When the application calls fork() from a signal handler and any of the fork handlers registered by pthread_atfork() calls a function that is not async-signal-safe, the behavior is undefined.

While the fork() function is async-signal-safe, there is no way for an implementation to determine whether the fork handlers established by pthread_atfork() are async-signal-safe. The fork handlers may attempt to execute portions of the implementation that are not async-signal- safe, such as those that are protected by mutexes, leading to a deadlock condition. It is therefore undefined for the fork handlers to execute functions that are not async-signal-safe when fork() is called from a signal handler.