WriteFile function (fileapi.h)

Writes data to the specified file or input/output (I/O) device.

This function is designed for both synchronous and asynchronous operation. For a similar function designed solely for asynchronous operation, see WriteFileEx.

Syntax

BOOL WriteFile(
  [in]                HANDLE       hFile,
  [in]                LPCVOID      lpBuffer,
  [in]                DWORD        nNumberOfBytesToWrite,
  [out, optional]     LPDWORD      lpNumberOfBytesWritten,
  [in, out, optional] LPOVERLAPPED lpOverlapped
);

Parameters

[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).

The hFile parameter must have been created with the write access. For more information, see Generic Access Rights and File Security and Access Rights.

For asynchronous write operations, hFile can be any handle opened with the CreateFile function using the FILE_FLAG_OVERLAPPED flag or a socket handle returned by the socket or accept function.

[in] lpBuffer

A pointer to the buffer containing the data to be written to the file or device.

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.

[in] nNumberOfBytesToWrite

The number of bytes to be written to the file or device.

A value of zero specifies a null write operation. The behavior of a null write operation depends on the underlying file system or communications technology.

Windows Server 2003 and Windows XP: Pipe write operations across a network are limited in size per write. The amount varies per platform. For x86 platforms it's 63.97 MB. For x64 platforms it's 31.97 MB. For Itanium it's 63.95 MB. For more information regarding pipes, see the Remarks section.

[out, optional] lpNumberOfBytesWritten

A pointer to the variable that receives the number of bytes written when using a synchronous hFile parameter. WriteFile sets this value to zero before doing any work or error checking. Use NULL for this parameter if this is an asynchronous operation to avoid potentially erroneous results.

This parameter can be NULL only when the lpOverlapped parameter is not NULL.

Windows 7: This parameter can not be NULL.

For more information, see the Remarks section.

[in, out, optional] lpOverlapped

A pointer to an OVERLAPPED structure is required if the hFile parameter was opened with FILE_FLAG_OVERLAPPED, otherwise this parameter can be NULL.

For an hFile that supports byte offsets, if you use this parameter you must specify a byte offset at which to start writing to the file or device. This offset is specified by setting the Offset and OffsetHigh members of the OVERLAPPED structure. For an hFile that does not support byte offsets, Offset and OffsetHigh are ignored.

To write to the end of file, specify both the Offset and OffsetHigh members of the OVERLAPPED structure as 0xFFFFFFFF. This is functionally equivalent to previously calling the CreateFile function to open hFile using FILE_APPEND_DATA access.

For more information about different combinations of lpOverlapped and FILE_FLAG_OVERLAPPED, see the Remarks section and the Synchronization and File Position section.

Return value

If the function succeeds, the return value is nonzero (TRUE).

If the function fails, or is completing asynchronously, the return value is zero (FALSE). To get extended error information, call the GetLastError function.

Remarks

The WriteFile function returns when one of the following conditions occur:

• The number of bytes requested is written.
• A read operation releases buffer space on the read end of the pipe (if the write was blocked). For more information, see the Pipes section.
• An asynchronous handle is being used and the write is occurring asynchronously.
• An error occurs.

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 either:

• CancelIo — This function cancels only 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 the CancelSynchronousIo function to cancel pending synchronous I/O operations.

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

The WriteFile function may fail with ERROR_NOT_ENOUGH_QUOTA, which means the calling process's buffer could not be page-locked. For more information, see SetProcessWorkingSetSize.

If part of the file is locked by another process and the write operation overlaps the locked portion, WriteFile fails.

When writing to a file, the last write time is not fully updated until all handles used for writing have been closed. Therefore, to ensure an accurate last write time, close the file handle immediately after writing to the file.

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 write to, reallocate, or free the output buffer that a write operation is using until the write operation completes. This can be particularly problematic when using an asynchronous file handle. Additional information regarding synchronous versus asynchronous file handles can be found later in the Synchronization and File Position section and Synchronous and Asynchronous I/O.

Note that the time stamps may not be updated correctly for a remote file. To ensure consistent results, use unbuffered I/O.

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.

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. For more information about console handles, see CreateFile.

When writing to a communications device, the behavior of WriteFile is determined by the current communication time-out as set and retrieved by using the SetCommTimeouts and GetCommTimeouts functions. Unpredictable results can occur if you fail to set the time-out values. For more information about communication time-outs, see COMMTIMEOUTS.

Although a single-sector write is atomic, a multi-sector write is not guaranteed to be atomic unless you are using a transaction (that is, the handle created is a transacted handle; for example, a handle created using CreateFileTransacted). Multi-sector writes that are cached may not always be written to the disk right away; therefore, specify FILE_FLAG_WRITE_THROUGH in CreateFile to ensure that an entire multi-sector write is written to the disk without potential caching delays.

