WSASendTo
The Windows Sockets
WSASendTo function sends data to a specific destination, using overlapped I/O where
applicable.
int WSASendTo (
SOCKET s,
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LPWSABUF lpBuffers,
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DWORD dwBufferCount,
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LPDWORD lpNumberOfBytesSent,
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DWORD dwFlags,
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const struct sockaddr FAR * lpTo,
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int iToLen,
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LPWSAOVERLAPPED lpOverlapped,
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LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionROUTINE
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);
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Parameters
s
[in] A descriptor identifying a connected socket which was created using
WSASocket with flag WSA_FLAG_OVERLAPPED.
lpBuffers
[in] A pointer to an array of
WSABUF structures. Each
WSABUF structure contains a pointer to a buffer and the length of the buffer. This
array must remain valid for the duration of the send operation.
dwBufferCount
[in] The number of
WSABUF structures in the
lpBuffers array.
lpNumberOfBytesSent
[out] A pointer to the number of bytes sent by this call if the I/O operation
completes immediately.
dwFlags
[in] Specifies the way in which the call is made.
lpTo
[in] An optional pointer to the address of the target socket.
iToLen
[in] The size of the address in
lpTo.
lpOverlapped
[in] A pointer to a WSAOVERLAPPED structure (ignored for nonoverlapped
sockets).
lpCompletionRoutine
[in] A pointer to the completion routine called when the send operation has
been completed (ignored for nonoverlapped sockets).
Remarks
This function provides functionality over and above the standard
sendto function in two important areas:
- It can be used in conjunction with overlapped sockets to perform overlapped
send operations.
- It allows multiple send buffers to be specified making it applicable to the
scatter/gather type of I/O.
WSASendTo is normally used on a connectionless socket specified by
s to send a datagram contained in one or more buffers to a specific peer socket
identified by the
lpTo parameter. On a connection-oriented socket, the
lpTo and
iToLen parameters are ignored; in this case, the
WSASendTo is equivalent to
WSASend.
For overlapped sockets (created using
WSASocket with flag WSA_FLAG_OVERLAPPED) this will occur using overlapped I/O, unless
both
lpOverlapped and
lpCompletionRoutine are NULL in which case the socket is treated as a nonoverlapped socket. A
completion indication will occur (invocation of the completion routine or setting
of an event object) when the supplied buffer(s) have been consumed by the
transport. If the operation does not complete immediately, the final completion
status is retrieved through the completion routine or
WSAGetOverlappedResult.
For nonoverlapped sockets, the last two parameters (
lpOverlapped, lpCompletionRoutine) are ignored and
WSASendTo adopts the same blocking semantics as
send. Data is copied from the supplied buffer(s) into the transport's buffer. If
the socket is nonblocking and stream oriented, and there is not sufficient space
in the transport's buffer,
WSASendTo will return with only part of the application's buffers having been consumed.
Given the same buffer situation and a blocking socket,
WSASendTo will block until all of the application's buffer contents have been consumed.
The array of
WSABUF structures pointed to by the
lpBuffers parameter is transient. If this operation is completed in an overlapped
manner, it is the sercvice provider's responsibility to capture these
WSABUF structures before returning from this call. This enables applications to
build stack-based
WSABUF arrays.
For message-oriented sockets, care must be taken not to exceed the maximum
message size of the underlying transport, which can be obtained by getting the
value of socket option SO_MAX_MSG_SIZE. If the data is too long to pass atomically
through the underlying protocol the error WSAEMSGSIZE is returned, and no data
is transmitted.
Note that the successful completion of a
WSASendTo does not indicate that the data was successfully delivered.
dwFlags may be used to influence the behavior of the function invocation beyond the
options specified for the associated socket. That is, the semantics of this
function are determined by the socket options and the
dwFlags parameter. The latter is constructed by or-ing any of the following values:
Value
| Meaning
|
MSG_DONTROUTE
| Specifies that the data should not be subject to routing. A Windows Sockets
service provider can choose to ignore this flag.
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MSG_OOB
| Send out-of-band data (stream-style socket such as SOCK_STREAM only).
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MSG_PARTIAL
| Specifies that lpBuffers only contains a partial message. Note that the error code WSAEOPNOTSUPP will
be returned by transports which do not support partial message transmissions.
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Overlapped socket I/O:
If an overlapped operation completes immediately,
WSASendTo returns a value of zero and the
lpNumberOfBytesSent parameter is updated with the number of bytes sent. If the overlapped
operation is successfully initiated and will complete later,
WSASendTo returns SOCKET_ERROR and indicates error code WSA_IO_PENDING. In this case,
lpNumberOfBytesSent is not updated. When the overlapped operation completes the amount of data
transferred is indicated either through the
cbTransferred parameter in the completion routine (if specified), or through the
lpcbTransfer parameter in
WSAGetOverlappedResult.
This function may be called from within the completion routine of a previous
WSARecv,
WSARecvFrom,
WSASend or
WSASendTo function. This permits time-sensitive data transmissions to occur entirely
within a preemptive context.
