?UDP 是 User Datagram Protocol 的簡稱， 中文名是用戶數據報協議，是 OSI（Open System Interconnection，開放式系統互聯） 參考模型中一種無連接的傳輸層協議，提供面向事務的簡單不可靠信息傳送服務，IETF RFC 768 是UDP的正式規范。UDP 在 IP 報文的協議號是17。UDP 協議與 TCP 協議一樣用于處理數據包，在 OSI 模型中，兩者都位于傳輸層，處于 IP 協議的上一層。UDP 有不提供數據包分組、組裝和不能對數據包進行排序的缺點，也就是說，當報文發送之后，是無法得知其是否安全完整到達的。UDP 用來支持那些需要在計算機之間傳輸數據的網絡應用。包括網絡視頻會議系統在內的眾多的客戶/服務器模式的網絡應用都需要使用 UDP 協議。UDP 報文沒有可靠性保證、順序保證和流量控制字段等，可靠性較差。但是正因為 UDP 協議的控制選項較少，在數據傳輸過程中延遲小、數據傳輸效率高，適合對可靠性要求不高的應用程序，或者可以保障可靠性的應用程序，如 DNS、TFTP、SNMP 等。


?UDP API 用于 UDP 通信。 它為實現無連接、面向數據報的通信提供了一個高級接口。具體的函數信息可以從下方得到：

UdpClose 函數
?主要是關閉 UDP Socket。函數成功執行后，傳遞的 socket 不再有效。具體的使用范例請在文章末尾查看。具體的使用結構如下：

?函數的返回值的情況如下：
??0: 函數執行成功
??WSA_INVALID_PARAMETER (87): 指定的 socket 無效
??SOCKET_ERROR (-1): 函數執行失敗。錯誤內容使用函數 IpGetLastError 查詢

UdpConnect 函數
?主要是將此 socket 連接到給定的遠程端點。socket 連接后，必須使用 UdpSend 而不是 UdpSendTo。在服務器端，當第一次在 socket 上接收數據時，可以連接到遠程端點。 之后，可以創建一個新的未連接的 Socket 以等待下一個傳入的 udp 連接。具體的使用結構如下：

?函數的返回值的情況如下：
??0: 函數執行成功
??WSA_INVALID_PARAMETER (87): 指定的 socket 無效
??SOCKET_ERROR (-1): 函數執行失敗。錯誤內容使用函數 IpGetLastError 查詢


?示例

// Client.can variables {UdpSocket gSocket1;UdpSocket gSocket2;char gBuffer[1500]; }on start {gSocket1 = UdpSocket::Open( IP_Endpoint(0.0.0.0:0) );gSocket2 = UdpSocket::Open( IP_Endpoint(0.0.0.0:0) );gSocket1.Connect(IP_Endpoint(192.168.1.3:40002));gSocket2.Connect(IP_Endpoint(192.168.1.3:40002)); }on key '1' {// send on the connected socketSendCommand(gSocket1, "Request");// wait for responsegSocket1.ReceiveFrom(gBuffer, elcount(gBuffer)); }on key '2' {// send on the connected socketSendCommand(gSocket2, "Request");// wait for responsegSocket1.ReceiveFrom(gBuffer, elcount(gBuffer)); }on key 'c' {SendCommand(gSocket1, "End");gSocket1.Close();SendCommand(gSocket2, "End");gSocket1.Close(); }SendCommand(UdpSocket socket, char command[]) {socket.Send(command, strlen(command)); }OnUdpReceiveFrom(dword socket, long result, IP_Endpoint remoteEndpoint, char buffer[], dword size) {// handle the response from the server here }//Server.can variables {// context to handle the state of the different connectionsstruct connectionContext{int connectionNumber;};dword gListeningSocket;struct connectionContext connections[long];char gBuffer[1500];dword gConnectionCount; }on start {gConnectionCount = 0;// open a socket and wait for the first datagListeningSocket = UdpOpen( IP_Endpoint(0.0.0.0:40002) );UdpReceiveFrom(gListeningSocket, gBuffer, elcount(gBuffer)); }OnUdpReceiveFrom(dword socket, long result, IP_Endpoint remoteEndpoint, char buffer[], dword size) {if(socket == gListeningSocket){// create a context for this connection and open a new socket for the// next connectionUdpConnect(socket, remoteEndpoint);connections[socket].connectionNumber = ++gConnectionCount;gListeningSocket = UdpOpen( IP_Endpoint(0.0.0.0:40002) );UdpReceiveFrom(gListeningSocket, gBuffer, elcount(gBuffer));}AnswerRequest(socket, buffer, size); }AnswerRequest(dword socket, char buffer[], dword size) {char response[100];if(strncmp(buffer, "Request", size) == 0){snprintf(response, elcount(response), "Response for connection #%d", connections[socket].connectionNumber);UdpSend(socket, response, strlen(response));UdpReceiveFrom(socket, gBuffer, elcount(gBuffer));}else if(strncmp(buffer, "End", size) == 0){connections.remove(socket);UdpClose(socket);} }

