What Happens to Detached Threads When the Main Thread Exits?

Fate of Detached Threads Upon Main Thread Termination

Q: What becomes of detached threads when the main() function exits without a reliable protocol for joining them?

The C standard leaves the outcome of this situation unspecified, leading to the question of whether it constitutes undefined behavior.

A: No, running out of main() with detached threads running is not explicitly defined as undefined behavior. According to the answer, detached threads continue executing but must avoid accessing variables of other threads or static objects. This is to ensure the thread does not trigger unexpected behavior during the destruction phase of static objects, when only code allowed in signal handlers can execute.

Q: Can detached threads be joined after being detached?

A: Yes, through the use of *_at_thread_exit functions like notify_all_at_thread_exit(). These functions allow signaling to occur at the very end of the detached thread's execution, after all other code has run. To avoid undefined behavior, detached threads should either be joined manually using the _at_thread_exit functions or execute only code that is safe for signal handlers.

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