Delpi FileStream Create Mode, FileOpen Access Mode

  fmOpenRead       = $0000;
  fmOpenWrite      = $0001;
  fmOpenReadWrite  = $0002;
  fmExclusive      = $0004; // when used with FileCreate, atomically creates the file only if it doesn't exist, fails otherwise

  fmShareCompat    = $0000 platform; // DOS compatibility mode is not portable
  fmShareExclusive = $0010;
  fmShareDenyWrite = $0020;
  fmShareDenyRead  = $0030 platform; // write-only not supported on all platforms
  fmShareDenyNone  = $0040;

constructor Create(const AFileName: string; Mode: Word); overload;

Call Create to instantiate a file stream for reading from or writing to the named file. Specify the name of the file and the way the file should be opened as parameters. 

function FileOpen(const FileName: string; Mode: LongWord): Integer;

Use FileOpen to open a file and obtain a file handle. The access mode value is constructed by or-ing one of the fmOpen constants with one of the fmShare constants defined in File open mode constants. If the return value is 0 or greater, the function was successful and the value is the file handle of the opened file. A return value of -1 indicates that an error occurred. 

The Mode parameter indicates how the file is to be opened. The Mode parameter consists of an open mode and (possibly) a share mode or'ed together. The open mode must be one of the following values:

Value	Meaning
fmCreate	Create a file with the given name. If a file with the given name exists, open the file in write mode.
fmOpenRead	Open the file for reading only.
fmOpenWrite	Open the file for writing only. Writing to the file completely replaces the current contents.
fmOpenReadWrite	Open the file to modify the current contents rather than replace them.

The share mode must be one of the following values:

Value	Meaning
fmShareCompat	Sharing is compatible with the way FCBs are opened.
fmShareExclusive	Other applications can not open the file for any reason.
fmShareDenyWrite	Other applications can open the file for reading but not for writing.
fmShareDenyRead	Other applications can open the file for writing but not for reading.
fmShareDenyNone	No attempt is made to prevent other applications from reading from or writing to the file.

If the file can not be opened, Create raises an exception.