Can't receive data from network using udp protocol - c++

Question

My aim is to write C++ program which receives data from network using udp protocol. OS - linux. I have few questions.
I want to make sure that I can use linux C libraries to write C++ program.
Is anywhere exists full tutorial which expailns step by step how to program using sockets in linux? ( I know that in network is many tutorials, but I'm looking for something which help me receive udp data from INTERNET, not from another program on the same device.).
I see in terminal ( using tcpdump) that my computer receive udp packages addressed to specified port. What's wrong with my program which looks like this
class Connection
{
private:
char * buffer;
int bufferSize;
int sock;
struct sockaddr_in server;
struct sockaddr_in other;
socklen_t addressLength;
public:
Connection(string address, int port, int bufferSize);
~Connection();
int reciveData();
};
Connection::Connection(string address, int port, int bufferSize)
{
this->addressLength = sizeof(this->server);
this->sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
if(this->sock == -1)
{
cout << "Socket error!" << endl;
exit(1);
}
memset(&(this->server), 0, sizeof(this->server));
this->server.sin_family = AF_INET;
this->server.sin_port = htons(port);
this->server.sin_addr.s_addr = htonl(INADDR_ANY);
if( bind(this->sock, (sockaddr *) &(this->server), this->addressLength) == -1 )
{
cout << "Bind socket error!" << endl;
exit(1);
}
this->bufferSize = bufferSize;
this->buffer = new char[bufferSize];
cout << "Socket created!" << endl;
}
int Connection::receiveData()
{
int returned;
fflush(stdout);
if( (returned =recvfrom(this->sock, this->buffer, this->bufferSize, 0, (sockaddr *) &(this->other), &this->addressLength)) == -1)
{
cout << "Receiving error!" << endl;
exit(1);
}
return returned;
}
When i call receiveData() nothing's happening - I think program waits for some udp package ( and i don't know why he does not receive it).
Could someone explain me difference between udp server and client program ?
Thanks a lot.

Answer 1

When calling recvfrom(), you are passing it a pointer to this->addressLength. On input, the length needs to be the complete size of the address buffer. On output, the actual address length is returned. So recvfrom() might be altering your addressLength member, affecting subsequent code. Use a local variable instead:
int Connection::receiveData()
{
int returned;
int addrlen = sizeof(this->other);
fflush(stdout);
returned = recvfrom(this->sock, this->buffer, this->bufferSize, 0, (sockaddr *) &(this->other), &addrLen);
if( returned == -1 )
{
cout << "Receiving error!" << endl;
exit(1);
}
return returned;
}
Alternatively, use a separate member variable instead:
class Connection
{
private:
char * buffer;
int bufferSize;
int sock;
struct sockaddr_in server;
struct sockaddr_in other;
socklen_t serverAddressLength;
socklen_t otherAddressLength;
public:
Connection(string address, int port, int bufferSize);
~Connection();
int reciveData();
};
Connection::Connection(string address, int port, int bufferSize)
{
this->serverAddressLength = sizeof(this->server);
this->sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
if(this->sock == -1)
{
cout << "Socket error!" << endl;
exit(1);
}
if(this->sock == -1)
{
cout << "Socket error!" << endl;
exit(1);
}
memset(&(this->server), 0, sizeof(this->server));
this->server.sin_family = AF_INET;
this->server.sin_port = htons(port);
this->server.sin_addr.s_addr = htonl(INADDR_ANY);
if( bind(this->sock, (sockaddr *) &(this->server), this->serverAddressLength) == -1 )
{
cout << "Bind socket error!" << endl;
exit(1);
}
this->bufferSize = bufferSize;
this->buffer = new char[bufferSize];
cout << "Socket created!" << endl;
}
int Connection::receiveData()
{
int returned;
fflush(stdout);
this->otherAddressLength = sizeof(this->other);
if( (returned =recvfrom(this->sock, this->buffer, this->bufferSize, 0, (sockaddr *) &(this->other), &this->otherAddressLength)) == -1)
{
cout << "Receiving error!" << endl;
exit(1);
}
return returned;
}