I have a C library:
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 ...
The implementation is found in a file named smart_string.c.
I need a guide for running the SmartString_new() function and accessing the fields of the returned struct-pointer.
Can anyone please show me how to do it?
Thanks!
Answering myself and sharing the knowledge with you:
First, need to create a shared library from the C file:
gcc -shared -fpic smart_string.c -o SmartString.so
Then, use the following Python code (see comments for explanation about each done action):
Note: char*, as appears in the above API is a C editable string, while const char* is a read-only string. Because the C API requires char* and not const char*, we have to pass to it a mutable string, so it can be edited by the C code. Python strings are immutable by default. Here we use therefore, the create_string_buffer() function
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
SmartString_new.argtypes = [c_char_p]. You might need to keep a reference to create_string_buffer(). my_str[0] is another way to dereference a pointer.