How to pass struct to a new thread (C++) - c++

Question

So, I am writing a small winsock app and I need to make a multi-client server.
I decided to use threads for every new connection, the problem is that I don't know how to pass multiple data to a thread, so I use struct.
Struct:
typedef struct s_par {
char lttr;
SOCKET clientSocket;
} par;
_stdcall:
unsigned __stdcall ClientSession(void *data) {
par param = data;
char ch = param.lttr;
SOCKET clntSocket = param.clientSocket;
// ..working with client
}
Main:
int main() {
unsigned seed = time (0);
srand(seed);
/*
..........
*/
SOCKET clientSockets[nMaxClients-1];
char ch = 'a' + rand()%26;
while(true) {
cout << "Waiting for clients(MAX " << nMaxClients << "." << endl;
while ((clientSockets[nClient] = accept(soketas, NULL, NULL))&&(nClient < nMaxClients)) {
par param;
// Create a new thread for the accepted client (also pass the accepted client socket).
if(clientSockets[nClient] == INVALID_SOCKET) {
cout << "bla bla" << endl;
exit(1);
}
cout << "Succesfull connection." << endl;
param.clientSocket = clientSockets[nClient];
param.lttr = ch;
unsigned threadID;
HANDLE hThread = (HANDLE)_beginthreadex(NULL, 0, &ClientSession, &param, 0, &threadID);
nClient++;
}
The problem is that I get errors with data type conversion. Maybe someone could suggest an easy fix with passing this struct to a thread?

Answer 1

With each round of your while-loop you're doing two ill-advised activites:
Passing the address of an automatic variable that will be destroyed with each cycle of the loop.
Leaking a thread HANDLE returned from _beginthreadex
Neither of those is good. Ideally your thread proc should look something like this:
unsigned __stdcall ClientSession(void *data)
{
par * param = reinterpret_cast<par*>(data);
char ch = param->lttr;
SOCKET clntSocket = param->clientSocket;
// ..working with client
delete param;
return 0U;
}
And the caller side should do something like this:
par *param = new par;
param->clientSocket = clientSockets[nClient];
param->lttr = ch;
...
HANDLE hThread = (HANDLE)_beginthreadex(NULL, 0, &ClientSession, param, 0, &threadID);
if (hThread != NULL)
CloseHandle(hThread);
else
delete param; // probably report error here as well
That should be enough to get you going. I would advise you may wish to take some time to learn about the C++11 Threading Model. It makes much of this considerably more elegant (and portable!).
Best of luck.