The sendto function sends data to a specific destination.

Syntax

int sendto(
  __in  SOCKET s,
  __in  const char *buf,
  __in  int len,
  __in  int flags,
  __in  const struct sockaddr *to,
  __in  int tolen
);

Parameters

s [in]

A descriptor identifying a (possibly connected) socket.

buf [in]

A pointer to a buffer containing the data to be transmitted.

len [in]

The length, in bytes, of the data pointed to by the buf parameter.

flags [in]

A set of flags that specify the way in which the call is made.

to [in]

An optional pointer to a sockaddr structure that contains the address of the target socket.

tolen [in]

The size, in bytes, of the address pointed to by the to parameter.

Return Value

If no error occurs, sendto returns the total number of bytes sent, which 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.

Remarks

The sendto function 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 using getsockopt to retrieve 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 used setsockopt with 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).

Note  If a socket is opened, a setsockopt call is made, and then a sendto call is made, Windows Sockets performs an implicit bind function call.

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

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

The sendto function 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, the to parameter overrides the destination address for that particular datagram only. On a connection-oriented socket, the to and tolen parameters are ignored, making sendto equivalent to send.

Example Code

The following example demonstrates the use of the sendto function.

#include <windows.h>
#include <stdio.h>
#include "winsock2.h"

void main() {
  
  WSADATA wsaData;
  SOCKET SendSocket;
  sockaddr_in RecvAddr;
  int Port = 27015;
  char SendBuf[1024];
  int BufLen = 1024;

  //---------------------------------------------
  // Initialize Winsock
  WSAStartup(MAKEWORD(2,2), &wsaData);

  //---------------------------------------------
  // Create a socket for sending data
  SendSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

  //---------------------------------------------
  // Set up the RecvAddr structure with the IP address of
  // the receiver (in this example case "123.456.789.1")
  // and the specified port number.
  RecvAddr.sin_family = AF_INET;
  RecvAddr.sin_port = htons(Port);
  RecvAddr.sin_addr.s_addr = inet_addr("123.456.789.1");

  //---------------------------------------------
  // Send a datagram to the receiver
  printf("Sending a datagram to the receiver...\n");
  sendto(SendSocket, 
    SendBuf, 
    BufLen, 
    0, 
    (SOCKADDR *) &RecvAddr, 
    sizeof(RecvAddr));

  //---------------------------------------------
  // When the application is finished sending, close the socket.
  printf("Finished sending. Closing socket.\n");
  closesocket(SendSocket);

  //---------------------------------------------
  // Clean up and quit.
  printf("Exiting.\n");
  WSACleanup();
  return;
}

For Sockets Using IP (Version 4)

To send a broadcast (on a SOCK_DGRAM only), the address pointed to by the to parameter can be constructed to contain the special IPv4 address INADDR_BROADCAST (defined in Winsock2.h), together with the intended port number. If the address pointed to by the to parameter contains the INADDR_BROADCAST address and intended port, then the broadcast will be sent out on all interfaces to that port.

If the broadcast should be sent out only on a specific interface, then the address pointed to by the to parameter should contain the subnet broadcast address for the interface and the intended port. For example, an IPv4 network address of 192.168.1.0 with a subnet mask of 255.255.255.0 would use a subnet broadcast address of 192.168.1.255.

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 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 client and server systems. The select, WSAAsyncSelect or WSAEventSelect function can be used to determine when it is possible to send more data.

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

The flags parameter can be used to influence the behavior of the function invocation beyond the options specified for the associated socket. The semantics of this function are determined by the socket options and the flags parameter. The latter is constructed by using the bitwise OR operator with any of the following values.

Requirements

See Also

Winsock Reference

Winsock Functions

recv

recvfrom

select

send

socket

WSAAsyncSelect

WSAEventSelect

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Build date: 11/12/2009