Python - ctypes - 如何调用函数和访问结构字段？

Question

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。

我需要一个指南来运行 SmartString_new() 函数并访问返回的结构指针的字段。

谁能告诉我怎么做？

谢谢!

Answer 1

回答我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解释，请参阅注释):

注意:char*，在上面的API中出现的是一个C可编辑的字符串，而const char*是一个只读的字符串。因为 C API 需要 char* 而不是 const char*，我们必须向它传递一个 mutable 字符串，所以它可以被编辑C代码。默认情况下，Python 字符串是不可变的。因此，我们在这里使用 create_string_buffer() 函数

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