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ReadFileEx info  Overview  Group

The ReadFileEx function reads data from a file asynchronously. It is designed solely for asynchronous operation, unlike the ReadFile function, which is designed for both synchronous and asynchronous operation. ReadFileEx lets an application perform other processing during a file read operation.

The ReadFileEx function reports its completion status asynchronously, calling a specified completion routine when reading is completed and the calling thread is in an alertable wait state.

BOOL ReadFileEx(

    HANDLE hFile,

// handle of file to read

    LPVOID lpBuffer,

// address of buffer

    DWORD nNumberOfBytesToRead,

// number of bytes to read

    LPOVERLAPPED lpOverlapped,

// address of offset


// address of completion routine



An open handle that specifies the file entity to be read from. This file handle must have been created with the FILE_FLAG_OVERLAPPED flag and must have GENERIC_READ access to the file.

Windows NT: hFile can be any handle opened with the FILE_FLAG_OVERLAPPED flag by the CreateFile function, or a socket handle returned by the socket or accept functions.

Windows 95: hFile can be a communications resource, mailslot, or named pipe handle opened with the FILE_FLAG_OVERLAPPED flag by CreateFile, or a socket handle returned by the socket or accept functions. Windows 95 does not support asynchronous operations on disk files.

Points to a buffer that receives the data read from the file.

This buffer must remain valid for the duration of the read operation. The application should not use this buffer until the read operation is completed.

Specifies the number of bytes to be read from the file.
Points to an OVERLAPPED data structure that supplies data to be used during the asynchronous (overlapped) file read operation.

If the file specified by hFile supports the concept of byte offsets, the caller of ReadFileEx must specify a byte offset within the file at which reading should begin. The caller specifies the byte offset by setting the OVERLAPPED structure’s Offset and OffsetHigh members.

If the file entity specified by hFile does not support the concept of byte offsets ¾ for example, if it is a named pipe ¾ the caller must set the Offset and OffsetHigh members to zero, or ReadFileEx fails.

The ReadFileEx function ignores the OVERLAPPED structure’s hEvent member. An application is free to use that member for its own purposes in the context of a ReadFileEx call. ReadFileEx signals completion of its read operation by calling, or queueing a call to, the completion routine pointed to by lpCompletionRoutine, so it does not need an event handle.

The ReadFileEx function does use the OVERLAPPED structure’s Internal and InternalHigh members. An application should not set these members.

The OVERLAPPED data structure pointed to by lpOverlapped must remain valid for the duration of the read operation. It should not be a variable that can go out of scope while the file read operation is in progress.

Points to the completion routine to be called when the read operation is complete and the calling thread is in an alertable wait state. For more information about the completion routine, see FileIOCompletionRoutine.

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 GetLastError.

If the function succeeds, the calling thread has an asynchronous I/O (input/output) operation pending: the overlapped read operation from the file. When this I/O operation completes, and the calling thread is blocked in an alertable wait state, the system calls the function pointed to by lpCompletionRoutine, and the wait state completes with a return code of WAIT_IO_COMPLETION.

If the function succeeds, and the file reading operation completes, but the calling thread is not in an alertable wait state, the system queues the completion routine call, holding the call until the calling thread enters an alertable wait state. For information about alertable waits and overlapped input/output operations, see Synchronization and Overlapped Input and Output.

If ReadFileEx attempts to read past the end of the file, the function returns zero, and GetLastError returns ERROR_HANDLE_EOF.


If a portion of the file specified by hFile is locked by another process, and the read operation specified in a call to ReadFileEx overlaps the locked portion, the call to ReadFileEx fails.

If ReadFileEx attempts to read data from a mailslot whose buffer is too small, the function returns FALSE, and GetLastError returns ERROR_INSUFFICIENT_BUFFER.

Applications must not read from nor write to the input buffer that a read operation is using until the read operation completes. A premature access to the input buffer may lead to corruption of the data read into that buffer.

The ReadFileEx function may fail if there are too many outstanding asynchronous I/O requests. In the event of such a failure, GetLastError can return ERROR_INVALID_USER_BUFFER or ERROR_NOT_ENOUGH_MEMORY.

To cancel all pending asynchronous I/O operations, use the CancelIo function. This function only cancels operations issued by the calling thread for the specified file handle. I/O operations that are canceled complete with the error ERROR_OPERATION_ABORTED.

If you are attempting to read from a floppy drive that does not have a floppy disk, the system displays a message box prompting the user to retry the operation. To prevent the system from displaying this message box, call the SetErrorMode function with SEM_NOOPENFILEERRORBOX.

If hFile is a handle to a named pipe or other file entity that doesn’t support the byte-offset concept, the Offset and OffsetHigh members of the OVERLAPPED structure pointed to by lpOverlapped must be zero, or ReadFileEx fails.

An application uses the MsgWaitForMultipleObjectsEx, WaitForSingleObjectEx, WaitForMultipleObjectsEx, and SleepEx functions to enter an alertable wait state. For more information about alertable waits and overlapped input/output, refer to those functions’ reference and Synchronization.

Windows 95: On this platform, neither ReadFileEx nor WriteFileEx can be used by the comm ports to communicate. However, you can use ReadFile and WriteFile to perform asynchronous communication.

See Also

CancelIo, CreateFile, FileIOCompletionRoutine, MsgWaitForMultipleObjectsEx, OVERLAPPED, ReadFile, SetErrorMode, SleepEx, WaitForMultipleObjectsEx, WaitForSingleObjectEx, WriteFileEx 


file: /Techref/os/win/api/win32/func/src/f71_12.htm, 10KB, , updated: 2001/6/26 10:35, local time: 2024/7/14 21:51,

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