TransmitFile
Notice This function
is a Microsoft-specific extension to the Windows Sockets specification. For
more information, see Microsoft Extensions and Windows Sockets 2.
The Windows
Sockets TransmitFile function transmits file data over a connected
socket handle. This function uses the operating system s cache manager to retrieve the file data, and
provides high-performance file data transfer over sockets.
BOOL TransmitFile(
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SOCKET hSocket, |
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HANDLE hFile, |
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DWORD nNumberOfBytesToWrite, |
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DWORD nNumberOfBytesPerSend, |
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LPOVERLAPPED lpOverlapped, |
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LPTRANSMIT_FILE_BUFFERS lpTransmitBuffers, |
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DWORD dwFlags |
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); |
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Parameters
hSocket
A handle to a
connected socket. The function will transmit the file data over this socket.
The socket
specified by hSocket must be a connection-oriented socket.
Sockets of
type SOCK_STREAM, SOCK_SEQPACKET, or SOCK_RDM are connection-oriented sockets.
The TransmitFile function does not support datagram sockets.
hFile
A handle to
an open file. The function transmits this file s data. The operating system reads the file data sequentially. You can
improve caching performance by opening the handle with the FILE_FLAG_SEQUENTIAL_SCAN.
nNumberOfBytesToWrite
The number of
bytes to transmit. The function completes when it has sent this many bytes, or
if an error occurs.
Set this
parameter to zero to transmit the entire file.
nNumberOfBytesPerSend
The size of
each block of data sent per send operation. This specification is for use by
the sockets layer of the operating system.
Set this
parameter to zero to have the sockets layer select a default send size.
This
parameter is useful for message protocols that have limitations on the size of
individual send requests.
lpOverlapped
Pointer to an
OVERLAPPED
structure. If the socket handle has been opened as overlapped, specify this
parameter in order to achieve an overlapped (aysnchronous) I/O operation. By
default, socket handles are opened as overlapped.
You can use lpOverlapped
to specify an offset within the file at which to start the file data transfer
by setting the Offset and OffsetHigh member of the OVERLAPPED
structure. If lpOverlapped is NULL, the transmission of data always
starts at the current byte offset in the file.
When lpOverlapped
is not NULL, the overlapped I/O might not finish before TransmitFile
returns. In that case, the TransmitFile function returns FALSE, and GetLastError
returns ERROR_IO_PENDING. This lets the caller continue processing while the
file transmission operation completes. The operating system will set the event
specified by the hEvent member of the OVERLAPPED structure, or
the socket specified by hSocket, to the signaled state upon completion
of the data transmission request.
lpTransmitBuffers
Pointer to a TRANSMIT_FILE_BUFFERS data structure that
contains pointers to data to send before and after the file data is sent. Set
this parameter to NULL if you only want to transmit the file data.
dwFlags
An attribute
that has three settings:
TF_DISCONNECT
Start a
transport-level disconnect after all the file data has been queued for
transmission.
TF_REUSE_SOCKET
Prepare the
socket handle to be reused. When the TransmitFile request completes, the socket
handle can be passed to the AcceptEx function. It is only valid if
TF_DISCONNECT is also specified.
TF_WRITE_BEHIND
Complete the TransmitFile
request immediately, without pending. If this flag is specified and TransmitFile
succeeds, then the data has been accepted by the system but not necessarily
acknowledged by the remote end. If TransmitFile returns TRUE, there will
be no completion port indication for the I/O. Do not use this setting with the
other two settings.
Return Values
If the
function succeeds, the return value is TRUE.
If the
function fails, the return value is FALSE. To get extended error information,
call GetLastError.
The function returns FALSE if an overlapped I/O operation is not complete
before TransmitFile returns. In that case, GetLastError returns
ERROR_IO_PENDING.
Remarks
The Windows
NT Server optimizes the TransmitFile function for high performance. The
Windows NT Workstation optimizes the function for minimum memory and resource
utilization. Expect better performance results when using TransmitFile
on Windows NT Server.