What happens to other threads when one thread forks()?
Nothing. Only the thread calling fork() gets duplicate. The child process has to start any new threads. The parents threads are left alone.
In POSIX when a multithreaded process forks, the child process looks exactly like a copy of the parent, but in which all the threads stopped dead in their tracks and disappeared.
This is very bad if the threads are holding locks.
For this reason, there is a crude mechanism called pthread_atfork in which you can register handlers for this situation.
Any properly written program module (and especially reusable middleware) which uses mutexes must call pthread_atfork to register some handlers, so that it does not misbehave if the process happens to call fork.
Besides mutex locks, threads could have other resources, such as thread-specific data squirreled away with pthread_setspecific which is only accessible to the thread (and the thread is responsible for cleaning it up via a destructor).
In the child process, no such destructor runs. The address space is copied, but the thread and its thread specific value is not there, so the memory is leaked in the child. This can and should be handled with pthread_atfork handlers also.
Quoting from http://thorstenball.com/blog/2014/10/13/why-threads-cant-fork/
If we call fork(2) in a multi-threaded environment the thread doing the call is now the main-thread in the new process and all the other threads, which ran in the parent process, are dead. And everything they did was left exactly as it was just before the call to fork(2).