Double buffering causes system to freeze

[leonrobinsonn]

Hey, I am currently working on implementing double buffering for my GOP driver. But after copying over the memory from the backbuffer to the framebuffer, it freezes.

Setting up the back buffer:

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BackBuffer = malloc(framebuffer->Width * framebuffer->Height * 4);

Drawing to the back buffer:

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void Graphics::draw_pixel(int x, int y, int color)
{
    *(unsigned int*)((unsigned int*)BackBuffer + x + (y * buffer->PixelsPerScanLine)) = color;
}

void Graphics::draw_rect(int x, int y, int w, int h, int color)
{
    for (int Y = y; Y < y + h; Y++)
    {
        for (int X = x; X < x + w; X++)
        {
            draw_pixel(X, Y, color);
        }
    }
}

Copying from the back buffer to frame buffer:

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memcpy((uint64_t*)buffer->BaseAddress, (uint64_t*)BackBuffer, buffer->Width * buffer->Height * 4);

memcpy function:

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void* memcpy(uint64_t* dest, uint64_t* source, int count)
{
    for (int i = 0; i < count; i++)
    {
        dest[i] = source[i];
    }
    return dest;
}

The drawing:

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g.draw_rect(0, 0, 50, 50, 0xFFFFFF);
g.update(); // Draws white square successfully but then freezes the system

// This doesn't draw!
g.draw_rect(0, 0, 50, 50, 0xFF0000);
g.update();

Not sure if I need to post any more information but I can if that's needed

[Octocontrabass]

Your memcpy function copies 8*count bytes, which means you're telling it to copy 8 times as much data as it should. Access beyond an object's boundaries is undefined behavior.

Also, it relies on accessing objects through incompatible pointer types, which is undefined behavior.

[nullplan]

In addition to the above, the way your memcpy function is declared, it invokes undefined behavior with GCC as compiler, since GCC requires memcpy to exist and be declared a certain way. Due to how GCC's optimizers work, I would suggest implementing the memory functions (memcpy(), memmove(), memcmp(), memset()) in assembler instead. They are not the most complicated functions in the world.

[leonrobinsonn]

Your memcpy function copies 8*count bytes, which means you're telling it to copy 8 times as much data as it should. Access beyond an object's boundaries is undefined behavior.

Also, it relies on accessing objects through incompatible pointer types, which is undefined behavior.

Yeah, this makes sense since I just divided the memcpy count by 8 and it seems to work fine

[Octocontrabass]

it seems to work fine

Undefined behavior may stop working at any point, for any reason (including no reason at all). You should fix the rest of the undefined behavior too.