UdpSend 函數
?主要是在連接的 UDP socket 上發送數據。 調用這個函數前需要先用 UdpConnect 連接socket。具體的使用結構如下：

?函數的返回值的情況如下：
??0: 函數執行成功
??WSA_INVALID_PARAMETER (87): 指定的 socket 無效
??SOCKET_ERROR (-1): 函數執行失敗。錯誤內容使用函數 IpGetLastError 查詢


?示例

variables {UdpSocket gSocket; }on start {gSocket = UdpSocket::Open( IP_Endpoint(0.0.0.0:0) );gSocket.Connect(ip_Endpoint(192.168.1.3:40002));gSocket.Send( "Request", 7 ); }

UdpReceiveFrom 函數
?主要是接收數據到指定的緩沖區。 如果接收操作沒有立即完成，操作將異步執行，函數將返回 SOCKET_ERROR (-1)。 使用 IpGetLastSocketError 獲取更具體的錯誤代碼。 如果特定的錯誤代碼是 WSA_IO_PENDING (997)，則 CAPL 回調 OnUdpReceiveFrom 將在完成時調用（成功與否），前提是它在同一個 CAPL 程序中實現。具體的使用范例請在文章末尾查看。具體的使用結構如下：

?函數的返回值的情況如下：
??0: 函數執行成功
??WSA_INVALID_PARAMETER (87): 指定的 socket 無效
??SOCKET_ERROR (-1): 函數執行失敗。錯誤內容使用函數 IpGetLastError 查詢

UdpSendTo 函數
?主要是將數據發送到指定位置。 如果發送操作沒有立即完成，操作將異步執行，函數將返回 SOCKET_ERROR (-1)。 使用 IpGetLastSocketError 獲取更具體的錯誤代碼。 如果具體的錯誤代碼是 WSA_IO_PENDING (997)，并且在完成（成功與否）時會調用 CAPL 回調 OnUdpSendTo，前提是它是在同一個 CAPL 程序中實現的。如果操作已被同步處理，則不會調用 CAPL 回調 OnUdpSendTo。具體的使用格式如下：

?函數的返回值的情況如下：
??0: 函數執行成功
??WSA_INVALID_PARAMETER (87): 指定的 socket 無效
??SOCKET_ERROR (-1): 函數執行失敗。錯誤內容使用函數 IpGetLastError 查詢


?示例

variables {UdpSocket gSocket;char gRxBuffer[1000]; }on start {gSocket = UdpSocket::Open( IP_Endpoint(0.0.0.0:40001) );gSocket.SendTo( IP_Endpoint(192.168.0.2:40002), "Hello", 5 );gSocket.ReceiveFrom( gRxBuffer, elcount( gRxBuffer) ); }OnUdpReceiveFrom( UdpSocket socket, long result, IP_Endpoint remoteEndpoint, char buffer[], dword size) {if (result == 0){write( "Received: %s", buffer );gSocket.ReceiveFrom( gRxBuffer, elcount( gRxBuffer) );} }

OnUdpReceiveFrom 函數
?主要是當 UDP socket 上的異步接收操作完成時調用。堆棧包含一個數據隊列，一旦數據位于該隊列中，該隊列就會被 UdpReceiveFrom 減少。因此，為了將來從數據隊列中接收額外的數據用于 socket，必須在回調中再次調用 UdpReceiveFrom。具體的使用格式如下：

