Why Are My C Function's Return Values Incorrect When Called from Python Using ctypes?

Incorrect Value Returned from C Function Called via Python Using Ctypes

When calling a C function from Python using Ctypes, ensuring the correct specification of argument and return types is crucial. Failure to do so can result in undefined behavior and incorrect values being returned.

In your case, you defined a C function that raises a number to a power but encountered incorrect results when calling it from Python. The culprit turns out to be the misspelling of the argtypes attribute. The corrected code is:

from ctypes import *

so_file = '/Users/.../test.so'
functions = CDLL(so_file)

functions.power.argtypes = [c_float, c_int]  # Correct spelling
functions.power.restype = c_float

print(functions.power(5,3))

By specifying argtypes and restype, Ctypes knows how to convert between Python and C data types, ensuring proper computations and value handling. This correction should resolve the issue and return the expected output.

Implications for Array Handling

As you mentioned, you aim to eventually call a C function that returns a two-dimensional C array. This involves additional considerations:

• C Arrays and Pointers: In C, arrays are essentially pointers to the first element. In this context, you'll need to manage pointers in Python using Ctypes' POINTER type.
• Data Conversion: Converting a multidimensional C array to a Python object requires careful consideration of memory management and data layout.
• Custom Data Structures: You may need to define custom data structures to represent the arrays in Python, ensuring efficient and convenient handling.

By carefully addressing these aspects, you should be able to successfully call C functions returning two-dimensional arrays from Python using Ctypes.

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