I am working with POSIX threads for a multi-threaded socket programming project. I have run into a situation where I need to detach a thread from the main program using setdetachstate(); however, later on I cancel the thread (I know that cancelling is generally bad practice, but I know what I'm doing (hopefully)). I need a method to check whether the thread is still alive or not, and after doing a bit of research, I found that waitpid() might work for my purposes even though I have a TID instead of a PID. However, after trying it out, both with and without ptraces, it didn't work. Another method that I have seen on the Internet everywhere is pthread_join(). While I agree that it is the optimal way to do it, as I said, my thread is detached, so it can't be joined.
As a side note, my goal is to find a way to wait for the function call pthread_cancel() to finish before executing any subsequent code, i.e.
pthread_t tid;
// ...
pthread_cancel(tid);
// wait until pthread with ID tid is cancelled
// more code here...
Originally, the reason why I need to check whether the detached pthread is alive was because I was planning on doing something like this: while(!pthread_dead(tid)); or something of this manner; however, if there is a solution that directly waits for the cancel to finish, that would be even better. Please try not to criticize my use of detached threads or pthread cancelling; I have contemplated many plans of action and this seems to be required no matter how I go about it (unless I'm doing a multiprocessed application, which I don't want to do). Unless I'm doing something absolutely syntactically or structurally abominable, I would appreciate it if you just answered my question.
Thank you!
P.S. I'm coding in C++.
Have you thought about using Actor model programming, or even better Communicating Sequential Processes?
These are really quite a good model for when you have a separate thread that needs to go off and do its own thing, and you need to be able to tell it something and get an answer back.
Your apparent need is to know that something asynchronous has completed (the termination of a separate thread) - there's nothing wrong with having that thread send you a direct acknowledgement of it's termination, rather than trying to have to determine whether or not it's still alive through slightly iffy means such as waitpid(). So say you chose ZeroMQ as your Actor model library; to "kill" that detached thread you'd send it a command down a ZeroMQ "socket". The recipient thread would receive that message, understand that it means "die", and do whatever clean up it needs to before terminating itself. Just before it terminates itself, it sends you back an acknowledgement on another "socket" that yes, it is dead (or at least about to be so, all necessary cleanup has already happened).
Actor model / CSP programming places an emphasis on having a loop responding to messages from one or more sources. Well, your own code snippet hints at a loop, waiting for the pthread_cancel() to take effect.
I've put "socket" in quotes as underneath a ZeroMQ socket can be a tcp socket, ipc, some in-process memory transfer, etc; it all behaves the same. In-proc is, naturally, quite quick.
The difference between Actor model and Communicating Sequential Processes is that in Actor model, when a message is sent there is no information available to the sender that it has been received, whilst in Communicating Sequential Processes a successful send = a completed read. Personally speaking I prefer the latter - your code then has complete knowledge as to where a message recipient has got to; a send/receive are an Execution Rendezvous. So when you send the "terminate" message, you know for sure that the recipient thread has received the message and is now acting on it. When the recipient sends it's "I'm dead" acknowledgement, it knows that the command thread has received that ack.
FYI, CSP is very useful in real time systems, not because it's faster but because your program can have much better knowledge as to whether it's kept up with the real time demand or not. Actor model lets you "hide" real time inadequacies as latency in communications links.