A process shall be created with a single thread. If a multi-threaded process calls fork(), the new process shall contain a replica of the calling thread and its entire address space, possibly including the states of mutexes and other resources. Consequently, to avoid errors, the child process may only execute async-signal-safe operations until such time as one of the exec functions is called. [THR]   Fork handlers may be established by means of the pthread_atfork() function in order to maintain application invariants across fork() calls.
  

  

  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 asynch-signal-safe, the behavior is undefined.