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Reputation: 15

DirectX 11, exception thrown when updating constant buffer with UpdateSubresource

So I am very new to DirectX and are trying to learn the basics but I'm running into some problem with my constant buffer. I'm trying to send a struct with three matrices to the vertex shader, but when I try to update the buffer with UpdateSubresource I get "Exception is thrown at 0x710B5DF3 (d3d11.dll) in Demo.exe: 0xC0000005: Access violation reading location 0x0000003C".

My struct:

struct Matracies
{
   DirectX::XMMATRIX projection;
   DirectX::XMMATRIX world;
   DirectX::XMMATRIX view;
};
Matracies matracies;

Buffer creation:

ID3D11Buffer*  ConstantBuffer = nullptr;
D3D11_BUFFER_DESC Buffer;
memset(&Buffer, 0, sizeof(Buffer));

Buffer.BindFlags = D3D11_BIND_CONSTANT_BUFFER;
Buffer.Usage = D3D11_USAGE_DEFAULT;
Buffer.ByteWidth = sizeof(Matracies);
Buffer.CPUAccessFlags = D3D11_CPU_ACCESS_WRITE;

D3D11_SUBRESOURCE_DATA data;
data.pSysMem = &matracies;
data.SysMemPitch = 0;
data.SysMemSlicePitch = 0;

Device->CreateBuffer(&Buffer, &data, &ConstantBuffer);
DeviceContext->VSSetConstantBuffers(0, 1, &ConstantBuffer);

Updating buffer:

DeviceContext->UpdateSubresource(ConstantBuffer, 0, 0, &matracies, 0, 0);

I am not sure what information is relevant to solve this so let me know if anything is missing.

Upvotes: 1

Views: 1570

Answers (1)

Chuck Walbourn
Chuck Walbourn

Reputation: 41087

Welcome to the wooly world of DirectX!

The first two steps in debugging any DirectX program are:

(1) Enable the Debug device. See this blog post. This will generate additional debug output at runtime which gives hints about problems like the one you have above.

(2) If a function returns an HRESULT, you must check that for success or failure at runtime. If it was safe to ignore the return value, it would return void. See this page.

If you had done either or both of the above, you would have caught the error returned from CreateBuffer above which resulted in ConstantBuffer still being a nullptr when you called UpdateSubresource.

The reason it failed is that you can't in general create a constant buffer that is both D3D11_USAGE_DEFAULT and D3D11_CPU_ACCESS_WRITE. DEFAULT usage memory is often in video memory that is not accessible to the CPU. Since you are using UpdateSubresource as opposed to Map, you should just use:

Buffer.CPUAccessFlags = 0;

You should take a look at DirectX Tool Kit and it's associated tutorials.

Upvotes: 3

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