fork and existing threads?
Threads on Linux nowadays try to stay POSIX compliant. Only the calling thread is replicated, not other threads (note that e.g. on Solaris you can choose what fork does depending on what library you link to)
From http://www.opengroup.org/onlinepubs/000095399/functions/fork.html (POSIX 2004):
A process shall be created with a single thread. If a multi-threaded process calls
fork(), the new process shall contain a replica of the calling thread and its entire address space, possibly including the states of mutexes and other resources. Consequently, to avoid errors, the child process may only execute async-signal-safe operations until such time as one of theexecfunctions is called. Fork handlers may be established by means of thepthread_atfork()function in order to maintain application invariants acrossfork()calls.
The POSIX 2018 specification of fork() is similar.
Threads are not inherited from a child process on a linux system using fork(). An in-depth source is here: http://linas.org/linux/threads-faq.html