WriteFile

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.

This function is designed for both synchronous and asynchronous operation. The WriteFileEx function is designed solely for asynchronous operation. It lets an application perform other processing during a file write operation.

BOOL WriteFile(
  HANDLE hFile,                    // handle to file
  LPCVOID lpBuffer,                // data buffer
  DWORD nNumberOfBytesToWrite,     // number of bytes to write
  LPDWORD lpNumberOfBytesWritten,  // number of bytes written
  LPOVERLAPPED lpOverlapped        // overlapped buffer
);

Parameters

hFile

[in] Handle to the file to be written to. The file handle must have been created with GENERIC_WRITE access to the file.

Windows NT/2000: For asynchronous write operations, 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 function.

Windows 95/98: For asynchronous write operations, hFile 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.

lpBuffer

[in] Pointer to the buffer containing the data to be written to the file.

nNumberOfBytesToWrite

[in] Specifies the number of bytes to write to the file.

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.

lpNumberOfBytesWritten

[out] Pointer to the variable that receives teh number of bytes written. WriteFile sets this value to zero before doing any work or error checking.

Windows NT/2000: If lpOverlapped is NULL, lpNumberOfBytesWritten cannot be NULL. 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 95/98: This parameter cannot be NULL.

lpOverlapped

[in] Pointer to an OVERLAPPED structure. This structure is required if hFile was opened with FILE_FLAG_OVERLAPPED.

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.

Windows NT/2000: 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.

Windows 95/98: For operations on files, disks, pipes, or mailslots, this parameter must be NULL; a pointer to an OVERLAPPED structure causes the call to fail. However, Windows 95/98 supports overlapped I/O on serial and parallel ports.

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.

Remarks

An application must meet certain requirements when working with files opened with FILE_FLAG_NO_BUFFERING:

• File access must begin at byte offsets within the file that are integer multiples of the volume's sector size. To determine a volume's sector size, call the GetDiskFreeSpace function.
• File access must be for numbers of bytes that are integer multiples of the volume's sector size. For example, if the sector size is 512 bytes, an application can request reads and writes of 512, 1024, or 2048 bytes, but not of 335, 981, or 7171 bytes.
• Buffer addresses for read and write operations must be sector aligned (aligned on addresses in memory that are integer multiples of the volume's sector size). One way to sector align buffers is to use the VirtualAlloc function to allocate the buffers. This function allocates memory that is aligned on addresses that are integer multiples of the system's page size. Because both page and volume sector sizes are powers of 2, memory aligned by multiples of the system's page size is also aligned by multiples of the volume's sector size.

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.

MAPI: For more information, see Syntax and Limitations for Win32 Functions Useful in MAPI Development.

Requirements

  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.

See Also

File I/O Overview, File I/O Functions, CancelIo, CreateFile, GetLastError, GetOverlappedResult, GetQueuedCompletionStatus, OVERLAPPED, ReadFile, SetEndOfFile, SetErrorMode, WriteFileEx