After a file has been opened, the file handle state flags set with a DosOpen can be queried and reset by calling DosQueryFHState and DosSetFHState . The handle returned by DosSetFHState is used for subsequent input and output to the file.

The following code fragment calls DosSetFHState to set the File Write-Through flag for an opened file. Writes to the file may go through the file system buffer, but the sectors are to be written before any synchronous write call returns. DosQueryFHState is called first to obtain the file handle state bits. Assume that the appropriate file handle has been placed into FileHandle already.

    #define INCL_DOSFILEMGR        /* File system values */
    #include <os2.h>
    #include <stdio.h>

    HFILE   hfFileHandle;         /* File handle        */
    ULONG   ulFileHandleState;    /* File handle state  */
    APIRET  ulrc;                 /* Return code        */

    ulrc = DosQueryFHState(hfFileHandle,
                           &FileHandleState);

    if (ulrc != 0) {
        printf("DosQueryFHState error: return code = %ld",
               ulrc);
        return;
    }

    ulFileHandleState |= OPEN_FLAGS_WRITE_THROUGH;

    ulrc = DosSetFHState(hfFileHandle,
                         ulFileHandleState);

    if (ulrc != 0) {
        printf("DosSetFHState error: return code = %ld",
               ulrc);
        return;
    }

Here are two scenarios involving the use of this function.

• An application requires that data be written in a specific order. To guarantee the order of the data written, it must perform separate synchronous write operations. The application can call DosSetFHState to set the Write-Through flag for the file. This action does not affect any previous asynchronous writes, whose data can remain in the buffers.

• An application cannot handle a certain critical error situation. DosSetFHState can be called to reset critica l error handling as OS/2's responsibility. The I/O function that caused the critical error must be called again so the error can recur, causing control to be passed to OS/2. In the case where asynchronous I/O is being done, the precise time the results of this function will be available to the application is unpredictable.