当整个过程终止时，未连接和未分离的线程会发生什么情况？

Question

It is expected that threads, on which pthread_detach() was not called, should be pthread_join() ed before the main thread returns from main() or calls exit() .

However, what happens when this requirement is not met? What happens when a process terminates when it still contains unjoined and not detached threads?

I would find it odd to learn that these other threads' resources will not be reclaimed until system reboot. However, if these resources will be reclaimed, then there may be little need to bother about joining or detaching, mightn't it?

Answer 1

It is up to the operating system. Typical modern operating systems will indeed reclaim the memory and descriptors (handles) used by abandoned threads. This is similar to how dynamically allocated memory works: typical modern systems will reclaim it when a process exits, even if the process never explicitly freed the memory. For certain unusual programs, this can be a meaningful performance optimization, because freeing lots of small resources takes time and the OS may be able to do it more quickly.

Answer 2

However, what happens when this requirement is not met? What happens when a process terminates when it still contains unjoined and not detached threads?

On any system with POSIX threads that is not ancient, the non-joined threads simply "evaporate" into space when the SYS_exit system call is performed by the main thread.

I would find it odd to learn that these other threads' resources will not be reclaimed until system reboot.

They will be.

However, if these resources will be reclaimed, then there may be little need to bother about joining or detaching, mightn't it?

It depends on what these threads do. The danger is at-exit data races.

In C++ , global variables are destructed (usually via atexit or equivalent registration mechanism), FILE handles are deleted, etc. etc.

If non-joined thread tries to access any such resource, it will likely crash with SIGSEGV , possibly producing core dump, and an unclean process exit code, both of which are often quite undesirable.