C / C++ / MFC

CString strValue;
strValue.Format(_T("%lf"), Value2Show);
pDC->TextOut(123, 456, strValue);

void CReadin2View::OnButton1() 
{
	// TODO: Add your control notification handler code here

	CFile myFile;

	myFile.Open("C:\\abc.dat", CFile::modeReadWrite);


	long length = myFile.GetLength();

	char * buf[length];

	myFile.Read(buf,length);

	buf[0] = 'p';

	buf[1]= 'k';

	myFile.Write(buf,length);	
}

Compiling...
readin2View.cpp
C:\readin2\readin2View.cpp(130) : error C2057: expected constant expression
C:\readin2\readin2View.cpp(130) : error C2466: cannot allocate an array of constant size 0
C:\readin2\readin2View.cpp(130) : error C2133: 'buf' : unknown size
C:\readin2\readin2View.cpp(134) : error C2440: '=' : cannot convert from 'const char' to 'char *'
        Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
C:\readin2\readin2View.cpp(136) : error C2440: '=' : cannot convert from 'const char' to 'char *'
        Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
Error executing cl.exe.

readin2View.obj - 5 error(s), 0 warning(s)

void CReadin2View::OnButton2() 
{
CFile myFile;

myFile("c:\\abc.dat", CFile::modeCreate |CFile::modeWrite	);

myFile.Write ( <code>....how to write some binary stuff to this file?</code>..)


}

char * buf;
buf = new char[length]

(!(fpIn = fopen(inputFile,"rb"))