Bitmap blur in c++ - c++

Question

iam writing a program to represent a blur efect on a bitmap.
Blur efect code:
for (xx = 0; xx < bitmapInfoHeader.biWidth; xx++)
{
for (yy = 0; yy <bitmapInfoHeader.biHeight; yy++)
{
avgB = avgG = avgR = 0;
Counter = 0;
for (x = xx; x < bitmapInfoHeader.biWidth && x < xx + blurSize; x++)
{
for (y = yy; y < bitmapInfoHeader.biHeight && y < yy + blurSize; y++)
{
avgB += bitmapImage[x *3 + y*bitmapInfoHeader.biWidth * 3 + 0]; //bitmapimage[x][y];
avgG += bitmapImage[x *3 + y*bitmapInfoHeader.biWidth * 3 + 1];
avgR += bitmapImage[x *3 + y*bitmapInfoHeader.biWidth * 3 + 2];
Counter++;
}
}
avgB = avgB / Counter;
avgG = avgG / Counter;
avgR = avgR / Counter;
bitmapImage[xx * 3 + yy*bitmapInfoHeader.biWidth * 3 + 0] = avgB;
bitmapImage[xx * 3 + yy*bitmapInfoHeader.biWidth * 3 + 1] = avgG;
bitmapImage[xx * 3 + yy*bitmapInfoHeader.biWidth * 3 + 2] = avgR;
}
}
And my output function which take the blured Array "bitmapImage" to a new file:
out = fopen(file,"wb");
fwrite(&bitmapInfoHeader, sizeof(char), sizeof(BITMAPINFOHEADER), out);
fwrite(&bitmapFileHeader, sizeof(char), sizeof(BITMAPFILEHEADER), out);
fseek(out, sizeof(char)*bitmapFileHeader.bfOffBits, SEEK_SET);
fwrite(bitmapImage, sizeof(char), bitmapInfoHeader.biSizeImage, out);
The reading function:
BITMAPINFOHEADER bitmapInfoHeader;
FILE *filePtr,*out;
BITMAPFILEHEADER bitmapFileHeader;
unsigned char *bitmapImage = nullptr;
filePtr = fopen(filename, "rb");
if (filePtr == NULL)
return NULL;
fread(&bitmapFileHeader, sizeof(BITMAPFILEHEADER), 1, filePtr);
fread(&bitmapInfoHeader, sizeof(BITMAPINFOHEADER), 1, filePtr);
fseek(filePtr, bitmapFileHeader.bfOffBits, SEEK_SET);
//bitmapImage = (unsigned char*)malloc(bitmapInfoHeader->biSizeImage);
bitmapImage = new unsigned char[bitmapInfoHeader.biSizeImage];
fread(bitmapImage, sizeof(char), bitmapInfoHeader.biSizeImage,filePtr);
*size = bitmapInfoHeader.biSizeImage;
When i compile the program a new bmp file is created but the image viewer cant show it and thorw an error.
So what am doing wrong here? is the blur algorytm good?