sendto function (winsock.h)

The sendto function sends data to a specific destination.


int sendto(
  [in] SOCKET         s,
  [in] const char     *buf,
  [in] int            len,
  [in] int            flags,
  [in] const sockaddr *to,
  [in] int            tolen


[in] s

A descriptor identifying a (possibly connected) socket.

[in] buf

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

[in] len

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

[in] flags

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

[in] to

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

[in] tolen

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.

Error code Meaning
A successful WSAStartup call 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. Call setsockopt with the SO_BROADCAST parameter to allow the use of the broadcast address.
An unknown flag was specified, or MSG_OOB was specified for a socket with SO_OOBINLINE enabled.
A blocking Windows Sockets 1.1 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 parameter is too small.
The connection has been broken due to keep-alive activity detecting a failure while the operation was in progress.
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, OOB 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 UDP 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 remote address is not a valid address, for example, ADDR_ANY.
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.
A socket operation was attempted to an unreachable host.
The connection has been dropped, because of a network failure or because the system on the other end went down without notice.


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. If the socket is connected, the getsockname function can be used to determine the local IP address and port associated with the socket.

If the socket is not connected, the
getsockname function can be used to determine the local port number associated with the socket but the IP address returned is set to the wildcard address for the given protocol (for example, INADDR_ANY or "" for IPv4 and IN6ADDR_ANY_INIT or "::" for IPv6).

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.

Note  When issuing a blocking Winsock call such as sendto, Winsock may need to wait for a network event before the call can complete. Winsock performs an alertable wait in this situation, which can be interrupted by an asynchronous procedure call (APC) scheduled on the same thread. Issuing another blocking Winsock call inside an APC that interrupted an ongoing blocking Winsock call on the same thread will lead to undefined behavior, and must never be attempted by Winsock clients.

Example Code

The following example demonstrates the use of the sendto function.
#ifndef UNICODE
#define UNICODE


#include <winsock2.h>
#include <Ws2tcpip.h>
#include <stdio.h>

// Link with ws2_32.lib
#pragma comment(lib, "Ws2_32.lib")

int main()

    int iResult;
    WSADATA wsaData;

    sockaddr_in RecvAddr;

    unsigned short Port = 27015;

    char SendBuf[1024];
    int BufLen = 1024;

    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != NO_ERROR) {
        wprintf(L"WSAStartup failed with error: %d\n", iResult);
        return 1;

    // Create a socket for sending data
    SendSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (SendSocket == INVALID_SOCKET) {
        wprintf(L"socket failed with error: %ld\n", WSAGetLastError());
        return 1;
    // Set up the RecvAddr structure with the IP address of
    // the receiver (in this example case "")
    // and the specified port number.
    RecvAddr.sin_family = AF_INET;
    RecvAddr.sin_port = htons(Port);
    RecvAddr.sin_addr.s_addr = inet_addr("");

    // Send a datagram to the receiver
    wprintf(L"Sending a datagram to the receiver...\n");
    iResult = sendto(SendSocket,
                     SendBuf, BufLen, 0, (SOCKADDR *) & RecvAddr, sizeof (RecvAddr));
    if (iResult == SOCKET_ERROR) {
        wprintf(L"sendto failed with error: %d\n", WSAGetLastError());
        return 1;
    // When the application is finished sending, close the socket.
    wprintf(L"Finished sending. Closing socket.\n");
    iResult = closesocket(SendSocket);
    if (iResult == SOCKET_ERROR) {
        wprintf(L"closesocket failed with error: %d\n", WSAGetLastError());
        return 1;
    // Clean up and quit.
    return 0;

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 with a subnet mask of would use a subnet broadcast address of

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.

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.
MSG_OOB Sends OOB data (stream-style socket such as SOCK_STREAM only).

Windows Phone 8: This function is supported for Windows Phone Store apps on Windows Phone 8 and later.

Windows 8.1 and Windows Server 2012 R2: This function is supported for Windows Store apps on Windows 8.1, Windows Server 2012 R2, and later.


Requirement Value
Minimum supported client Windows 8.1, Windows Vista [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header winsock.h (include Winsock2.h)
Library Ws2_32.lib
DLL Ws2_32.dll

See also



Winsock Functions

Winsock Reference