What Happens to Detached Threads When the `main()` Function Exits?

Detached Threads and the Consequences of Main() Exit

Consider a scenario where a detached thread is launched and continues execution after its controlling std::thread object goes out of scope. The standard's potential silence on the subject has sparked concern: what becomes of a detached thread when the main() function exits?

Undefined Behavior or Defined Effects?

The standard lacks explicit guidelines on this matter. However, the response to the original question provides clarification:

• Detached threads continue running upon main() exit, as no explicit provisions for termination are specified in the standard.
• This behavior is well-defined, provided the detached thread avoids accessing automatic or thread-local variables of other threads or static objects.

Joining Detached Threads and Signal Handlers

Addressing the second question, the response asserts that detached threads can be joined via the *_at_thread_exit family of functions. These functions enable signaling the completion of a thread's execution while ensuring that all destructors have been invoked.

However, employing condition variables or semaphores alone is insufficient for reliable joining. The standard enforces limitations on code execution during signal handling, excluding the use of condition variables unless explicitly permitted by the implementation.

Consequences for Program Behaviour

In light of these observations, the standard's constraints on thread behavior upon main() exit dictate specific actions for developers:

• For safety, join detached threads manually using *_at_thread_exit functions or restrict their execution to code permissible within signal handlers.
• Failure to comply with these guidelines may result in undefined behavior when static object destruction is complete.

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