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File open mode constants are used to control the access mode to a file or stream. 
 
Unit
 
SysUtils
 
Category
 
file management routines
 
Delphi syntax:
 
On Windows:
 
  const
 
    fmCreate         = $FFFF;
    fmOpenRead       = $0000;
    fmOpenWrite      = $0001;
    fmOpenReadWrite  = $0002;
 
    fmShareCompat    = $0000 platform;
    fmShareExclusive = $0010;
    fmShareDenyWrite = $0020;
    fmShareDenyRead  = $0030 platform;
    fmShareDenyNone  = $0040;
 
On Linux:
 
  const
 
    fmOpenRead       = O_RDONLY;
    fmOpenWrite      = O_WRONLY;
    fmOpenReadWrite  = O_RDWR;
    fmShareExclusive = $0010;
    fmShareDenyWrite = $0020;
    fmShareDenyNone  = $0030;
 
C++ syntax:
 
static const Word fmCreate = 0xffff;
 
static const Shortint fmOpenRead = 0x0;
static const Shortint fmOpenWrite = 0x1;
static const Shortint fmOpenReadWrite = 0x2;
static const Shortint fmShareCompat = 0x0;
static const Shortint fmShareExclusive = 0x10;
static const Shortint fmShareDenyWrite = 0x20;
static const Shortint fmShareDenyRead = 0x30;
static const Shortint fmShareDenyNone = 0x40;
 
Description
 
The file open mode constants are used when a file or stream is opened to control how it can be shared.
 
The TFileStream constructor has a Mode parameter that you can set to one of these constants:
 
Constant    Definition
 
fmCreate    If the file exists, open for write access, otherwise, create a new file. Unlike the other constants, which are declared in the SysUtils unit, this constant is declared in tge classes unit.
fmOpenRead  Open for read access only.
fmOpenWrite Open for write access only.
fmOpenReadWrite Open for read and write access.
fmShareCompat   Compatible with the way FCBs are opened. Do not use this mode in cross-platform applications.
fmShareExclusive    Read and write access is denied.
 
fmShareDenyWrite    Write access is denied.
fmShareDenyRead Read access is denied. Do not use this mode in cross-platform applications.
fmShareDenyNone Allows full access for others.