What Happens to Detached Threads After the Main Function Terminates in C ?

What Happens to a Detached Thread Upon Main Function Termination?

When a std::thread is detached using detach(), it continues executing independently after the main thread exits. However, there are specific conditions and consequences to consider in this scenario.

Undefined Behavior without Reliable Joining

If the program lacks a reliable mechanism to join detached threads before main() exits, their behavior becomes undefined. Essentially, std::thread::detach() cannot be used unless the main thread runs indefinitely.

Defined Effects in the C Standard

Despite the absence of explicit language in the standard regarding detached threads in this context, their continuation is well-defined. Detached threads may continue running indefinitely, provided they do not interact with certain types of variables:

• Automatic or thread-local variables of other threads
• Static objects

Limitations and Potential Issues

Once the destruction of static objects is complete, execution enters a restricted mode where only atomic library code is allowed. This poses a challenge for detached threads if they utilize other C standard library components, such as condition variables.

Joining Detached Threads

By design, detached threads cannot be joined using std::thread::join(). However, they can be joined asynchronously using functions from the *_at_thread_exit family (e.g., notify_all_at_thread_exit()). These functions facilitate signaling and synchronization when a detached thread terminates its execution.

Avoiding Undefined Behavior

To avoid undefined behavior, detached threads should either be joined manually using *_at_thread_exit functions or restricted to executing code that is safe within a signal handler. This ensures that detached threads do not interact with critical variables or engage in unsafe operations.

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