CFile and reading data

Hi all

I am trying to get to grips with file handling under MFC and it has me slightly baffled. I am using a CFile object to interface with the file - but my problem is that the buffer reads the entire 1024 bytes.. This of course has the nasty side effect of printing garbage in the messagebox. Is there a better way of reading the file such that you only read the data in the file?

Code:

  BYTE buffer[0x0400];	// 1KB buffer
  CFile file (_T("File.txt"), CFile::modeRead);
  DWORD dwBytesRemaining = file.GetLength();

  while(dwBytesRemaining)
  {
      UINT nBytesRead = file.Read(buffer, sizeof(buffer));
      dwBytesRemaining -= nBytesRead;
  }

  MessageBox((char*)buffer);

Thanks.

I have just been playing with this code and realised it is possible that the problem is I am trying to read a text file using the wrong functions and data types. Is it possible to effectively read a text file using a CFile object with a BYTE array or is it advisable to only read binary data in this fashion?

Code:

  CStdioFile file (_T("File.txt"), CStdioFile::modeRead);
  CString buffer;
  CString string;

  while (file.ReadString(buffer))
      string += buffer;

  MessageBox(string);

Here's an example of what you are trying to do....you can use CFile::Read....but CStdioFile::ReadLine workds too

You need to observe MFC exception handling for both the constructor and the ReadLine.......also...you can then use a CString which makes the memory allocation simple....

Exceptions in MFC are horrible...they are created on the heap and thrown as pointers....so you must manually delete them....far simpler to throw a reference to a stack based exception in my opinion (and also that of people like Scott Meyers), but this is how MFC is made, so you must call CException::Delete on the exception pointer to avoid a memory leak

Code:

void CTestMFCDlg::Read()
{
	try
	{
	
		CStdioFile file(_T("File.txt"), CFile::modeRead);
		CString str,mainstr = _T("");

		while(file.ReadString(str))
		{
			mainstr += str;
			mainstr += _T("\n");
		}
		MessageBox(mainstr);
	}
	catch(CException* e)//Catch by pointer as exceptions in MFC are crap
	{
		MessageBox(_T("Error - unable to open file"));
		e->Delete();//Lame...very lame...but needed

	}

}

Thanks Fordy, that is excellent. I understand that fully. But I now have to understand why I cannot achieve the same results using CFile.

I think my problem is that I am somewhat stubborn, and I must understand why something doesn't work. I am attempting to retry the code using the CFile class and the CFile::Read method. Similar operations should work so I have just edited the code snippet Fordy provided:

This is what is in my data file (File.txt):

This file contains data
This file contains data
This file contains data
This file contains data
This file contains data
This file contains data
This file contains data
This file contains data
This file contains data

Code:

    try
   {
        CFile file(_T("File.txt"), CFile::modeRead);
        CString mainstr = _T("");
        char buffer[10];
    
        while(file.Read(buffer, sizeof(buffer)))
        {
             TRACE(_T("%s"),buffer);
             mainstr += buffer;
             mainstr += _T("\n");
        }
        MessageBox(mainstr);
    }
    catch(CException* e)
    {
        MessageBox(_T("Error - unable to open file"));
        e->Delete();
    }

And this is the debug output - which leads me to believe I am touching memory I shouldn't somewhere.

This fileÌÌÌÌÌÌÌÌÌÌÌ$ÿ.|ÌÌÌÌÌÌÌÌl3|| containsÌÌÌÌÌÌÌÌÌÌÌ data
ThÌÌÌÌÌÌÌÌÌÌÌis file cÌÌÌÌÌÌÌÌÌÌÌontains dÌÌÌÌÌÌÌÌÌÌÌata
ThisÌÌÌÌÌÌÌÌÌÌÌ file conÌÌÌÌÌÌÌÌÌÌÌtains datÌÌÌÌÌÌÌÌÌÌÌa
This fÌÌÌÌÌÌÌÌÌÌÌile contaÌÌÌÌÌÌÌÌÌÌÌins data
ÌÌÌÌÌÌÌÌÌÌÌ
This filÌÌÌÌÌÌÌÌÌÌÌe containÌÌÌÌÌÌÌÌÌÌÌs data
TÌÌÌÌÌÌÌÌÌÌÌhis file ÌÌÌÌÌÌÌÌÌÌÌcontains ÌÌÌÌÌÌÌÌÌÌÌdata

Remember that CFile::Read is a raw operation....it doesnt NULL of the buffer it retrieves

Have a look at this;

Code:

void CTestMFCDlg::Read()
{
    try
   {
        CFile file(_T("File.txt"), CFile::modeRead);
        CString mainstr = _T("");
        char buffer[10];
		UINT unRead;
   
		unRead = file.Read(buffer, sizeof(buffer)-1);
        while(unRead)
        {
			buffer[unRead] = '\0';
            TRACE(_T("%s"),buffer);
            mainstr += buffer;
            mainstr += _T("\n");
			unRead = file.Read(buffer, sizeof(buffer)-1);
        }
        MessageBox(mainstr);
    }
    catch(CException* e)
    {
        MessageBox(_T("Error - unable to open file"));
        e->Delete();
    }

}

Also, dump the " mainstr += _T("\n");" line....Read will add the newline from the file

And maybe make the read buffer bigger...this will result in less function calls

Thank you, Thank you. I got it.

Thanks again for the help.