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OVERLAPPED

The OVERLAPPED structure contains information used in asynchronous input and output (I/O).

typedef struct _OVERLAPPED {
ULONG_PTR Internal;
ULONG_PTR InternalHigh;
DWORD Offset;
DWORD OffsetHigh;
HANDLE hEvent; } OVERLAPPED;

Members

Internal
Reserved for operating system use. This member, which specifies a system-dependent status, is valid when the GetOverlappedResult function returns without setting the extended error information to ERROR_IO_PENDING.
InternalHigh
Reserved for operating system use. This member, which specifies the length of the data transferred, is valid when the GetOverlappedResult function returns TRUE.
Offset
File position at which to start the transfer. The file position is a byte offset from the start of the file. The calling process sets this member before calling the ReadFile or WriteFile function. This member is ignored when reading from or writing to named pipes and communications devices and should be zero.
OffsetHigh
High-order word of the byte offset at which to start the transfer. This member is ignored when reading from or writing to named pipes and communications devices and should be zero.
hEvent
Handle to an event set to the signaled state when the operation has been completed. The calling process must set this member either to zero or a valid event handle before calling any overlapped functions. To create an event object, use the CreateEvent function.

Functions such as WriteFile set the event to the nonsignaled state before they begin an I/O operation.

Remarks

You can use the HasOverlappedIoCompleted macro to determine whether an asynchronous I/O operation has completed. You can use the CancelIo function to cancel an asynchronous I/O operation.

Requirements

Windows NT/2000/XP: Included in Windows NT 3.1 and later.
Windows 95/98/Me: Included in Windows 95 and later.
Header: Declared in Winbase.h; include Windows.h.

See Also

Synchronization Overview, Synchronization Structures, CancelIo, GetOverlappedResult, HasOverlappedIoCompleted, ReadFile, WriteFile

Platform SDK Release: August 2002
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