Why Does Omitting the 's' in `argtypes` Cause Incorrect Return Values When Using ctypes?

Misspecifying Option Parameters by Omitting the Terminal 's' in C function called from Python via ctypes Can Cause Incorrect Return Values

When interfacing with a C function from Python using ctypes, it's crucial to correctly specify the argument types and return type of the function. Failure to do so, or even misspelling the option names, can result in undefined behavior.

In the provided example, the issue arises from misspecifying the argtype option. The error involves omitting the terminal 's' in the argument type specification, resulting in an incorrect type being applied to the function's argument list.

To rectify this issue, simply correct the argtype specification by adding the missing 's'. The corrected code will look like:

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))

By correctly specifying the argtype and restype options, the ctypes mechanism will properly convert the arguments and return value between Python and C types. This ensures the accurate invocation and execution of the C function in the Python environment.

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