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

C Function Returning Incorrect Value When Called from Python via Ctypes

In an attempt to invoke a C function from Python utilizing Ctypes, a mismatch in the expected result emerged. The function in question elevated a given number to a specified power, returning the result as expected when called directly in C. However, upon invoking it from within Python, a different, incorrect value was obtained.

The shared file housing the C function was produced using the command:

$ gcc -fPIC -shared -o test.so test.c

Following some permutations of the C function's configuration, the expected value was indeed returned in some instances, indicating that the issue did not stem from an inherent flaw in the C code. Notably, when the function was simplified to compute a simple square (i.e., return x*x instead of employing a for loop), Python returned the correct result.

The ultimate goal was to access a two-dimensional C array from within Python.

#include <stdio.h>

float power(float x, int exponent)
{
    float val = x;
    for(int i=1; i<exponent; i++){
        val = val*x;
    }
    return val;
}

from ctypes import *

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

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

print(functions.power(5,3))

When calling the function from C, the anticipated output of 125.0 was observed. However, Python yielded a value of 0.0 instead.

Upon further investigation, the discrepancy was traced to an error in specifying the C function's argument types within Python. Instead of argtype, the incorrect spelling of argtypes was inadvertently used.

functions.power.argtypes = [c_float, c_int]

Once this typo was corrected, the correct value (125.0) was successfully returned when calling the function from Python.

The above is the detailed content of Why Does My C Function Return an Incorrect Value When Called from Python Using ctypes?. For more information, please follow other related articles on the PHP Chinese website!