Access violation writing location, in my loop

Question

The exact error I am getting is

First-chance exception at 0x0096234a in chp2.exe: 0xC0000005: Access violation writing location 0x002b0000.
Windows has triggered a breakpoint in chp2.exe.

And the breakpoint stops here

for(DWORD i = 0; i < m; ++i)
{
    //we are start at the top of z
    float z = halfDepth - i*dx;
    for(DWORD j = 0; j < n; ++j)
    {
        //to the left of us
        float x = -halfWidth + j*dx;
        float y = 0.0f;
        vertices[i*n+j].pos = D3DXVECTOR3(x, y, z); //<----- Right here
        vertices[i*n+j].color = D3DXVECTOR4(1.0f, 0.0f, 0.0f, 0.0f);
    }
}

I am not sure what I am doing wrong. below is the code in its entirety

#include "MyGame.h"
//#include "CubeVector.h"
/* This code sets a projection and shows a turning cube. What has been added is the project, rotation and
a rasterizer to change the rasterization of the cube. The issue that was going on was something with the effect file
which was causing the vertices not to be rendered correctly.*/
typedef struct 
{
    ID3D10Effect* pEffect;
    ID3D10EffectTechnique* pTechnique;

    //vertex information
    ID3D10Buffer* pVertexBuffer;
    ID3D10Buffer* pIndicesBuffer;
    ID3D10InputLayout* pVertexLayout;

    UINT numVertices;
    UINT numIndices;
}ModelObject;

ModelObject modelObject;
// World Matrix
D3DXMATRIX                  WorldMatrix;
// View Matrix
D3DXMATRIX                  ViewMatrix;
// Projection Matrix
D3DXMATRIX                  ProjectionMatrix;
ID3D10EffectMatrixVariable* pProjectionMatrixVariable = NULL;

//grid information
#define NUM_COLS 16
#define NUM_ROWS 16

#define CELL_WIDTH 32
#define CELL_HEIGHT 32

#define NUM_VERTSX (NUM_COLS + 1)
#define NUM_VERTSY (NUM_ROWS + 1)




bool MyGame::InitDirect3D()
{
    if(!DX3dApp::InitDirect3D())
    {
        return false;
    }

    D3D10_RASTERIZER_DESC rastDesc;
    rastDesc.FillMode = D3D10_FILL_WIREFRAME;
    rastDesc.CullMode = D3D10_CULL_FRONT;
    rastDesc.FrontCounterClockwise = true;
    rastDesc.DepthBias = false;
    rastDesc.DepthBiasClamp = 0;
    rastDesc.SlopeScaledDepthBias = 0;
    rastDesc.DepthClipEnable = false;
    rastDesc.ScissorEnable = false;
    rastDesc.MultisampleEnable = false;
    rastDesc.AntialiasedLineEnable = false;

    ID3D10RasterizerState *g_pRasterizerState;
    mpD3DDevice->CreateRasterizerState(&rastDesc, &g_pRasterizerState);
    mpD3DDevice->RSSetState(g_pRasterizerState);

    // Set up the World Matrix
    //The first line of code creates your identity matrix. Second line of code
    //second combines your camera position, target location, and which way is up respectively
    D3DXMatrixIdentity(&WorldMatrix);
    D3DXMatrixLookAtLH(&ViewMatrix, new D3DXVECTOR3(200.0f, 60.0f, -20.0f), new D3DXVECTOR3(200.0f, 50.0f, 0.0f), new D3DXVECTOR3(0.0f, 1.0f, 0.0f));
    // Set up the projection matrix
    D3DXMatrixPerspectiveFovLH(&ProjectionMatrix, (float)D3DX_PI * 0.5f, (float)mWidth/(float)mHeight, 0.1f, 100.0f);

    if(!CreateObject())
    {
        return false;
    }

    return true;
}

//These are actions that take place after the clearing of the buffer and before the present
void MyGame::GameDraw()
{

    static float rotationAngle = 0.0f;

    // create the rotation matrix using the rotation angle
    D3DXMatrixRotationY(&WorldMatrix, rotationAngle);
    rotationAngle += (float)D3DX_PI * 0.0f;

    // Set the input layout
    mpD3DDevice->IASetInputLayout(modelObject.pVertexLayout);

    // Set vertex buffer
    UINT stride = sizeof(VertexPos);
    UINT offset = 0;
    mpD3DDevice->IASetVertexBuffers(0, 1, &modelObject.pVertexBuffer, &stride, &offset);
    mpD3DDevice->IASetIndexBuffer(modelObject.pIndicesBuffer, DXGI_FORMAT_R32_UINT, 0);

    // Set primitive topology
    mpD3DDevice->IASetPrimitiveTopology(D3D10_PRIMITIVE_TOPOLOGY_TRIANGLELIST);

    // Combine and send the final matrix to the shader
    D3DXMATRIX finalMatrix = (WorldMatrix * ViewMatrix * ProjectionMatrix);
    pProjectionMatrixVariable->SetMatrix((float*)&finalMatrix);


    // make sure modelObject is valid


    // Render a model object
    D3D10_TECHNIQUE_DESC techniqueDescription;
    modelObject.pTechnique->GetDesc(&techniqueDescription);

    // Loop through the technique passes
    for(UINT p=0; p < techniqueDescription.Passes; ++p)
    {
        modelObject.pTechnique->GetPassByIndex(p)->Apply(0);
        // draw the cube using all 36 vertices and 12 triangles
        mpD3DDevice->DrawIndexed(modelObject.numIndices,0,0);
    }
}

