std::future.wait_for blocking forever

Question

I am crating SSL socket and when I try to connect to that socket my program hangs.

I found some issue that server does not send any data so SSL connection is waiting for the any response.

I decided to create future and kill it after timeout. But it is still hanging.

If you see to the code, below part is not executed: cout<<"TEST";

{

     std::future<void> future = std::async(std::launch::async, 
     [&](){ 
              err = SSL_connect (m_ssl); 
          });

     std::future_status status;
     status = future.wait_for(std::chrono::seconds(t_timeout));
}

cout<<"TEST";

To simulate stopping response from server I just run:

for item in $(seq 1 1000); do sudo nc -i 100000ms -l 443 >> log.out; done;

How do I kill the SSL connection? I want to continue execution of code after kill the future.

EDIT: I do not want to open another question for that. Answer : So i am sure now that this is because of future destructor. It is waiting for finish of code from the future body.

Question 2: How Can I fix above issue? I want to execute code after scope which future is in.

Is it OK to create thread and wait until timeout or ssl some mutex is unlocked?

Answer 1

The problem with your code is that when the future goes out of scope, its destructor might block as reported in the documentation for std::future<T>::~future :

It says explicitly:

... will not block for the shared state to become ready, except that it may block if all of the following are true: the shared state was created by a call to std::async, the shared state is not yet ready, and this was the last reference to the shared state.

In your case:

1. the shared state is not yet ready: SSL hangs forever
2. that is the only (and so last) reference to the shared state.

---

The solution to your problem is that you have to make the scope of variable std::future<void> future such that it outlives the part of the code you want not to be blocked.

Check these answers:

• main thread waits for std::async to complete
• Can I use std::async without waiting for the future limitation?

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