Boost TCP client to connect to multiple servers - c++

Question

I want my TCP client to connect to multiple servers(each server has a separate IP and port).
I am using async_connect. I can successfully connect to different servers but the read/write fails since the server's corresponding tcp::socket object is not available.
Can you please suggest how I could store each server's socket in some data structure? I tried saving the IP, socket to a std::map, but the first server's socket object is not available in memory and the app crashes. I tried making the socket static, but it does not help either.
Please help me!!
Also, I hope I am logically correct in making a single TCP client connect to 2 different servers.
I am sharing below the simplified header & cpp file.
class TCPClient: public Socket
{
public:
TCPClient(boost::asio::io_service& io_service,
boost::asio::ip::tcp::endpoint ep);
virtual ~TCPClient();
void Connect(boost::asio::ip::tcp::endpoint ep, boost::asio::io_service &ioService, void (Comm::*SaveClientDetails)(std::string,void*),
void *pClassInstance);
void TransmitData(const INT8 *pi8Buffer);
void HandleWrite(const boost::system::error_code& err,
size_t szBytesTransferred);
void HandleConnect(const boost::system::error_code &err,
void (Comm::*SaveClientDetails)(std::string,void*),
void *pClassInstance, std::string sIPAddr);
static tcp::socket* CreateSocket(boost::asio::io_service &ioService)
{ return new tcp::socket(ioService); }
static tcp::socket *mSocket;
private:
std::string sMsgRead;
INT8 i8Data[MAX_BUFFER_LENGTH];
std::string sMsg;
boost::asio::deadline_timer mTimer;
};
tcp::socket* TCPClient::mSocket = NULL;
TCPClient::TCPClient(boost::asio::io_service &ioService,
boost::asio::ip::tcp::endpoint ep) :
mTimer(ioService)
{
}
void TCPClient::Connect(boost::asio::ip::tcp::endpoint ep,
boost::asio::io_service &ioService,
void (Comm::*SaveServerDetails)(std::string,void*),
void *pClassInstance)
{
mSocket = CreateSocket(ioService);
std::string sIPAddr = ep.address().to_string();
/* To send connection request to server*/
mSocket->async_connect(ep,boost::bind(&TCPClient::HandleConnect, this,
boost::asio::placeholders::error, SaveServerDetails,
pClassInstance, sIPAddr));
}
void TCPClient::HandleConnect(const boost::system::error_code &err,
void (Comm::*SaveServerDetails)(std::string,void*),
void *pClassInstance, std::string sIPAddr)
{
if (!err)
{
Comm* pInstance = (Comm*) pClassInstance;
if (NULL == pInstance)
{
break;
}
(pInstance->*SaveServerDetails)(sIPAddr,(void*)(mSocket));
}
else
{
break;
}
}
void TCPClient::TransmitData(const INT8 *pi8Buffer)
{
sMsg = pi8Buffer;
if (sMsg.empty())
{
break;
}
mSocket->async_write_some(boost::asio::buffer(sMsg, MAX_BUFFER_LENGTH),
boost::bind(&TCPClient::HandleWrite, this,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}
void TCPClient::HandleWrite(const boost::system::error_code &err,
size_t szBytesTransferred)
{
if (!err)
{
std::cout<< "Data written to TCP Client port! ";
}
else
{
break;
}
}