If you write directly to a volume that has a mounted file system, you must first obtain exclusive access to the volume. Otherwise, you risk causing data corruption or system instability, because your application's writes may conflict with other changes coming from the file system and leave the contents of the volume in an inconsistent state. To prevent these problems, the following changes have been made in Windows Vista and later:

• A write on a volume handle will succeed if the volume does not have a mounted file system, or if one of the following conditions is true:
  • The sectors to be written to are boot sectors.
  • The sectors to be written to reside outside of file system space.
  • You have explicitly locked or dismounted the volume by using FSCTL_LOCK_VOLUME or FSCTL_DISMOUNT_VOLUME.
  • The volume has no actual file system. (In other words, it has a RAW file system mounted.)
• A write on a disk handle will succeed if one of the following conditions is true:
  • The sectors to be written to do not fall within a volume's extents.
  • The sectors to be written to fall within a mounted volume, but you have explicitly locked or dismounted the volume by using FSCTL_LOCK_VOLUME or FSCTL_DISMOUNT_VOLUME.
  • The sectors to be written to fall within a volume that has no mounted file system other than RAW.

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

If hFile was opened with FILE_FLAG_OVERLAPPED, the following conditions are in effect:

• The lpOverlapped parameter must point to a valid and unique OVERLAPPED structure, otherwise the function can incorrectly report that the write operation is complete.
• The lpNumberOfBytesWritten parameter should be set to NULL. To get the number of bytes written, use the GetOverlappedResult function. If the hFile parameter is associated with an I/O completion port, you can also get the number of bytes written by calling the GetQueuedCompletionStatus function.

In Windows Server 2012, this function is supported by the following technologies.

Synchronization and File Position

If hFile is opened with FILE_FLAG_OVERLAPPED, it is an asynchronous file handle; otherwise it is synchronous. The rules for using the OVERLAPPED structure are slightly different for each, as previously noted.

Considerations for working with asynchronous file handles:

• WriteFile may return before the write operation is complete. In this scenario, WriteFile returns FALSE and the GetLastError function returns ERROR_IO_PENDING, which allows the calling process to continue while the system completes the write operation.
• The lpOverlapped parameter must not be NULL and should be used with the following facts in mind:
  • Although the event specified in the OVERLAPPED structure is set and reset automatically by the system, the offset that is specified in the OVERLAPPED structure is not automatically updated.
  • WriteFile resets the event to a non-signaled state when it begins the I/O operation.
  • The event specified in the OVERLAPPED structure is set to a signaled state when the write operation is complete; until that time, the write operation is considered pending.
  • Because the write operation starts at the offset that is specified in the OVERLAPPED structure, and WriteFile may return before the system-level write operation is complete (write pending), neither the offset nor any other part of the structure should be modified, freed, or reused by the application until the event is signaled (that is, the write completes).

Considerations for working with synchronous file handles:

• If lpOverlapped is NULL, the write operation starts at the current file position and WriteFile does not return until the operation is complete, and the system updates the file pointer before WriteFile returns.
• If lpOverlapped is not NULL, the write operation starts at the offset that is specified in the OVERLAPPED structure and WriteFile does not return until the write operation is complete. The system updates the OVERLAPPED Internal and InternalHigh fields and the file pointer before WriteFile returns.

For more information, see CreateFile and Synchronous and Asynchronous I/O.

Pipes

If an anonymous pipe is being used and the read handle has been closed, when WriteFile attempts to write using the pipe's corresponding write handle, the function returns FALSE and GetLastError returns ERROR_BROKEN_PIPE.

If the pipe buffer is full when an application uses the WriteFile function to write to a pipe, the write operation may not finish immediately. The write operation will be completed when a read operation (using the ReadFile function) makes more system buffer space available for the pipe.

When writing to a non-blocking, byte-mode pipe handle with insufficient buffer space, WriteFile returns TRUE with *lpNumberOfBytesWritten < nNumberOfBytesToWrite.

For more information about pipes, see Pipes.

Transacted Operations

If there is a transaction bound to the file handle, then the file write is transacted. For more information, see About Transactional NTFS.

Examples

For some examples, see Creating and Using a Temporary File and Opening a File for Reading or Writing.

The following C++ example shows how to align sectors for unbuffered file writes. The Size variable is the size of the original data block you are interested in writing to the file. For additional rules regarding unbuffered file I/O, see File Buffering.

#include <windows.h>

#define ROUND_UP_SIZE(Value,Pow2) ((SIZE_T) ((((ULONG)(Value)) + (Pow2) - 1) & (~(((LONG)(Pow2)) - 1))))
#define ROUND_UP_PTR(Ptr,Pow2)  ((void *) ((((ULONG_PTR)(Ptr)) + (Pow2) - 1) & (~(((LONG_PTR)(Pow2)) - 1))))

int main()
{
   // Sample data
   unsigned long bytesPerSector = 65536; // obtained from the GetFreeDiskSpace function.
   unsigned long size = 15536; // Buffer size of your data to write.
   
   // Ensure you have one more sector than Size would require.
   size_t sizeNeeded = bytesPerSector + ROUND_UP_SIZE(size, bytesPerSector);
   
   // Replace this statement with any allocation routine.
   auto buffer = new uint8_t[sizeNeeded];
   
   // Actual alignment happens here.
   auto bufferAligned = ROUND_UP_PTR(buffer, bytesPerSector);

   // ... Add code using bufferAligned here.
   
   // Replace with corresponding free routine.
   delete buffer;
}

Requirements

See also

CancelIo

CancelIoEx

CancelSynchronousIo

CreateFile

CreateFileTransacted

File Management Functions

GetLastError

GetOverlappedResult

GetQueuedCompletionStatus

ReadFile

SetEndOfFile

WriteFileEx