please dont rip this site Prev Next

WriteFileEx info  Overview  Group

The WriteFileEx function writes data to a file. It is designed solely for asynchronous operation, unlike WriteFile, which is designed for both synchronous and asynchronous operation.

WriteFileEx reports its completion status asynchronously, calling a specified completion routine when writing is completed and the calling thread is in an alertable wait state.

BOOL WriteFileEx(

    HANDLE hFile, 

// handle to output file

    LPCVOID lpBuffer, 

// pointer to input buffer

    DWORD nNumberOfBytesToWrite, 

// number of bytes to write

    LPOVERLAPPED lpOverlapped, 

// pointer to async. i/o data


// ptr. to completion routine



An open handle that specifies the file entity to be written to. This file handle must have been created with the FILE_FLAG_OVERLAPPED flag and with GENERIC_WRITE 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 the buffer containing the data to be written to the file.

This buffer must remain valid for the duration of the write operation. The caller must not use this buffer until the write operation is completed.

Specifies the number of bytes to write to the file.

If nNumberOfBtyesToWrite is zero, this function does nothing; in particular, it does not truncate the file. For additional discussion, see the following Remarks section.

Points to an OVERLAPPED data structure that supplies data to be used during the overlapped (asynchronous) write operation.

For files that support byte offsets, you must specify a byte offset at which to start writing to the file. You specify this offset by setting the Offset and OffsetHigh members of the OVERLAPPED structure. For files that do not support byte offsets ¾ named pipes, for example ¾ you must set Offset and OffsetHigh to zero, or WriteFileEx fails.

The WriteFileEx 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 WriteFileEx call. WriteFileEx signals completion of its writing operation by calling, or queueing a call to, the completion routine pointed to by lpCompletionRoutine, so it does not need an event handle.

The WriteFileEx function does use the Internal and InternalHigh members of the OVERLAPPED structure. You should not change the value of these members.

The OVERLAPPED data structure must remain valid for the duration of the write operation. It should not be a variable that can go out of scope while the write operation is pending completion.

Points to a completion routine to be called when the write operation has been completed and the calling thread is in an alertable wait state. For more information about this 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 WriteFileEx function succeeds, the calling thread has an asynchronous I/O (input/output) operation pending: the overlapped write operation to the file. When this I/O operation finishes, and the calling thread is blocked in an alertable wait state, the operating system calls the function pointed to by lpCompletionRoutine, and the wait completes with a return code of WAIT_IO_COMPLETION.

If the function succeeds and the file-writing operation finishes, but the calling thread is not in an alertable wait state, the system queues the call to *lpCompletionRoutine, holding the call until the calling thread enters an alertable wait state. See Synchronization for more information about alertable wait states and overlapped input/output operations.


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

Applications must not read from nor write to the output buffer that a write operation is using until the write operation completes. Premature access of the output buffer may lead to corruption of the data written from that buffer.

The WriteFileEx function may fail, returning the messages ERROR_INVALID_USER_BUFFER or ERROR_NOT_ENOUGH_MEMORY if there are too many outstanding asynchronous I/O requests.

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 write to 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 byte offsets, the Offset and OffsetHigh members of the OVERLAPPED structure pointed to by lpOverlapped must be zero, or the WriteFileEx function fails.

An application uses the WaitForSingleObjectEx, WaitForMultipleObjectsEx, MsgWaitForMultipleObjectsEx, SignalObjectAndWait, and SleepEx functions to enter an alertable wait state. Refer to Synchronization for more information about alertable wait states and overlapped input/output operations.

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

See Also

CancelIo, CreateFile, FileIOCompletionRoutine, MsgWaitForMultipleObjectsEx, OVERLAPPED, ReadFileEx, SetEndOfFile, SetErrorMode, SleepEx, SignalObjectAndWait, WaitForMultipleObjectsEx, WaitForSingleObjectEx, WriteFile 


file: /Techref/os/win/api/win32/func/src/f92_22.htm, 9KB, , updated: 2001/6/26 10:35, local time: 2024/7/14 22:22,

 ©2024 These pages are served without commercial sponsorship. (No popup ads, etc...).Bandwidth abuse increases hosting cost forcing sponsorship or shutdown. This server aggressively defends against automated copying for any reason including offline viewing, duplication, etc... Please respect this requirement and DO NOT RIP THIS SITE. Questions?
Please DO link to this page! Digg it! / MAKE!

<A HREF=""> WriteFileEx</A>

After you find an appropriate page, you are invited to your to this massmind site! (posts will be visible only to you before review) Just type a nice message (short messages are blocked as spam) in the box and press the Post button. (HTML welcomed, but not the <A tag: Instead, use the link box to link to another page. A tutorial is available Members can login to post directly, become page editors, and be credited for their posts.

Link? Put it here: 
if you want a response, please enter your email address: 
Attn spammers: All posts are reviewed before being made visible to anyone other than the poster.
Did you find what you needed?


Welcome to!


Welcome to!