The Windows Sockets sendto function sends data to a specific destination.

int sendto (


const char FAR * buf,

int len,

int flags,

const struct sockaddr FAR * to,

int tolen




[in] A descriptor identifying a socket.


[in] A buffer containing the data to be transmitted.


[in] The length of the data in buf.


[in] Specifies the way in which the call is made.


[in] An optional pointer to the address of the target socket.


[in] The size of the address in to.


is used to write outgoing data on a socket. For message-oriented sockets, care must be taken not to exceed the maximum packet size of the underlying subnets, 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.

The to parameter can be any valid address in the socket's address family, including a broadcast or any multicast address. To send to a broadcast address, an application must have setsockopt SO_BROADCAST enabled. Otherwise, sendto will fail with the error code WSAEACCES. For TCP/IP, an application can send to any multicast address (without becoming a group member).

If the socket is unbound, unique values are assigned to the local association by the system, and the socket is marked as bound. An application can use getsockname to determine the local socket name in this case.

Note that the successful completion of a sendto does not indicate that the data was successfully delivered.

sendto is normally used on a connectionless socket to send a datagram to a specific peer socket identified by the to parameter. Even if the connectionless socket has been previously connected to a specific address, to overrides the destination address for that particular datagram only. On a connection-oriented socket, the to and tolen parameters are ignored; in this case, the sendto is equivalent to send.

For sockets using IP:

To send a broadcast (on a SOCK_DGRAM only), the address in the to parameter should be constructed using the special IP address INADDR_BROADCAST (defined in WINSOCK2.H) together with the intended port number. It is generally inadvisable for a broadcast datagram to exceed the size at which fragmentation can occur, which implies that the data portion of the datagram (excluding headers) should not exceed 512 bytes.

If no buffer space is available within the transport system to hold the data to be transmitted, sendto will block unless the socket has been placed in a nonblocking I/O mode. On nonblocking stream-oriented sockets, the number of bytes written can be between 1 and the requested length, depending on buffer availability on both the local and foreign hosts. The select, WSAAsyncSelect or WSAEventSelect call can be used to determine when it is possible to send more data.

Calling sendto with a len of zero is legal and, in this case, sendto will return zero as a valid return value. For message-oriented sockets, a zero-length transport datagram is sent.

Flags can 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 flags parameter. The latter is constructed by or-ing any of the following values:

Specifies that the data should not be subject to routing. A Windows Sockets service provider can choose to ignore this flag.
Send out-of-band data (stream-style socket such as SOCK_STREAM only. Also see Out-Of-Band data for a discussion of this topic.)

Return Values

If no error occurs, sendto returns the total number of bytes sent. (Note that this can be less than the number indicated by len.) Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError.

Error Codes

A successful WSAStartup must occur before using this function.
The network subsystem has failed.
The requested address is a broadcast address, but the appropriate flag was not set.
An unknown flag was specified, or MSG_OOB was specified for a socket with SO_OOBINLINE enabled.
The (blocking) call was canceled through WSACancelBlockingCall.
A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
The buf or to parameters are not part of the user address space, or the tolen argument is too small.
The connection has been broken due to the remote host resetting.
No buffer space is available.
The socket is not connected (connection-oriented sockets only)
The descriptor is not a socket.
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, or the socket is unidirectional and supports only receive operations.
The socket has been shut down; it is not possible to sendto on a socket after shutdown has been invoked with how set to SD_SEND or SD_BOTH.
The socket is marked as nonblocking and the requested operation would block.
The socket is message oriented, and the message is larger than the maximum supported by the underlying transport.
The remote host cannot be reached from this host at this time.
The virtual circuit was terminated due to a time-out or other failure. The application should close the socket as it is no longer usable.
The virtual circuit was reset by the remote side executing a "hard" or "abortive" close. For UPD sockets, the remote host was unable to deliver a previously sent UDP datagram and responded with a "Port Unreachable" ICMP packet. The application should close the socket as it is no longer usable.
The specified address is not available from the local machine.
Addresses in the specified family cannot be used with this socket.
A destination address is required.
The network cannot be reached from this host at this time.
The connection has been dropped, because of a network failure or because the system on the other end went down without notice.

See Also

, recvfrom, select, send, socket, WSAAsyncSelect, WSAEventSelect

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