| Platform SDK: Files and I/O |
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 or canceled and the calling thread is in an alertable wait state.
BOOL WriteFileEx( HANDLE hFile, // handle to output file LPCVOID lpBuffer, // data buffer DWORD nNumberOfBytesToWrite, // number of bytes to write LPOVERLAPPED lpOverlapped, // overlapped buffer LPOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine // completion routine );
Windows NT/ 2000: 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.
Windows 95/98: This parameter can be a communications resource opened with the FILE_FLAG_OVERLAPPED flag by CreateFile, or a socket handle returned by socket or accept. You cannot perform asynchronous write operations on mailslots, named pipes, or disk files.
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.
If nNumberOfBtyesToWrite is zero, this function does nothing; in particular, it does not truncate the file. For additional discussion, see the following Remarks section.
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, Offset and OffsetHigh are ignored.
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 queuing 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.
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.
When using WriteFileEx 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 WriteFileEx 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.
An application must meet certain requirements when working with files opened with FILE_FLAG_NO_BUFFERING:
If part of the output file is locked by another process, and the specified write operation overlaps the locked portion, the WriteFileEx function fails.
Accessing the output buffer while a write operation is using the buffer may lead to corruption of the data written from that buffer. Applications must not read from, write to, reallocate, or free the output buffer that a write operation is using until the write operation completes.
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.
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 95/98: 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.
Windows NT/2000: Requires Windows NT 3.1 or later.
Windows 95/98: Requires Windows 95 or later.
Header: Declared in Winbase.h; include Windows.h.
Library: Use Kernel32.lib.
File I/O Overview, File I/O Functions, CancelIo, CreateFile, FileIOCompletionRoutine, MsgWaitForMultipleObjectsEx, OVERLAPPED, ReadFileEx, SetEndOfFile, SetErrorMode, SleepEx, SignalObjectAndWait, WaitForMultipleObjectsEx, WaitForSingleObjectEx, WriteFile