read from an istream object

Hi,

This is what I am trying to do - If the filename is given I have to read
from the file else I have to read from cin. The program below compiles fine
but while reading from cin, it reads some part of the stream fine but
encounters an EOF prematuerly. If I read from the cin in a file I see the
full data. So I know that cin does get all the data, just that the read
function behaves in an unexpected way.

    std::istream* myFile; // stream object to read the cin
    std::filebuf* myFileBuf; // specifies a streamBuffer

........

if (!myFileName)
    {
            myFile = &std::cin;
    }
    else
    {
        myFileBuf = new std::filebuf;
        if (!(myFileBuf->open (myFileName, std::ios::in | std::ios::binary)))
        {
            status = My_FileOpenErr;
        }
        else
        {
            myFile = new std::istream (myFileBuf);
        }
    }

..........

Then I have the read function
if (!(myFile->read (reinterpret_cast<char*>(mydata), nBytes)))
 {
        if (myFile->eof())
        {
            status = My_EndOfFile;
        }
        else
        {
            status = My_IOErr;
        }
    }

Please help,
Thanks in advance.
M