ctypes struct returned from library

问题

Given a simple C file:

#include <stdio.h>

typedef struct point {
    int x;
    int y;
} POINT; 

POINT get_point() 
{
    POINT p = {1, 2};
    return p;
}

And I have a simple python file:

from ctypes import *
import os

lib_name = '/testlib.so' 
test_lib = CDLL(os.getcwd() + lib_name)

class POINT(Structure):
    _fields_ = [('x', c_int),
                ('y', c_int)]

# Sets p1 to the integer 1
p1 = test_lib.get_point()
# Sets p2 to the struct POINT with values {1, 0}
p2 = POINT(test_lib.get_point())

How can I set my returned value to the struct POINT with values {1, 2}?

回答1:

What you ar asking is nto the sole problem in your example. Just to answer just what you asked first: you have to annotate the C function return type, so that ctypes know it is a memory address - otherwise it is a (4 byte) integer by default (while in any 64 bit OS, pointers are 8 bytes long).

Then you can create Python side POINT structures by using the (hidden) "from_address" method in your POINT class:

test_lib.get_point.restype = c_void_p
p = POINT.from_address(test_lib.get_point())

print(p.x, p.y)

Before that works, however, you have a more fundamental issue on the C side: the POINT structure you declare on your example only exists while get_point is running, and is deallocated afterwards. The code above would lead to a segmentation fault.

Your C code have to allocate memory properly. And also, you should take provisions to deallocate data structures you allocate in C - otherwise you will have memory leaks as each call to the function in C allocates more memory and you don't free that. (Notice that this memory won't be freed by itself when the Python POINT object goes out of scope).

Your C code could be like this:

#include <stdlib.h>
#include <stdio.h>

typedef struct point {
    int x;
    int y;
} POINT;

POINT *get_point()
{
    POINT *p;
    POINT initial = {1, 2};
    p = malloc(sizeof(POINT));
    *p = initial;
    return p;
}

void free_point(POINT *p)
{
    free(p);
}

And with this Python part:

from ctypes import *
import os

lib_name = '/testlib.so'
test_lib = CDLL(os.getcwd() + lib_name)

class POINT(Structure):
    _fields_ = [('x', c_int),
                ('y', c_int)]

test_lib.get_point.restype = c_void_p

p1 = POINT.from_address( test_lib.get_point())
print (p1.x, p1.y)

test_lib.free_point(byref(p1))
del p1

everything should just work.

(just so that this answer is a complete ctypes example, I will add the GCC commands to build the testlib file:

gcc -c -fPIC test.c -o test.o
gcc test.o -shared -o testlib.so

)

来源：https://stackoverflow.com/questions/38661635/ctypes-struct-returned-from-library

标签

python

python-2.7

wrapper

ctypes