?示例

variables {UdpSocket gSocket;char gRxBuffer[1000]; }on start {gSocket = UdpSocket::Open( IP_Endpoint(0.0.0.0:40001) );gSocket.SendTo( IP_Endpoint(192.168.0.2:40002), "Hello", 5 );gSocket.ReceiveFrom( gRxBuffer, elcount( gRxBuffer) ); }OnUdpReceiveFrom( UdpSocket socket, long result, IP_Endpoint remoteEndpoint, char buffer[], dword size) {if (result == 0){write( "Received: %s", buffer );gSocket.ReceiveFrom( gRxBuffer, elcount( gRxBuffer) );} }

OnUdpSendTo 函數
?主要是當 UDP 套接字上的異步發送操作完成時調用。具體的使用格式如下
? 示例

// --------------------------------------------------- // node global variables. // --------------------------------------------------- variables {const int gMTU = 1500; // tcp mtuconst int gSERVER_REPLY_WELCOME = 1; // server reply for welcome.const int gSERVER_REPLY_ANSWER = 2; // server reply for answer.const dword gIPV4_STR_SIZE = 16; // IPv4 string sizeconst dword gINVALID_SOCKET = ~0; // invalid socket constantdword gListenPort = 12345; // port to send udp to.dword gListenSocket = gINVALID_SOCKET; // server side: server listen socketdword gServerClientSocket = gINVALID_SOCKET; // server side: a client's socketchar gServerTcpBuffer[gMTU]; // tcp receive buffer of server }// --------------------------------------------------- // Interaction: Server disconnects client... // --------------------------------------------------- on key 'x' {serverDisconnectClient(); }// --------------------------------------------------- // on measurement start. (initialize and start demo) // --------------------------------------------------- on start {serverStart(); }// --------------------------------------------------- // before measurement stops. // --------------------------------------------------- on preStop {// close server ( this implies closing all server and client sockets. )serverClose(); }// --------------------------------------------------- // Callback when client connects to server's listen socket. // --------------------------------------------------- void OnTcpListen( dword socket, long result) {dword errCode;writeLineEx(1, 1, " [ S: OnTcpListen called. (result: %d)]", result);if (gServerClientSocket != gINVALID_SOCKET){writeLineEx(1, 2, "S: A client is already connected. Not accepting.");return;}if (result == 0){if (socket == gListenSocket){write("S: Incoming client.", socket);gServerClientSocket = TcpAccept( gListenSocket );if (gServerClientSocket == gINVALID_SOCKET){errCode = IpGetLastSocketError(gListenSocket);if (errCode == 87){writeLineEx( 1, 3, "S: TcpAccept: Invalid listen socket given.");// handle error...}else{writeLineEx( 1, 3, "S: TcpAccept: Socket error: %d", IpGetLastSocketError(gListenSocket) );// handle error...}}else{write("S: Accepted client on socket %d to socket %d.", gListenSocket, gServerClientSocket);// start to receive data on the server's client socket...startReceive(gServerClientSocket, gServerTcpBuffer);// do more stuff on server to initialize the incoming accepted client...// welcome client.serverSend(gSERVER_REPLY_WELCOME, gServerClientSocket);}}else{writeLineEx( 1, 3, "S: OnTcpListen: Unexpected connection on socket %d. (result:%d)", socket, result);// handle error...}} }// --------------------------------------------------- // When asynchronous TcpSend completes... // --------------------------------------------------- void OnTcpSend( dword socket, long result, char buffer[], dword size) {writeLineEx(1, 1, " [ S: OnTcpSend called. (result: %d)]", result);if (result == 0){if (socket != gINVALID_SOCKET){if (socket == gServerClientSocket){write("S: Server sent %d bytes to client done. (socket %d, result: %d)", size, socket, result);}}} }// --------------------------------------------------- // When receiving data on socket... // --------------------------------------------------- void OnTcpReceive( dword socket, long result, dword address, dword port, char buffer[], dword size) {writeLineEx(1, 1, " [ S: OnTcpReceive called. (result: %d)]", result);if (result == 0){if (socket == gServerClientSocket){// server receives from client...write("S: Server received %d bytes from client: %s (result: %d)", size, buffer, result);// check client's request...if (strstr(buffer, "REQUEST") >= 0){write("S: Client request OK, sending answer.");serverSend(gSERVER_REPLY_ANSWER, gServerClientSocket); // answer}// continue receiving data.startReceive(gServerClientSocket, gServerTcpBuffer);}else if (socket != gINVALID_SOCKET){writeLineEx(1, 3, " [ S: UNIMPLEMENTED: Received %d bytes on socket %d from 0x%x:%d with data: %s (result: %d) ]", size, socket, address, port, buffer, result);}} }// --------------------------------------------------- // TCP socket receives a close notification // (remote closed) // --------------------------------------------------- void OnTcpClose( dword socket, long result) {if (socket == gServerClientSocket){TcpClose(gServerClientSocket);gServerClientSocket = gINVALID_SOCKET;writeLineEx(1, 1, " [ S: OnTcpClose called. (socket: %d, result: %d) ]", socket, result);} }// --------------------------------------------------- // server disconnects client // --------------------------------------------------- void serverDisconnectClient() {if (gServerClientSocket != gINVALID_SOCKET){write("S: Server disconnects client. (socket %d)", gServerClientSocket);TcpClose(gServerClientSocket);gServerClientSocket = gINVALID_SOCKET;} }// --------------------------------------------------- // start receiving on given socket into given buffer. // --------------------------------------------------- void startReceive ( dword socket, char buffer[] ) {long result;result = TcpReceive( socket, buffer, elcount(buffer) );if (result == -1){result = IpGetLastSocketError(socket);if (result != 997) // not asynchronous{// failurewriteLineEx( 1, 3, "S: TcpReceive error %d", result);}}else if (result != 0) // synchronous sending failed{// failurewriteLineEx( 1, 3, "S: TcpReceive error %d", result);} }// --------------------------------------------------- // start server // --------------------------------------------------- void serverStart() {writeLineEx(1, 1, " [ S: Open TCP server socket at port %d... ]", gListenPort);gListenSocket = TcpOpen(0, gListenPort);if (gListenSocket == gINVALID_SOCKET){writeLineEx( 1, 3, " [ S: TcpOpen: Error opening TCP Socket on port %d. (Error %d) ]", gListenPort, IpGetLastError() );// handle error...}else{if (TcpListen( gListenSocket ) != 0){writeLineEx(1, 3, " [ S: TcpListen: Error listening on socket. ]");TcpClose(gListenSocket);gListenSocket = gINVALID_SOCKET;// handle error...}else{writeLineEx(1, 1, " [ S: Start listening on server socket %d... ]", gListenSocket);}} }// --------------------------------------------------- // close server // --------------------------------------------------- void serverClose() {if (gListenSocket != gINVALID_SOCKET){writeLineEx(1, 1, " [ Server shutdown. ]");serverDisconnectClient();TcpClose(gListenSocket);gListenSocket = gINVALID_SOCKET;} }// --------------------------------------------------- // server sends data to client // --------------------------------------------------- void serverSend(int replyNr, dword socket) {if (socket != gINVALID_SOCKET){writeLineEx(1, 1, "S: Sending data to client. (socket %d)", socket);switch(replyNr){case gSERVER_REPLY_WELCOME:{sendTcpData( socket, " Server data: WELCOME.");break;}case gSERVER_REPLY_ANSWER:{sendTcpData(socket, "Server data: ANSWER.");break;}default:{writeLineEx(1, 3, " [ serverSend: UNSUPPORTED REPLY NR %d ]", replyNr);break;}}} }// --------------------------------------------------- // send tcp data. // --------------------------------------------------- void sendTcpData( dword socket, char data[] ) {long result;dword size;size = elcount(data);result = TcpSend(socket, data, size);if (result == 0){// sending took place immediately.writeLineEx(1, 1, " [ S: Synchronous sending: '%s' on socket %d ]", data, socket);OnTcpSend(socket, result, data, size); // trigger callback manually}else{if (result == -1){result = IpGetLastSocketError(socket);if (result == 997){// sending is done asynchronously.writeLineEx(1, 1, " [ S: Asynchronous sending: '%s' on socket %d ]", data, socket);// => OnTcpSend is called when done sending.}else{writeLineEx( 1, 3, " [ S: sendTcpData: Error sending data. (%d) ]", result);}}else{writeLineEx( 1, 3, " [ S: sendTcpData: Error sending data. (%d) ]", result);}} }

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