The WriteFile function writes data to a file and is designed for both synchronous and asynchronous operation. The function starts writing data to the file at the position indicated by the file pointer. After the write operation has been completed, the file pointer is adjusted by the number of bytes actually written, except when the file is opened with FILE_FLAG_OVERLAPPED. If the file handle was created for overlapped input and output (I/O), the application must adjust the position of the file pointer after the write operation is finished.
BOOL WriteFile( HANDLE hFile, // handle to file to write to LPCVOID lpBuffer, // pointer to data to write to file DWORD nNumberOfBytesToWrite, // number of bytes to write LPDWORD lpNumberOfBytesWritten, // pointer to number of bytes written LPOVERLAPPED lpOverlapped // pointer to structure for overlapped I/O );
A value of zero specifies a null write operation. A null write operation does not write any bytes but does cause the time stamp to change.
Named pipe write operations across a network are limited to 65,535 bytes.
Windows NT: If lpOverlapped is NULL, lpNumberOfBytesWritten cannot be NULL.
Windows NT: If lpOverlapped is not NULL, lpNumberOfBytesWritten can be NULL. If this is an overlapped write operation, you can get the number of bytes written by calling GetOverlappedResult. If hFile is associated with an I/O completion port, you can get the number of bytes written by calling GetQueuedCompletionStatus.
Windows 95 and Windows 98: This parameter cannot be NULL.
If hFile was opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must not be NULL. It must point to a valid OVERLAPPED structure. If hFile was opened with FILE_FLAG_OVERLAPPED and lpOverlapped is NULL, the function can incorrectly report that the write operation is complete.
If hFile was opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the write operation starts at the offset specified in the OVERLAPPED structure and WriteFile may return before the write operation has been completed. In this case, WriteFile returns FALSE and the GetLastError function returns ERROR_IO_PENDING. This allows the calling process to continue processing while the write operation is being completed. The event specified in the OVERLAPPED structure is set to the signaled state upon completion of the write operation.
If hFile was not opened with FILE_FLAG_OVERLAPPED and lpOverlapped is NULL, the write operation starts at the current file position and WriteFile does not return until the operation has been completed.
If hFile was not opened with FILE_FLAG_OVERLAPPED and lpOverlapped is not NULL, the write operation starts at the offset specified in the OVERLAPPED structure and WriteFile does not return until the write operation has been completed.
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.
An application must meet certain requirements when working with files opened with FILE_FLAG_NO_BUFFERING:
If part of the file is locked by another process and the write operation overlaps the locked portion, this 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.
Characters can be written to the screen buffer using WriteFile with a handle to console output. The exact behavior of the function is determined by the console mode. The data is written to the current cursor position. The cursor position is updated after the write operation.
The system interprets zero bytes to write as specifying a null write operation and WriteFile does not truncate or extend the file. To truncate or extend a file, use the SetEndOfFile function.
When writing to a nonblocking, byte-mode pipe handle with insufficient buffer space, WriteFile returns TRUE with *lpNumberOfBytesWritten < nNumberOfBytesToWrite.
When an application uses the WriteFile function to write to a pipe, the write operation may not finish if the pipe buffer is full. The write operation is completed when a read operation (using the ReadFile function) makes more buffer space available.
If the anonymous read pipe handle has been closed and WriteFile attempts to write using the corresponding anonymous write pipe handle, the function returns FALSE and GetLastError returns ERROR_BROKEN_PIPE.
The WriteFile function may fail with ERROR_INVALID_USER_BUFFER or ERROR_NOT_ENOUGH_MEMORY whenever 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.
Windows CE: Windows CE does not support asynchronous write operations. The lpOverlapped parameter is ignored and should be set to NULL before calling WriteFile.
Unlike the MS-DOS operating system, Windows CE interprets zero bytes to write as specifying a null write operation and WriteFile does not truncate or extend the file. To truncate or extend a file, use the SetEndOfFile function.
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Requires version 1.0 or later.
Header: Declared in winbase.h.
Import Library: Use kernel32.lib.
File I/O Overview, File Functions, CancelIo, CreateFile, GetLastError, GetOverlappedResult, GetQueuedCompletionStatus, OVERLAPPED, ReadFile, SetEndOfFile, SetErrorMode, WriteFileEx