WriteFileGather

The WriteFileGather function gathers data from a set of buffers and writes the data to a file.

The WriteFileGather function starts writing data to the file at a position specified by an OVERLAPPED structure.

The WriteFileGather function operates asynchronously.

BOOL WriteFileGather(
  HANDLE hFile,              // handle to a file to write data to
  FILE_SEGMENT_ELEMENT aSegmentArray [],
                             // pointer to an array of buffer 
                             // pointers
  DWORD nNumberOfBytesToWrite,  // number of bytes to write
  LPDWORD lpReserved,        // reserved; must be NULL
  LPOVERLAPPED lpOverlapped  // pointer to an asynchronous I/O data 
                             // structure
);
 

Parameters

hFile

An open handle to the file to write to.

This file handle must have been created using GENERIC_WRITE to specify write access to the file, FILE_FLAG_OVERLAPPED to specify asynchronous I/O, and FILE_FLAG_NO_BUFFERING to specify non-cached I/O.

aSegmentArray

Pointer to an array of FILE_SEGMENT_ELEMENT pointers to buffers. The function gathers the data it writes to the file from this set of buffers.

Each buffer should be the size of a system memory page. Each buffer should be aligned on a system memory page size boundary.

A FILE_SEGMENT_ELEMENT pointer is a 64-bit value. The WriteFileGather function uses all 64 bits. Since the operating systems do not currently support 64-bit memory addressing, you must explicitly zero the upper 32 bits of each FILE_SEGMENT_ELEMENT pointer.

The function gathers the data from the buffers in a sequential manner: it writes data to the file from the first buffer, then from the second buffer, then from the next, until there is no more data to write.

The final element of the array should be a NULL pointer.

nNumberOfBytesToWrite

Specifies the number of bytes to write to the file.

If nNumberOfBytesToWrite is zero, the function performs a null write operation. A null write operation does not write any bytes to the file, but it does cause the file's time stamp to change.

Note that this behavior differs from file writing functions on the MS-DOS platform, where a write count of zero bytes truncates a file. WriteFileGather does not truncate or extend the file. To truncate or extend a file on Windows NT, use the SetEndOfFile function..

lpReserved

This parameter is reserved for future use. You must set it to NULL.

lpOverlapped

Pointer to an OVERLAPPED data structure.

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

The WriteFileGather function starts writing data to the file at a position specified by the Offset and OffsetHigh members of the OVERLAPPED structure.

The WriteFileGather function may return before the write operation has completed. In that case, the WriteFileGather function returns the value zero, and the GetLastError function returns the value ERROR_IO_PENDING. This asynchronous operation of WriteFileGather lets the calling process continue while the write operation completes. You can call the GetOverlappedResult, HasOverlappedIoCompleted, or GetQueuedCompletionStatus function to obtain information about the completion of the write operation.

Return Values

If the function succeeds, the return value is nonzero.

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

If the function returns before the write operation has completed, the function returns zero, and GetLastError returns ERROR_IO_PENDING.

Remarks

If part of the file specified by hFile is locked by another process, and the write operation overlaps the locked portion, the WriteFileGather function fails.

QuickInfo

  Windows NT: Requires version 4.0 SP2 or later.
  Windows: Unsupported.
  Windows CE: Unsupported.
  Header: Declared in winbase.h.
  Import Library: Use kernel32.lib.

See Also

File I/O Overview, File Functions, CreateFile, GetOverlappedResult, GetQueuedCompletionStatus, HasOverlappedIoCompleted, OVERLAPPED, ReadFile, ReadFileEx, ReadFileScatter, ReadFileVlm