FlushFileBuffers function (fileapi.h)

Article
10/13/2021

Flushes the buffers of a specified file and causes all buffered data to be written to a file.

Syntax

BOOL FlushFileBuffers(
  [in] HANDLE hFile
);

Parameters

[in] hFile

A handle to the open file.

The file handle must have the GENERIC_WRITE access right. For more information, see File Security and Access Rights.

If hFile is a handle to a communications device, the function only flushes the transmit buffer.

If hFile is a handle to the server end of a named pipe, the function does not return until the client has read all buffered data from the pipe.

Return value

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

The function fails if hFile is a handle to the console output. That is because the console output is not buffered. The function returns FALSE, and GetLastError returns ERROR_INVALID_HANDLE.

Remarks

Typically the WriteFile and WriteFileEx functions write data to an internal buffer that the operating system writes to a disk or communication pipe on a regular basis. The FlushFileBuffers function writes all the buffered information for a specified file to the device or pipe.

Due to disk caching interactions within the system, the FlushFileBuffers function can be inefficient when used after every write to a disk drive device when many writes are being performed separately. If an application is performing multiple writes to disk and also needs to ensure critical data is written to persistent media, the application should use unbuffered I/O instead of frequently calling FlushFileBuffers. To open a file for unbuffered I/O, call the CreateFile function with the FILE_FLAG_NO_BUFFERING and FILE_FLAG_WRITE_THROUGH flags. This prevents the file contents from being cached and flushes the metadata to disk with each write. For more information, see CreateFile.

To flush all open files on a volume, call FlushFileBuffers with a handle to the volume. The caller must have administrative privileges. For more information, see Running with Special Privileges.

When opening a volume with CreateFile, the lpFileName string should be the following form: \.\x: or \?\Volume{GUID}. Do not use a trailing backslash in the volume name, because that indicates the root directory of a drive.

In Windows 8 and Windows Server 2012, this function is supported by the following technologies.

Technology	Supported
Server Message Block (SMB) 3.0 protocol	Yes
SMB 3.0 Transparent Failover (TFO)	Yes
SMB 3.0 with Scale-out File Shares (SO)	Yes
Cluster Shared Volume File System (CsvFS)	Yes
Resilient File System (ReFS)	Yes

Examples

For an example, see Multithreaded Pipe Server.

Requirements

Requirement	Value
Minimum supported client	Windows XP [desktop apps \| UWP apps]
Minimum supported server	Windows Server 2003 [desktop apps \| UWP apps]
Target Platform	Windows
Header	fileapi.h (include Windows.h)
Library	Kernel32.lib
DLL	Kernel32.dll