Reading array returned by c function in ctypes

Question

I’ve got some C code, which I’m trying to access from ctypes in python. A particular function looks something like this:

float *foo(void) {
    static float bar[2];
    // Populate bar
    return bar;
}

I know this isn’t an ideal way to write C, but it does the job in this case. I’m struggling to write the python to get the two floats contained in the response. I’m fine with return values that are single variables, but I can’t work out from the ctypes docs how to handle a pointer to an array.

Any ideas?

Asked By: TimD

||

Source

Answer 1

Specify restypes as [POINTER][1](c_float):

import ctypes

libfoo = ctypes.cdll.LoadLibrary('./foo.so')
foo = libfoo.foo
foo.argtypes = ()
foo.restype = ctypes.POINTER(ctypes.c_float)
result = foo()
print(result[0], result[1])

Answered By: falsetru

Answer 2

Thanks to @falsetru, believe I found a somehow better solution, which takes into account the fact that the C function returns a pointer to two floats:

import ctypes

libfoo = ctypes.cdll.LoadLibrary('./foo.so')
foo = libfoo.foo
foo.argtypes = ()
foo.restype = ctypes.POINTER(ctypes.c_float * 2)
result = foo().contents

print('length of the returned list: ' + len(result))
print(result[0], result[1])

Answered By: Olivier Michel

Reading array returned by c function in ctypes

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