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I have the following C code

int num_detections = zarray_size(detections);

apriltag_detection_t tag_detections[num_detections];
for (int i = 0; i < zarray_size(detections); i++) {
    apriltag_detection_t *det;
    zarray_get(detections, i, &det);
    tag_detections[i] = det;
}

defined in a function that is being called by Python via ctypes. I want to be able to pass back tag_detections (an array of struct pointers) back to my Python function, and be able to access the structs as Python objects. Here's what I have on the Python side.

libtag36h11_detector = ctypes.CDLL('tag36h11_detector/libtag36h11_detector.so')
_scan_frame = libtag36h11_detector.scan_frame
_scan_frame.argtypes = (ctypes.c_int, ctypes.c_int, ndpointer(ctypes.c_uint8, flags="C_CONTIGUOUS"))
...
def scan_frame(frame):
    height, width, channels = frame.shape
    frame = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    frame = frame.flatten()
    _scan_frame(width, height, frame)

Search results have turned up ways to do the other way around (passing an array of Python objects to an array of C structs)

Improve this question
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  • 1
    Perhaps the reason you haven't found anything is because doing so isn't a good (or feasible) approach to the problem—for example since Python doesn't have pointers. Commented Sep 30, 2017 at 17:43
  • Okay, thanks. How do I accomplish something similar? Commented Sep 30, 2017 at 17:43
  • I'm no expert on the subject, but you may have to pass back an array of the structs themselves, which can be handled with Python's built-in ctypes or struct modules. Commented Sep 30, 2017 at 17:47
  • Do you have any resources I could use to learn how to do that? Commented Sep 30, 2017 at 17:57
  • You now know what to look for, so go for it. Here's something I just found in about 20 seconds: Python C API How to pass array of structs from C to Python. The section in the ctypes documentation about pointers may also be useful. Commented Sep 30, 2017 at 19:18
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