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send function

Applies to: desktop apps only

The send function sends data on a connected socket.

Syntax

Copy

int send(
  __in  SOCKET s,
  __in  const char *buf,
  __in  int len,
  __in  int flags
);

Parameters

s [in]

A descriptor identifying a connected socket.

buf [in]

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

len [in]

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

flags [in]

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

Return value

If no error occurs, send returns the total number of bytes sent, which can be less than the number requested to be sent in the len parameter. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError.

Error code	Meaning
WSANOTINITIALISED	A successful WSAStartup call must occur before using this function.
WSAENETDOWN	The network subsystem has failed.
WSAEACCES	The requested address is a broadcast address, but the appropriate flag was not set. Call setsockopt with the SO_BROADCAST socket option to enable use of the broadcast address.
WSAEINTR	A blocking Windows Sockets 1.1 call was canceled through WSACancelBlockingCall.
WSAEINPROGRESS	A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
WSAEFAULT	The buf parameter is not completely contained in a valid part of the user address space.
WSAENETRESET	The connection has been broken due to the keep-alive activity detecting a failure while the operation was in progress.
WSAENOBUFS	No buffer space is available.
WSAENOTCONN	The socket is not connected.
WSAENOTSOCK	The descriptor is not a socket.
WSAEOPNOTSUPP	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.
WSAESHUTDOWN	The socket has been shut down; it is not possible to send on a socket after shutdown has been invoked with how set to SD_SEND or SD_BOTH.
WSAEWOULDBLOCK	The socket is marked as nonblocking and the requested operation would block.
WSAEMSGSIZE	The socket is message oriented, and the message is larger than the maximum supported by the underlying transport.
WSAEHOSTUNREACH	The remote host cannot be reached from this host at this time.
WSAEINVAL	The socket has not been bound with bind, or an unknown flag was specified, or MSG_OOB was specified for a socket with SO_OOBINLINE enabled.
WSAECONNABORTED	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.
WSAECONNRESET	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.
WSAETIMEDOUT	The connection has been dropped, because of a network failure or because the system on the other end went down without notice.

Remarks

The send function is used to write outgoing data on a connected socket.

For message-oriented sockets (address family of AF_INET or AF_INET6, type of SOCK_DGRAM, and protocol of IPPROTO_UDP, for example), care must be taken not to exceed the maximum packet size of the underlying provider. The maximum message packet size for a provider can be obtained by calling getsockopt with the optname parameter set to SO_MAX_MSG_SIZE to retrieve the value of socket option. If the data is too long to pass atomically through the underlying protocol, the error WSAEMSGSIZE is returned, and no data is transmitted.

The successful completion of a send function does not indicate that the data was successfully delivered and received to the recipient. This function only indicates the data was successfully sent.

If no buffer space is available within the transport system to hold the data to be transmitted, send will block unless the socket has been placed in 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 computers. The select, WSAAsyncSelect or WSAEventSelect functions can be used to determine when it is possible to send more data.

Calling send with a len parameter of zero is permissible and will be treated by implementations as successful. In such cases, send 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 beyond the options specified for the associated socket. The semantics of the send function are determined by any options previously set on the socket specified in the s parameter and the flags parameter passed to the send function.

Note When issuing a blocking Winsock call such as send, 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 send function.

C++

Copy


#ifndef UNICODE
#define UNICODE
#endif

#define WIN32_LEAN_AND_MEAN

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

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

#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT 27015

int main() {

    //----------------------
    // Declare and initialize variables.
    int iResult;
    WSADATA wsaData;

    SOCKET ConnectSocket = INVALID_SOCKET;
    struct sockaddr_in clientService; 

    int recvbuflen = DEFAULT_BUFLEN;
    char *sendbuf = "Client: sending data test";
    char recvbuf[DEFAULT_BUFLEN] = "";

    //----------------------
    // 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 connecting to server
    ConnectSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (ConnectSocket == INVALID_SOCKET) {
        wprintf(L"socket failed with error: %ld\n", WSAGetLastError());
        WSACleanup();
        return 1;
    }

    //----------------------
    // The sockaddr_in structure specifies the address family,
    // IP address, and port of the server to be connected to.
    clientService.sin_family = AF_INET;
    clientService.sin_addr.s_addr = inet_addr( "127.0.0.1" );
    clientService.sin_port = htons( DEFAULT_PORT );

