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.
| SOCKET hSocket,
| HANDLE hFile,
| DWORD nNumberOfBytesToWrite,
| DWORD nNumberOfBytesPerSend,
| LPOVERLAPPED lpOverlapped,
| LPTRANSMIT_FILE_BUFFERS lpTransmitBuffers,
| DWORD dwFlags
A handle to a connected socket. The function will transmit the file data over
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.
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.
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.
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
This parameter is useful for message protocols that have limitations on the
size of individual send requests.
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
member of the OVERLAPPED
structure. If lpOverlapped
is NULL, the transmission of data always starts at the current byte offset in
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.
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
An attribute that has three settings:
Start a transport-level disconnect after all the file data has been queued for
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.
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.
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
returns. In that case, GetLastError
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.
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