Answer 1

You seem to know your problem: the socket object is unavailable. That's 100% by choice. You chose to make it static, of course there will be only one instance.
Also, I hope I am logically correct in making a single TCP client connect to 2 different servers.
It sounds wrong to me. You can redefine "client" to mean something having multiple TCP connections. In that case at the very minimum you expect a container of tcp::socket objects to hold those (or, you know, a Connection object that contains the tcp::socket.
BONUS: Demo
For fun and glory, here's what I think you should be looking for.
Notes:
no more new, delete
no more void*, reinterpret casts (!!!)
less manual buffer sizing/handling
no more bind
buffer lifetimes are guaranteed for the corresponding async operations
message queues per connection
connections are on a strand for proper synchronized access to shared state in multi-threading environments
I added in a connection max idle time timeout; it also limits the time taken for any async operation (connect/write). I assumed you wanted something like this because (a) it's common (b) there was an unused deadline_timer in your question code
Note the technique of using shared pointers to have Comm manage its own lifetime. Note also that _socket and _outbox are owned by the individual Comm instance.
Live On Coliru
#include <boost/asio.hpp>
#include <deque>
#include <iostream>
using INT8 = char;
using boost::asio::ip::tcp;
using boost::system::error_code;
//using SaveFunc = std::function<void(std::string, void*)>; // TODO abolish void*
using namespace std::chrono_literals;
using duration = std::chrono::high_resolution_clock::duration;
static inline constexpr size_t MAX_BUFFER_LENGTH = 1024;
using Handle = std::weak_ptr<class Comm>;
class Comm : public std::enable_shared_from_this<Comm> {
public:
template <typename Executor>
explicit Comm(Executor ex, tcp::endpoint ep, // ex assumed to be strand
duration max_idle)
: _ep(ep)
, _max_idle(max_idle)
, _socket{ex}
, _timer{_socket.get_executor()}
{
}
~Comm() { std::cerr << "Comm closed (" << _ep << ")\n"; }
void Start() {
post(_socket.get_executor(), [this, self = shared_from_this()] {
_socket.async_connect(
_ep, [this, self = shared_from_this()](error_code ec) {
std::cerr << "Connect: " << ec.message() << std::endl;
if (!ec)
DoIdle();
else
_timer.cancel();
});
DoIdle();
});
}
void Stop() {
post(_socket.get_executor(), [this, self = shared_from_this()] {
if (not _outbox.empty())
std::cerr << "Warning: some messages may be undelivered ("
<< _ep << ")" << std::endl;
_socket.cancel();
_timer.cancel();
});
}
void TransmitData(std::string_view msg) {
post(_socket.get_executor(),
[this, self = shared_from_this(), msg = std::string(msg.substr(0, MAX_BUFFER_LENGTH))] {
_outbox.emplace_back(std::move(msg));
if (_outbox.size() == 1) { // no send loop already active?
DoSendLoop();
}
});
}
private:
// The DoXXXX functions are assumed to be on the strand
void DoSendLoop() {
DoIdle(); // restart max_idle even after last successful send
if (_outbox.empty())
return;
boost::asio::async_write(
_socket, boost::asio::buffer(_outbox.front()),
[this, self = shared_from_this()](error_code ec, size_t xfr) {
std::cerr << "Write " << xfr << " bytes to " << _ep << " " << ec.message() << std::endl;
if (!ec) {
_outbox.pop_front();
DoSendLoop();
} else
_timer.cancel(); // causes Comm shutdown
});
}
void DoIdle() {
_timer.expires_from_now(_max_idle); // cancels any pending wait
_timer.async_wait([this, self = shared_from_this()](error_code ec) {
if (!ec) {
std::cerr << "Timeout" << std::endl;
_socket.cancel();
}
});
}
tcp::endpoint _ep;
duration _max_idle;
tcp::socket _socket;
boost::asio::high_resolution_timer _timer;
std::deque<std::string> _outbox;
};
class TCPClient {
boost::asio::any_io_executor _ex;
std::deque<Handle> _comms;
public:
TCPClient(boost::asio::any_io_executor ex) : _ex(ex) {}
void Add(tcp::endpoint ep, duration max_idle = 3s)
{
auto pcomm = std::make_shared<Comm>(make_strand(_ex), ep, max_idle);
pcomm->Start();
_comms.push_back(pcomm);
// optionally garbage collect expired handles:
std::erase_if(_comms, std::mem_fn(&Handle::expired));
}
void TransmitData(std::string_view msg) {
for (auto& handle : _comms)
if (auto pcomm = handle.lock())
pcomm->TransmitData(msg);
}
void Stop() {
for (auto& handle : _comms)
if (auto pcomm = handle.lock())
pcomm->Stop();
}
};
int main() {
using std::this_thread::sleep_for;
boost::asio::thread_pool ctx(1);
TCPClient c(ctx.get_executor());
c.Add({{}, 8989});
c.Add({{}, 8990}, 1s); // shorter timeout for demo
c.TransmitData("Hello world\n");
c.Add({{}, 8991});
sleep_for(2s); // times out second connection
c.TransmitData("Three is a crowd\n"); // only delivered to 8989 and 8991
sleep_for(1s); // allow for delivery
c.Stop();
ctx.join();
}
Prints (on Coliru):
for p in {8989..8991}; do netcat -t -l -p $p& done
sleep .5; ./a.out
Hello world
Connect: Success
Connect: Success
Hello world
Connect: Success
Write 12 bytes to 0.0.0.0:8989 Success
Write 12 bytes to 0.0.0.0:8990 Success
Timeout
Comm closed (0.0.0.0:8990)
Write Three is a crowd
17Three is a crowd
bytes to 0.0.0.0:8989 Success
Write 17 bytes to 0.0.0.0:8991 Success
Comm closed (0.0.0.0:8989)
Comm closed (0.0.0.0:8991)
The output is a little out of sequence there. Live local demo: