Running an old threads app on a current system

I have an app (that I wrote) that uses threads. It was compiled with
"Linux threads" - such that when run on the original system, you ended
up with 3 processes showing up in "ps".

Now, I'm running the app on a new system, which is, I believe, running
"Posix threads", and have noticed that you no longer have multiple
processes running. I find this surprising; I had assumed you had to
recompile it in order to make it run with Posix threads. But, it seems
to "just work". But what I find really strange is the output of
"pstree", which is like this:

myapp(2551)---{myapp}(2563)

2551 is the pid of myapp (the only one). There is no process 2563.
So, what's up with that?

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