Ubuntu Linux中的阵列复制实际上不起作用

Question

I mean, this thing makes me feel like a naive programmer :')
I'll explain it.
For my buttons controller input I use two arrays, one for the current button pressed and one for the last button pressed. The thing I'm doing is, for every frame, copying the content of the "current" array to the "previous" array and then use glfw to get the buttons state into the "current" array.
Here's the code i'm talking about:

for(int j=0; j<Input::joyButtonsCount[i]; j++)
    Input::joyPrevButtons[i][j] = Input::joyCurrButtons[i][j];
Input::joyCurrButtons[i] = (char*)glfwGetJoystickButtons(i, &Input::joyButtonsCount[i]);

Now, under Windows all works correctly, but in Linux it doesn't. It results in the prev and curr arrays to be the same.
Can someone explain me why it occurs?
If you need this information I am using g++ under Linux and MinGW under Windows.

Answer 1

From what I see glfwGetJoystickButtons returns a pointer to some internal array. You are assigning it to Input::joyCurrButtons[i] and in next iteration you assume it will have a previous values. This might not be true (maybe its implementation defined), as system might have updated this array internally with new values. This way your first loop will copy to joyPrevButtons new values and not the previous ones.

glfwGetJoystickButtons returns a const pointer to make sure API user will not try to modify its values. So you should not cast it to non-const. Also its often a sign that you should not keep this pointer, but use it only to copy values instead.

My suggestion for you is not to store in pointer from glfwGetJoystickButtons in Input::joyCurrButtons[i] , but instead copy values from glfwGetJoystickButtons to Input::joyCurrButtons[i] array.

Answer 2

Considering the code snippet you provided, there is no reason why the array copy does not perform the same way in Linux as in Windows. Your problem is likely elsewhere.

I suggest that you check the following points:

• is Input::joyButtonsCount[i] properly defined on both platform ?
• double check on both platforms that Input::joyCurrButtons[i] properly changes when you press button #i.
• check on both platform if the copy happens at time you expect; it's likely that, on Linux, the copy happens before you expect, so that when you check values, they are the same (because already copied earlier); then you will have to understand why copy happens before expected time.

Answer 3

For everyone else in my situation (such as noobs like me :P) I'll post the problem.
It seems that I had to copy the pointer to an array and not get the pointer and use that as my input data.
Here's how this code is now:

Input::joyPrevButtons[i] = (char*)memcpy(Input::joyPrevButtons[i], Input::joyCurrButtons[i], Input::joyButtonsCount[i]);
Input::joyCurrButtons[i] = (char*)memcpy(Input::joyCurrButtons[i], glfwGetJoystickButtons(i, &Input::joyButtonsCount[i]), Input::joyButtonsCount[i]);

Obviously I had to include the string header

#include <string.h>

But I still don't know why it doesn't work on Linux as it works on Windows.