    //----------------------
    // Connect to server.
    iResult = connect( ConnectSocket, (SOCKADDR*) &clientService, sizeof(clientService) );
    if (iResult == SOCKET_ERROR) {
        wprintf(L"connect failed with error: %d\n", WSAGetLastError() );
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
  }

    //----------------------
    // Send an initial buffer
    iResult = send( ConnectSocket, sendbuf, (int)strlen(sendbuf), 0 );
    if (iResult == SOCKET_ERROR) {
        wprintf(L"send failed with error: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    printf("Bytes Sent: %d\n", iResult);

    // shutdown the connection since no more data will be sent
    iResult = shutdown(ConnectSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        wprintf(L"shutdown failed with error: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    // Receive until the peer closes the connection
    do {

        iResult = recv(ConnectSocket, recvbuf, recvbuflen, 0);
        if ( iResult > 0 )
            wprintf(L"Bytes received: %d\n", iResult);
        else if ( iResult == 0 )
            wprintf(L"Connection closed\n");
        else
            wprintf(L"recv failed with error: %d\n", WSAGetLastError());

    } while( iResult > 0 );


    // close the socket
    iResult = closesocket(ConnectSocket);
    if (iResult == SOCKET_ERROR) {
        wprintf(L"close failed with error: %d\n", WSAGetLastError());
        WSACleanup();
        return 1;
    }

    WSACleanup();
    return 0;
}

Example Code

For a another example that uses the send function, see Getting Started With Winsock.

Notes for IrDA Sockets

The Af_irda.h header file must be explicitly included.

Requirements

Minimum supported client	Windows 2000 Professional
Minimum supported server	Windows 2000 Server
Header	Winsock2.h
Library	Ws2_32.lib
DLL	Ws2_32.dll

This statement is WRONG. Correct one is: “The successful completion of a send function indicates that the data was successfully copied to the socket send buffer”. It is possible that the data wasn’t sent. For example, sender has 64k send buffer, while receiver has 32k receive buffer. It is possible that sender receives a zero receive window after it sends 32k data. At this time, send is called successfully, while the data is only in the send buffer, but hasn’t be sent.

History

5/17/2012
Jeris

Invalid pointer error

I called GlobalAlloc with a large size (0x00400800) and I called send, but it returns -1 and Error is WSAEFAULT. I checked the buffer, buf there was no fault or error. and then I called send Function with len to 2, and It runned completely and returns no error.

History

6/24/2011
throm1739

Send function returns 0

send function can return 0 (which is not an error) if the socket is blocking , a timeout is set on the bloking socket and timeout expired.

History

9/7/2009
yyaaiirr

7/12/2010
Stanley Roark

Maximum length

If you try to send really large amounts of data (100mb+) then send returns -1 with WSAGetLastError() of 0. 10mb seems fine. This can trip you up if you're porting from other platforms without this limitation.

History

12/18/2009
Tony Hoyle

blocking send() and delayed ack

I have an (time critical) application where a sender sends large blocks of data in constant time intervals using a blocking call to send(). The receiver doesn't return anything. Sometimes nothing is sent for about 200ms (which means that we have to drop packets an the sender side), I used wireshark to log the network traffic and saw that always when the 200ms pause occured the TCP ACK from the sender was delayed. I assume that we sometimes send an even number of packets, causing the ACK to be sent immediatly and sometimes an uneven number of packets, causing the TCP delayed ACK mechanism to delay the ACK for 200 ms. And i assume that the blocked send() returns only after it received an ACK for the last sent packet - is this true? If I set the TcpAckFrequency registry key to 1 on the receiver side, the problem disappears. TCP_NODELAY is set on both sides.

I'm looking for a better solution than setting the registry key, should it be possible to resolve the issue using overlapped I/O?

History

12/10/2008
Ralph_G_1

8/5/2009
Thomas Lee

buf parameter type

The current specifications for POSIX and others demand that the buf parameter be of type const void *, instead of const char *. The current parameter type requires an explicit cast when trying to send non-char data...

History

7/2/2008
Medinoc

send function

Syntax

Parameters

Return value

Remarks

Example Code

Example Code

Notes for IrDA Sockets

Requirements

See also