ReadFileEx function (fileapi.h)

Reads data from the specified file or input/output (I/O) device. It reports its completion status asynchronously, calling the specified completion routine when reading is completed or canceled and the calling thread is in an alertable wait state.

To read data from a file or device synchronously, use the ReadFile function.


BOOL ReadFileEx(
  [in]            HANDLE                          hFile,
  [out, optional] LPVOID                          lpBuffer,
  [in]            DWORD                           nNumberOfBytesToRead,
  [in, out]       LPOVERLAPPED                    lpOverlapped,
  [in]            LPOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine


[in] hFile

A handle to the file or I/O device (for example, a file, file stream, physical disk, volume, console buffer, tape drive, socket, communications resource, mailslot, or pipe).

This parameter 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 function.

This handle also must have the GENERIC_READ access right. For more information on access rights, see File Security and Access Rights.

[out, optional] lpBuffer

A pointer to a buffer that receives the data read from the file or device.

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.

[in] nNumberOfBytesToRead

The number of bytes to be read.

[in, out] lpOverlapped

A pointer to an OVERLAPPED data structure that supplies data to be used during the asynchronous (overlapped) file read operation.

For files that support byte offsets, you must specify a byte offset at which to start reading from the file. You specify this offset by setting the Offset and OffsetHigh members of the OVERLAPPED structure. For files or devices that do not support byte offsets, Offset and OffsetHigh are ignored.

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 queuing 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 must remain valid for the duration of the read operation. It should not be a variable that can go out of scope while the read operation is pending completion.

[in] lpCompletionRoutine

A pointer 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 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.

If the function succeeds, the calling thread has an asynchronous I/O 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 About Synchronization.

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


When using ReadFileEx you should check GetLastError even when the function returns "success" to check for conditions that are "successes" but have some outcome you might want to know about. For example, a buffer overflow when calling ReadFileEx will return TRUE, but GetLastError will report the overflow with ERROR_MORE_DATA. If the function call is successful and there are no warning conditions, GetLastError will return ERROR_SUCCESS.

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 either:

  • CancelIo—this function only cancels operations issued by the calling thread for the specified file handle.
  • CancelIoEx—this function cancels all operations issued by the threads for the specified file handle.
Use CancelSynchronousIo to cancel pending synchronous I/O operations.

I/O operations that are canceled complete with the error ERROR_OPERATION_ABORTED.

If part 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.

When attempting to read data from a mailslot whose buffer is too small, ReadFileEx returns FALSE, and GetLastError returns ERROR_INSUFFICIENT_BUFFER.

Accessing the input buffer while a read operation is using the buffer may lead to corruption of the data read into that buffer. Applications must not read from, write to, reallocate, or free the input buffer that a read operation is using until the read operation completes.

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, see About Synchronization.

There are strict requirements for successfully working with files opened with CreateFile using FILE_FLAG_NO_BUFFERING. For details see File Buffering.

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

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. For additional information, see About Transactional NTFS.


For an example, see Named Pipe Server Using Completion Routines.


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

See also





File Management Functions