How To Force Files to Be Flushed to Disk

MORE INFORMATION

To flush the C runtime buffers, you need a call to fflush for files that are opened with fopen or a call to the flush function for C++ ofstream objects. Flushing the operating system's disk cache is a little more difficult; it depends on the operating system in use.

16-bit Operating Systems - MS-DOS or Windows 3.1

In MS-DOS or Windows 3.1 running Smartdrv.exe version 4.0 or later, you have two choices. You can use the _commit C runtime function or link with Commode.obj and use the fflush C runtime function.

32-bit Windows Operating Systems

In 32-bit versions of Windows, the operating system has built-in disk caching. The only way to force a file to be flushed to disk is by linking to Commode.obj.

Commode.obj is designed to affect the way the C Runtime handles files. When you link to this .obj file, a call to the C runtime function fflush also forces the operating system to flush its cache to disk, making the call to _commit unnecessary.

Sample Code

   /* Compile options needed: none

      This sample code is designed to show some of the more
      common ways to flush both the C runtime buffers and the
      operating system cache
   */ 

   #include <stdio.h>
   #include <ofstream.h>

   void DoSomeCFileIO();
   void DoSomeCXXFileIO();

   void main()
   {
     DoSomeCFileIO()
     DoSomeCXXFileIO();
   }

   void DoSomeCFileIO()
   {
    FILE* CFileBuf;

   // Open CFileBuf for output & perform some writes

    fflush(CFileBuf);
    _commit(_fileno(CFileBuf));

   // The call to fflush will cause the C Runtime to flush
   // the buffers associated with CFileBuf to the
   // Operating system.
   // The call to _commit will tell Smartdrv.exe to flush
   // its cache to the disk.
   // The _commit function requires a file handle, hence the
   // call to _fileno
   // _commit will only function on 16-bit operating systems.
   // On 32-bit operating systems, you need to link to Commode.obj
   }

   void DoSomeCXXFileIO()
   {
    ofstream CXXFileBuf;

   // Open CXXFileBuf & perform some writes

    CXXFileBuf.flush();
    _commit(CXXFileBuf.rdbuf()->fd());

   // The call to flush causes the C Runtime to flush
   // the buffer associated with CXXFileBuf to the operating system.
   // 
   // The call to _commit tells Smartdrv.exe to flush
   // its cache to the disk.
   // 
   // The _commit function requires a file handle, hence the
   // call to ofstream.rdbuf()->fd()
   // _commit will only function on 16-bit operating systesm.
   // On 32-bit operating systems, you will need to link to Commode.obj.
   }