//Render actually incapsulates Gamedraw, so you can call data before you actually clear the buffer or after you 
//present data
void MyGame::Render()
{
    DX3dApp::Render();
}

bool MyGame::CreateObject()
{
    //dx will represent the width and the height of the spacing of each vector
    float dx = 1;
    //Below are the number of vertices
    //m is the vertices of each row. n is the columns
    DWORD m = 30;
    DWORD n = 30;
    //This get the width of the entire land
    //30 - 1 = 29 rows * 1 = 29 * 0.5 = 14.5
    float halfWidth = (n-1)*dx*0.5f;
    float halfDepth = (m-1)*dx*0.5f;
    float vertexsize = m * n;
    VertexPos vertices[80];

    for(DWORD i = 0; i < m; ++i)
    {
        //we are start at the top of z
        float z = halfDepth - i*dx;
        for(DWORD j = 0; j < n; ++j)
        {
            //to the left of us
            float x = -halfWidth + j*dx;
            float y = 0.0f;
            vertices[i*n+j].pos = D3DXVECTOR3(x, y, z);
            vertices[i*n+j].color = D3DXVECTOR4(1.0f, 0.0f, 0.0f, 0.0f);
        }
    }

    int k = 0;

    DWORD indices[540];
    for(DWORD i = 0; i < n-1; ++i)
    {
        for(DWORD j = 0; j < n-1; ++j)
        {
            indices[k]      = (i * n) + j;
            indices[k + 1]  = (i * n) + j + 1;
            indices[k + 2]  = (i + 1) * n  + j;

            indices[k + 3]  = (i + 1) * n  + j;
            indices[k + 4]  = (i * n) + j + 1;
            indices[k + 5]  = (i + 1) * n  + j+ 1;

            k += 6;
        }
    }




    //Create Layout
    D3D10_INPUT_ELEMENT_DESC layout[] = {
        {"POSITION",0,DXGI_FORMAT_R32G32B32_FLOAT, 0 , 0, D3D10_INPUT_PER_VERTEX_DATA, 0},
        {"COLOR",0,DXGI_FORMAT_R32G32B32A32_FLOAT, 0 , 12, D3D10_INPUT_PER_VERTEX_DATA, 0}
    };

    UINT numElements = (sizeof(layout)/sizeof(layout[0]));
    modelObject.numVertices = sizeof(vertices)/sizeof(VertexPos);

    //Create buffer desc
    D3D10_BUFFER_DESC bufferDesc;
    bufferDesc.Usage = D3D10_USAGE_DEFAULT;
    bufferDesc.ByteWidth = sizeof(VertexPos) * modelObject.numVertices;
    bufferDesc.BindFlags = D3D10_BIND_VERTEX_BUFFER;
    bufferDesc.CPUAccessFlags = 0;
    bufferDesc.MiscFlags = 0;

    D3D10_SUBRESOURCE_DATA initData;
    initData.pSysMem = vertices;
    //Create the buffer

    HRESULT hr = mpD3DDevice->CreateBuffer(&bufferDesc, &initData, &modelObject.pVertexBuffer);
    if(FAILED(hr))
        return false;

    modelObject.numIndices = sizeof(indices)/sizeof(DWORD);

    bufferDesc.ByteWidth = sizeof(DWORD) * modelObject.numIndices;
    bufferDesc.BindFlags = D3D10_BIND_INDEX_BUFFER;

    initData.pSysMem = indices;

    hr = mpD3DDevice->CreateBuffer(&bufferDesc, &initData, &modelObject.pIndicesBuffer);
    if(FAILED(hr))
        return false;


    /////////////////////////////////////////////////////////////////////////////
    //Set up fx files
    LPCWSTR effectFilename = L"effect.fx";
    modelObject.pEffect = NULL;

     hr = D3DX10CreateEffectFromFile(effectFilename,
        NULL,
        NULL,
        "fx_4_0",
        D3D10_SHADER_ENABLE_STRICTNESS,
        0,
        mpD3DDevice,
        NULL,
        NULL,
        &modelObject.pEffect,
        NULL,
        NULL);

    if(FAILED(hr))
        return false;

    pProjectionMatrixVariable = modelObject.pEffect->GetVariableByName("Projection")->AsMatrix();
    //Dont sweat the technique. Get it!
    LPCSTR effectTechniqueName = "Render";

    modelObject.pTechnique = modelObject.pEffect->GetTechniqueByName(effectTechniqueName);
    if(modelObject.pTechnique == NULL)
        return false;


    //Create Vertex layout
    D3D10_PASS_DESC passDesc;
    modelObject.pTechnique->GetPassByIndex(0)->GetDesc(&passDesc);

    hr = mpD3DDevice->CreateInputLayout(layout, numElements,
        passDesc.pIAInputSignature,
        passDesc.IAInputSignatureSize,
        &modelObject.pVertexLayout);
    if(FAILED(hr))
        return false;

    return true;
}

Answer 1

Most likely, you're writing past the end of the vertices array or trying to read from invalid variables (empty). Make sure your array is of the proper size.

Answer 2

If you have 30 vertices in each row and 30 in each column how come vertices is defined as

VertexPos vertices[80];

This line:

vertices[i*n+j].pos = D3DXVECTOR3(x, y, z);

writes past the end of the vertices array whenever i * n + j > 79 (e.g, if i = 10 then i * n + j >= 300)

Answer 3

Your vertices array has size of 80. In worst case, You are trying to write in vertices[i*n+j], where i=30, n=30, j=30, so You are trying to address vertices[930].

Not intended, is it ?