Passing byte array from Unity to Android (C++) for modification

Firstly, you did not provide any information about your configuration. What is your scripting backend: Mono or IL2CPP?

Secondly, why don't you call C++ code directly from C#?

1) Go to: [File] > [build Settings] > [Player Settings] > [Player] and turn on [Allow 'unsafe' Code] property.

2) After building the library, copy the output .so file(s) into the Assets/Plugins/Android directory in your Unity project.

C# code:

using UnityEngine;
using UnityEngine.UI;
using System.Runtime.InteropServices;
using System;


public class CallNativeCode : MonoBehaviour
{
    [DllImport("NativeCode")]
    unsafe private static extern bool PostprocessNative(int width, int height, short* data_out);

    public short[] dataShortOut;
    public Text TxtOut;

    public void Update()
    {
        dataShortOut = new short[100];
        bool o = true;

        unsafe
        {
            fixed (short* ptr = dataShortOut)
            {
                o = PostprocessNative(10, 10, ptr);
            }
        }

        TxtOut.text = "Function out:  " + o + " Array element 0: " + dataShortOut[0];
    }
}

C++ code:

#include <stdint.h>
#include <android/log.h>

#define LOG(...) __android_log_print(ANDROID_LOG_VERBOSE, "0xBFE1A8", __VA_ARGS__)


extern "C"
{
    bool PostprocessNative(int width, int height, short *data_out)
    {
        for (int y = 0; y < height; ++y)
        {
            for (int x = 0; x < width; ++x)
            {
                data_out[x + y * width] = 10;
            }
        }

        LOG("Log: %d", data_out[0]);

        // Changing the return value here is correctly reflected in C#.
        return false;
    }
}