the 'Structure' class
# Structure, c_uint, c_char_p, etc. were imported from ctypes
class SmartString(Structure):
    _fields_=[("string_len",c_uint),
              ("alloc_len",c_uint),
              ("str",c_char_p),
              ("str_terminator", c_char_p)]


# Loading the C shared lib I've just compiled
smartstring_lib = ctypes.CDLL('SmartString.so')

# Defining pointer to the SmartString_new() function
SmartString_new = smartstring_lib.SmartString_new

# Declaring the function return type - a pointer to a SmartString object - just like in the C code
SmartString_new.restype = POINTER(SmartString)

# Declaring list of parameter types. In this case, the list contains only one item,
# as the function has only one parameter
SmartString_new.argtypes = [c_char_p]

# Calling the SmartString_new() function. Expecting to get a pointer to SmartString object into 'my_str'
# The API requires a MUTABLE string, so create_string_buffer() is used here
# The reference to this string is not saved, as I don't care if it is modified by the C code
my_str = SmartString_new(create_string_buffer('my nice string'))

# Printing fields of the dereferenced returned value (dereferencing is done using '.contents')
print my_str.contents.string_len
print my_str.contents.alloc_len
print my_str.contents.str
print my_str.contents.str_terminator