File size in bytes

[aamir121a]

I was wondering if there is a more efficent way to calculate actual size in bytes of any file,
here is how I am doing.
win32 API which will do it for me. accuracy is important as I will be reading the entire file in to a shared buffer.

fstream reader("myfile",std:ios:binary| std::ios::in);
unsigned int counter =0;
unsigned char myReader;
while(!reader.eof)
{
   reader.read((char *)&myReader,sizeof(unsigned char));
   counter++
}
reader.close();
unsigned int filesize = counter;

I am hoping how many times the loops run will tell me the actual size in bytes. Once I know what the size is then I can allocate memory and read the entire file all at once in to the shared buffer.