How to create a C struct from Python using ctypes?

Question

I have a similar problem to this question. I'm basically trying to create C structs from Python using ctypes.

In C I have:

typedef struct Point {
    int x;
    int y;
} Point ;

Point* makePoint(int x, int y){
    Point *point = (Point*) malloc(sizeof (Point));
    point->x = x;
    point->y = y;
    return point;
}

void freePoint(Point* point){
    free(point);
}

While in Python I have:

    class Point(ct.Structure):
        _fields_ = [
            ("x", ct.c_int64),
            ("y", ct.c_int64),
        ]


    lib = ct.CDLL("SRES.dll")

    lib.makePoint.restype = ct.c_void_p

    pptr = lib.makePoint(4, 5)
    print(pptr)

    p = Point.from_address(pptr)
    print(p)
    print(p.x)
    print(p.y)

Currently this outputs a bunch of pointers:

2365277332448
<__main__.Point object at 0x00000226D2C55340>
21474836484
-8646857406049613808

How can I instead make this output return the numbers I put in, i.e.

2365277332448
<__main__.Point object at 0x00000226D2C55340>
4
5

ghchoi · Accepted Answer · 2021-01-26 14:03:27Z

2

The problem was c_int64. After changing it to c_int32, it works fine. You may take c_int64 as long from C side.

Additionally, you can do

lib.makePoint.restype = ct.POINTER(Point)
p = lib.makePoint(4, 5)
print(p.contents.x)
print(p.contents.y)

edited Jan 26, 2021 at 14:03

answered Jan 26, 2021 at 13:17

ghchoi

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7 Comments

CiaranWelsh Over a year ago

I get this: AttributeError: 'Point' object has no attribute 'contents'

ghchoi Over a year ago

@CiaranWelsh Ah... Then, p['x'] and p['y']. Have you tried them?

ghchoi Over a year ago

@CiaranWelsh pptr.contents.x may work. I think...

Shawn Over a year ago

Best not to rely on c builtin types for word length on the C-side of the equation. stdint.h has int32_t/int64_t and others for this purpose.

Mark Tolonen Over a year ago

Also, ctypes has c_int to parallel C int. Use matching types.

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