Question

Given only thread id, is it possible to decide from there whether the thread is still alive or not? I’m talking about C++ on linux.

Answer 1

From the manpage:

DESCRIPTION

   The pthread_kill() function shall request that a signal  be  deliv-
   ered to the specified thread.

   As in kill(), if sig is zero, error checking shall be performed but
   no signal shall actually be sent.

so:

bool isalive(int threadid)
{
  return pthread_kill(threadid, 0) != ESRCH;
}