How to convert ThreadId to (LWP) PID?

There are 2 functions:

forkIO :: IO () -> IO ThreadId
forkOS :: IO () -> IO ThreadId

and the thread ID is data ThreadId - that's all what it is available from it (internally it's data ThreadId = ThreadId ThreadId#). So, its "argument" is not available but it is related to an address, not to PID. At the same time Haskell threads (ie, POSIX threads) are mapped to some LWP in Linux (no idea about other OSes, like Mac, Windows), I suppose as N:M. I can list them in Linux box with ps -AL:

........
15001 15001 ?        00:00:00 haskell-languag
15001 15177 ?        00:00:02 ghc_ticker
15001 15178 ?        00:00:56 haskell-langu:w
15001 15179 ?        00:00:47 haskell-langu:w
15001 15180 ?        00:01:00 haskell-langu:w
15001 15181 ?        00:00:56 haskell-langu:w
15001 15182 ?        00:00:07 haskell-langu:w
15001 15183 ?        00:00:57 haskell-langu:w
........

Is there a way to find PID (possibly LWP) from ThreadId? And related question: will it be cross-platform (to make sense for other OSes) - on OsX, Windows, if such way exists?

Unbound threads, those created by forkIO, are not tied to any specific POSIX / OS thread. They might be run on an OS thread for a while, and then continue on another OS thread. Even if you found a way to get their current PID, that information can become out-of-date quickly.

Bound threads, those created by forkOS, are instead tied to a specific POSIX / OS thread. These threads have access to thread-local state, and can call C libraries that make use of such state. I can't find a way to get the PID from the thread ID, but I guess you might get the current PID in the thread and advertise it to the other threads in some way (e.g. using shared state variables like MVars).

Even with bound threads, I believe the OS thread with that PID is not exclusively dedicated to the bound thread. The mapping Haskell_threads:OS_threads is still not 1:1.