Why Does My C Function Return Incorrect Values When Called from Python Using ctypes?

C Function Called from Python via ctypes Returns Incorrect Value

A C function called via Python using the ctypes module may return an incorrect value, despite functioning correctly when invoked directly in C. This disparity can arise when certain configuration issues are overlooked.

Argument and Return Value Types

To ensure accurate value conversion between Python and C, it is essential to specify the argument and return value types correctly. In Python, using ctypes, this is accomplished through:

• argtypes: A sequence of CTypes types representing each argument of the function, arranged in the order they appear in the function declaration. For a single argument, a one-element sequence is required.
• restype: A single CTypes type representing the return type of the function.

Misconfiguring Argument Types

An error commonly encountered when calling a C function from Python via ctypes is misconfiguring the argument types. For instance, specifying all argument types as int (default) for functions expecting other data types can lead to undefined behavior and incorrect results.

Example

Consider a C function power that calculates a given number raised to a given power. The expected C function prototype is:

float power(float x, int exponent);

The corresponding Python implementation using ctypes would be:

from ctypes import *

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

functions.power.argtypes = [c_float, c_int]
functions.power.restype = c_float

print(functions.power(5, 3))

In this example, the argument types and return type are properly specified, ensuring accurate value conversion.

Correcting the Error

To resolve the issue described in the problem, where the function returns an incorrect value, it is crucial to ensure that the argument and return value types are correctly specified in the Python code. Specifically, verifying that the argtypes and restype attributes of the function object are set with the proper CTypes types will prevent miscalculations and undefined behavior.

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