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ReadFileScatter function

Reads data from a file and stores it in an array of buffers.

The function starts reading data from the file at a position that is specified by an OVERLAPPED structure. The ReadFileScatter function operates asynchronously.


BOOL WINAPI ReadFileScatter(
  _In_       HANDLE               hFile,
  _In_       FILE_SEGMENT_ELEMENT aSegmentArray[],
  _In_       DWORD                nNumberOfBytesToRead,
  _Reserved_ LPDWORD              lpReserved,
  _Inout_    LPOVERLAPPED         lpOverlapped


hFile [in]

A handle to the file to be read.

The file handle must be created with the GENERIC_READ right, and the FILE_FLAG_OVERLAPPED and FILE_FLAG_NO_BUFFERING flags. For more information, see File Security and Access Rights.

aSegmentArray [in]

A pointer to an array of FILE_SEGMENT_ELEMENT buffers that receives the data. For a description of this union, see Remarks.

Each element can receive one page of data.

Note  To determine the size of a system page, use GetSystemInfo.

The array must contain enough elements to store nNumberOfBytesToRead bytes of data, plus one element for the terminating NULL. For example, if there are 40 KB to be read and the page size is 4 KB, the array must have 11 elements that includes 10 for the data and one for the NULL.

Each buffer must be at least the size of a system memory page and must be aligned on a system memory page size boundary. The system reads one system memory page of data into each buffer.

The function stores the data in the buffers in sequential order. For example, it stores data into the first buffer, then into the second buffer, and so on until each buffer is filled and all the data is stored, or there are no more buffers.

nNumberOfBytesToRead [in]

The total number of bytes to be read from the file. Each element of aSegmentArray contains a one-page chunk of this total. Because the file must be opened with FILE_FLAG_NO_BUFFERING, the number of bytes must be a multiple of the sector size of the file system where the file is located.


This parameter is reserved for future use and must be NULL.

lpOverlapped [in, out]

A pointer to an OVERLAPPED data structure.

The ReadFileScatter function requires a valid OVERLAPPED structure. The lpOverlapped parameter cannot be NULL.

The ReadFileScatter function starts reading data from the file at a position that is specified by the Offset and OffsetHigh members of the OVERLAPPED structure.

The ReadFileScatter function may return before the read operation is complete. In that scenario, the ReadFileScatter function returns the value 0 (zero), and the GetLastError function returns the value ERROR_IO_PENDING. This asynchronous operation of ReadFileScatter lets the calling process continue while the read operation completes. You can call the GetOverlappedResult, HasOverlappedIoCompleted, or GetQueuedCompletionStatus functions to obtain information about the completion of the read operation. For more information, see Synchronous and Asynchronous I/O.

Return value

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero (0). To get extended error information, call the GetLastError function.

If ReadFileScatter attempts to read past the end-of-file (EOF), the call to GetOverlappedResult for that operation returns FALSE and GetLastError returns ERROR_HANDLE_EOF.

If the function returns before the read operation is complete, the function returns zero (0), and GetLastError returns ERROR_IO_PENDING.


This function is not supported for 32-bit applications by WOW64 on Itanium-based systems.

The FILE_SEGMENT_ELEMENT union is defined as follows:

typedef union _FILE_SEGMENT_ELEMENT {
    PVOID64 Buffer;
    ULONGLONG Alignment;

Assigning a pointer to the Buffer member will sign-extend the value if the code is compiled as 32-bits; this can break large-address aware applications running on systems configured with 4-Gigabyte Tuning or running on under WOW64 on 64-bit Windows. Therefore, use the PtrToPtr64 macro when assigning pointers to Buffer.

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


Server Message Block (SMB) 3.0 protocol


SMB 3.0 Transparent Failover (TFO)


SMB 3.0 with Scale-out File Shares (SO)


Cluster Shared Volume File System (CsvFS)


Resilient File System (ReFS)



Transacted Operations

If there is a transaction bound to the file handle, then the function returns data from the transacted view of the file. A transacted read handle is guaranteed to show the same view of a file for the duration of the handle.


Minimum supported client

Windows XP [desktop apps only]

Minimum supported server

Windows Server 2003 [desktop apps only]


FileAPI.h (include Windows.h);
WinBase.h on Windows Server 2008 R2, Windows 7, Windows Server 2008, Windows Vista, Windows Server 2003 and Windows XP (include Windows.h)





See also

File Management Functions