The
lpOverlapped parameter must be valid for the duration of the overlapped operation. If
multiple I/O operations are simultaneously outstanding, each must reference a
separate overlapped structure. The WSAOVERLAPPED structure has the following form:
typedef struct _WSAOVERLAPPED {
DWORD Internal; // reserved
DWORD InternalHigh; // reserved
DWORD Offset; // reserved
DWORD OffsetHigh; // reserved
WSAEVENT hEvent;
} WSAOVERLAPPED, FAR * LPWSAOVERLAPPED;
If the
lpCompletionRoutine parameter is NULL, the
hEvent field of
lpOverlapped is signaled when the overlapped operation completes if it contains a valid
event object handle. An application can use
WSAWaitForMultipleEvents or
WSAGetOverlappedResult to wait or poll on the event object.
If
lpCompletionRoutine is not NULL, the
hEvent field is ignored and can be used by the application to pass context
information to the completion routine.
The completion routine follows the same rules as stipulated for Win32 file I/O
completion routines. The completion routine will not be invoked until the
thread is in an alertable wait state such as can occur when the function
WSAWaitForMultipleEvents with the
fAlertable parameter set to TRUE is invoked.
Transport providers allow an application to invoke send and receive operations
from within the context of the socket I/O completion routine, and guarantee
that, for a given socket, I/O completion routines will not be nested. This
permits time-sensitive data transmissions to occur entirely within a preemptive
context.
The prototype of the completion routine is as follows:
- oid CALLBACK CompletionROUTINE(
IN DWORD
dwError,
IN DWORD cbTransferred,
IN LPWSAOVERLAPPED lpOverlapped,
IN DWORD dwFlags
);
CompletionRoutine is a placeholder for an application-defined or library-defined function name.
dwError specifies the completion status for the overlapped operation as indicated by
lpOverlapped.
cbTransferred specifies the number of bytes sent. Currently there are no flag values
defined and
dwFlags will be zero. This function does not return a value.
Returning from this function allows invocation of another pending completion
routine for this socket. All waiting completion routines are called before the
alertable thread's wait is satisfied with a return code of WSA_IO_COMPLETION.
The completion routines may be called in any order, not necessarily in the same
order the overlapped operations are completed. However, the posted buffers are
guaranteed to be sent in the same order they are supplied.
Return Values
If no error occurs and the send operation has completed immediately,
WSASendTo returns zero. Note that in this case, the completion routine will have
already been scheduled, and to be called once the calling thread is in the alertable
state. Otherwise, a value of SOCKET_ERROR is returned, and a specific error
code may be retrieved by calling
WSAGetLastError. The error code WSA_IO_PENDING indicates that the overlapped operation has
been successfully initiated and that completion will be indicated at a later
time. Any other error code indicates that the overlapped operation was not
successfully initiated and no completion indication will occur.
Error Codes
WSANOTINITIALISED
| A successful WSAStartup must occur before using this function.
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WSAENETDOWN
| The network subsystem has failed.
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WSAEACCES
| The requested address is a broadcast address, but the appropriate flag was not
set.
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WSAEINTR
| The (blocking) call was canceled through WSACancelBlockingCall.
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WSAEINPROGRESS
| A blocking Windows Sockets 1.1 call is in progress, or the service provider is
still processing a callback function.
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WSAEFAULT
| The lpBuffers or lpTo parameters are not part of the user address space, or the lpTo argument is too small.
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WSAENETRESET
| The connection has been broken due to the remote host resetting.
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WSAENOBUFS
| The Windows Sockets provider reports a buffer deadlock.
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WSAENOTCONN
| The socket is not connected (connection-oriented sockets only)
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WSAENOTSOCK
| The descriptor is not a socket.
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WSAEOPNOTSUPP
| MSG_OOB was specified, but the socket is not stream style such as type
SOCK_STREAM, out-of-band data is not supported in the communication domain associated
with this socket, MSG_PARTIAL is not supported, or the socket is unidirectional
and supports only receive operations.
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WSAESHUTDOWN
| The socket has been shut down; it is not possible to WSASendTo on a socket after shutdown has been invoked with how set to SD_SEND or SD_BOTH.
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WSAEWOULDBLOCK
| Overlapped sockets: There are too many outstanding overlapped I/O requests.
Nonoverlapped sockets: The socket is marked as nonblocking and the send operation
cannot be completed immediately.
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WSAEMSGSIZE
| The socket is message oriented, and the message is larger than the maximum
supported by the underlying transport.
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WSAEINVAL
| The socket has not been bound with bind, or the socket is not created with the overlapped flag.
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WSAECONNABORTED
| The virtual circuit was terminated due to a time-out or other failure.
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WSAECONNRESET
| The virtual circuit was reset by the remote side.
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WSAEADDRNOTAVAIL
| The specified address is not available from the local machine.
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WSAEAFNOSUPPORT
| Addresses in the specified family cannot be used with this socket.
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WSAEDESTADDRREQ
| A destination address is required.
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WSAENETUNREACH
| The network cannot be reached from this host at this time.
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WSA_IO_PENDING
| An overlapped operation was successfully initiated and completion will be
indicated at a later time.
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WSA_OPERATION_ABORTED
| The overlapped operation has been canceled due to the closure of the socket,
or the execution of the SIO_FLUSH command in WSAIoctl.
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See Also
WSACloseEvent,
WSACreateEvent,
WSAGetOverlappedResult,
WSASocket,
WSAWaitForMultipleEvents
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