Strange behaviour in a simple Client/Server Countdown C Application - c

Question

I am trying to implement a simple countdown application in C using UDP sockets.
I have a very strange problem with the server part of the application: it should receive a number from a client and then send different numbers for the countdown. So if, for example, a user types 5 in the client, then the server should receive 5 and send 4, 3, 2, 1 and 0 to the client. Here's my code:
#define BUFFERSIZE 512
#define PORT 55123
void ClearWinSock()
{
#if defined WIN32
WSACleanup();
#endif
}
int main()
{
#if defined WIN32
WSADATA wsaData;
WORD wVersionRequested;
wVersionRequested = MAKEWORD(2, 2);
if(WSAStartup(wVersionRequested, &wsaData) != 0)
{
printf("Error: unable to initialize the socket!\n");
return -1;
}
#endif
int mainSocket = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
if(mainSocket < 0)
{
printf( "Error: unable to create the socket!\n");
ClearWinSock();
return -1;
}
struct sockaddr_in serverSockAddrIn;
memset(&serverSockAddrIn, 0, sizeof(serverSockAddrIn));
serverSockAddrIn.sin_family = AF_INET;
serverSockAddrIn.sin_port = htons(PORT);
serverSockAddrIn.sin_addr.s_addr = inet_addr("127.0.0.1");
if(bind(mainSocket, (struct sockaddr*) &serverSockAddrIn, sizeof(serverSockAddrIn)) < 0)
{
fprintf(stderr, "Error: unable to bind the socket!\n");
ClearWinSock();
return -1;
}
char buffer[BUFFERSIZE];
struct sockaddr_in clientAddress;
unsigned int clientAddressLength;
int recvMessageSize;
while(1)
{
clientAddressLength = sizeof(clientAddress);
recvMessageSize = recvfrom(mainSocket, buffer, BUFFERSIZE, 0, (struct sockaddr*) &clientAddress, &clientAddressLength);
int countdownValue;
sscanf(buffer, "%d", &countdownValue);
printf("\nNumber received: %d\n", countdownValue);
int index;
for(index = countdownValue - 1; index >= 0; --index)
{
itoa(index, buffer, 10);
int outputStringLength = strlen(buffer);
if(sendto(mainSocket, buffer, outputStringLength, 0, (struct sockaddr*) &clientAddress, sizeof(clientAddress)) != outputStringLength)
{
printf("Error: unable to send the message!");
}
}
}
ClearWinSock();
return 0;
}
Now the problem is that if I, for example, send the number 5 from the client, sometimes the server works correctly and sometimes it says "Number received: 5", doesn't send anything and then it says "Number received: 0" for 5 times.
I think I am doing something wrong in using the sockets. Or maybe it's something which involves cleaning the buffer, don't know! I can't reproduce the error because with the same input sometimes it acts in a way and sometimes in the other.

Answer 1

Are both your client and your server both listening on the same port? If so, you might want to consider having them listen on different ports (e.g. client sends to X and listens to port Y; server sends to port Y and listens to port X) so that they don't interfere with each other or accidentally receive their own sent-packets when both client and server are running on the same host.
Alternatively, you can instruct both client and server to share the same port by always executing the following code before calling bind():
const int trueValue = 1;
setsockopt(mainSocket, SOL_SOCKET, SO_REUSEADDR, (const char *) &trueValue, sizeof(trueValue));