Trying to send a short message using TCP or UDP sockets and then read the short response through the same socket

I am using Dynamic C V9.62 to develop on a BL2600 SBC.

Essentially, I want to open a socket to a remote instrument (IP:PORT), send a short message to that instrument, and then read the instrument’s reply through the same socket. I am new to socket programming and none of the examples I tried gave me enough insight on how to do this.

Thanks in advance for any assistance.

UDP Server: C:\DCRABBIT_10.72\Samples cpip\udp\UDP_SRV.C

UDP Client: C:\DCRABBIT_10.72\Samples cpip\udp\Udp_cli.c

A good example for TCP is the Modbus TCP Master and slave libraries which use small packets over TCP/IP. These can be found in the Lib\Rabbit4000\Modbus folder.

On the UDP side of things, the Dynamic C BACnet stack at uses UDP to send small to medium packets over UDP.

These examples don’t use the same socket to send and receive data. Would be good to have an example of a client that sends a string to the server, and the server replies on the same socket (the client waits for the reply).

The key is the call to “tcp_tick(NULL)” after receiving the string from the client.

CLIENT (Rabbit):


  • Please see the function help (Ctrl-H) on TCPCONFIG for instructions on
  • compile-time network configuration.
    #define TCPCONFIG 5


  • Define the number of socket buffers that will be allocated for
  • UDP sockets. We only need one UDP socket, so one socket buffer
  • will be enough.


  • UDP demo configuration

/* what local UDP port to use - we receive packets only sent to this port */
#define LOCAL_PORT 10001


  • If REMOTE_IP is set to -1, we will accept packets from anybody.
  • If it is set to 0, we will accept packets from anybody, but
  • the first host to connect to us will complete the socket with
  • their IP address and port number. At that point, the local socket
  • will be limited to that host only.
  • If it is set to an IP address, the socket will only accept packets
  • from that IP.

#define REMOTE_IP -1 // accept packets from all hosts
//#define REMOTE_IP 0 // accept packets from first host
//#define REMOTE_IP IPADDR(192,168,2,71) // accept from this addr only


  • End of configuration section *

#memmap xmem
#use “dcrtcp.lib”

udp_Socket sock;

/* receive one packet (heartbeat) */
int receive_packet(void)
static char buf[128];
int len;
longword remoteIP;
word remotePort;
udp_Socket remoteSock;

	memset(buf, 0, sizeof(buf));

// receive the packet

//len = udp_recv(&sock, buf, sizeof(buf));
len = udp_recvfrom(&sock, buf, sizeof(buf), &remoteIP, &remotePort);
if (-1 == len) return 0; // no packet read. return

printf("Received %i bytes -> %s

", len, buf);

tcp_tick(NULL);                                           // *** VERY IMPORTANT ***

len = udp_sendto(&sock, buf, len, remoteIP, remotePort);
printf("Sent back %i bytes to remote %lX, port %u
", len, remoteIP, (int)remotePort);

return 1;


void main()
// Start network and wait for interface to come up (or error exit).

if(!udp_open(&sock, LOCAL_PORT, REMOTE_IP, 0, NULL)) {
	printf("udp_open failed!


/* receive heartbeats */
for( ; ; ) {


SERVER (PC-SIDE, Visual C++):

static void RunClient(const char* sServerIPAddr)
int socket_desc;
struct sockaddr_in server_addr;
char server_message[2000] = { 0 };
int server_struct_length = sizeof(server_addr);

socket_desc = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

if (socket_desc < 0) { printf("Error while creating socket
"); return; }

// Set port and IP:
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(10001);
server_addr.sin_addr.s_addr = inet_addr(sServerIPAddr);

// Get input from the user:
std::cout << "Enter message: ";
std::string client_message;
std::getline(std::cin, client_message);

// Send the message to server:
if (sendto(socket_desc, client_message.c_str(), client_message.size(), 0, (struct sockaddr*)&server_addr, server_struct_length) < 0)
printf("Unable to send message

// Receive the server’s response:
if (recvfrom(socket_desc, server_message, sizeof(server_message), 0, (struct sockaddr*)&server_addr, &server_struct_length) < 0)
printf("Error while receiving server’s msg

std::cout << "Server’s response: " << server_message << std::endl;

// Close the socket: