Multithreading in c++ ending unexpectledly - c++

Question

I'm a newbie in multithread, so the problem is: I'm trying to make multiple threads send message to a server, ever thread represents a device. The server who I'm testing has 10k ports open so every socket has his exclusively port. The problem is the program subtly stops and don't send all messages from all devices. I'm using a vector of futures to simulate the devices, that way:
vector<future<void>> futures;
map<string,vector<string> >::iterator it = mapDevtoMessage.begin();
for(it = mapDevtoMessage.begin(); it != mapDevtoMessage.end(); it++){
futures.push_back(async(launch::async,[&]{ send_messages(it->second, it->first);}));
std::this_thread::sleep_for(std::chrono::milliseconds(10));
}
for(auto &its : futures){
its.wait();
}
The code for send_messages is:
void send_messages (vector<string>& vec, string name){
mtx.lock();
PORT++;
mtx.unlock();
int sock = 0, client_fd;
struct sockaddr_in serv_addr;
if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
printf("\n Socket creation error \n");
}
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(PORT);
if (inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr) <= 0) {
printf("\nInvalid address/ Address not supported \n");
}
if ((client_fd
= connect(sock, (struct sockaddr*)&serv_addr,
sizeof(serv_addr)))
< 0) {
printf("\nConnection Failed \n");
}
for(auto &i : vec){
string val = hexToASCII(i);
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
send(sock, val.c_str(), 100, 0);
mtx.lock();
printf("\nmessage %s from %s send\n", i.c_str() , name.c_str());
mtx.unlock();
}
// closing the connected socket
close(client_fd);
}
I added the for loop with the wait() method because the program keep terminating unexpectledly without any error or anything. So I added, but the program continues to subtle terminate. I tried an endless loop also. I may have missed a simple thing (I'm a noob), please let me know.