This example writes to a file named "DOSMAN.DAT", resets the buffer, and changes the file size.

#define INCL_DOSFILEMGR          /* File Manager values */
#define INCL_DOSERRORS           /* DOS Error values    */
#include <os2.h>
#include <stdio.h>
#include <string.h>

int main(VOID) {
   HFILE  hfFileHandle   = 0L;     /* Handle for file being manipulated */
   ULONG  ulAction       = 0;      /* Action taken by DosOpen */
   ULONG  ulWrote        = 0;      /* Number of bytes written by DosWrite */
   UCHAR  uchFileName[20]  = "dosman.dat",     /* Name of file */
          uchFileData[4]   = "DATA";            /* Data to write to file */
   APIRET rc             = NO_ERROR;            /* Return code */

   /* Open the file dosman.dat.  Use an existing file or create a new */
   /* one if it doesn't exist.                                      */
   rc = DosOpen(uchFileName, &hfFileHandle, &ulAction, 4L,
                FILE_ARCHIVED | FILE_NORMAL,
                OPEN_ACTION_CREATE_IF_NEW | OPEN_ACTION_OPEN_IF_EXISTS,
                OPEN_FLAGS_NOINHERIT | OPEN_SHARE_DENYNONE  |
                OPEN_ACCESS_READWRITE, 0L);
   if (rc != NO_ERROR) {
      printf("DosOpen error: return code = %u\n", rc);
      return 1;
   }

   rc = DosWrite (hfFileHandle, (PVOID) uchFileData,
                  sizeof (uchFileData), &ulWrote);
   if (rc != NO_ERROR) {
      printf("DosWrite error: return code = %u\n", rc);
      return 1;
   }

   rc = DosResetBuffer (hfFileHandle);
   if (rc != NO_ERROR) {
      printf("DosResetBuffer error: return code = %u\n", rc);
      return 1;
   } /* endif */

   rc = DosSetFileSize (hfFileHandle, 8L);    /* Change file size */
   if (rc != NO_ERROR) {
      printf("DosSetFileSize error: return code = %u\n", rc);
      return 1;
   }

   return NO_ERROR;
}