 Modern operating systems also provide system calls that allow a process's thread to create other threads and wait for them to terminate ("join" them) in a similar fashion.An operating system may provide variations of the "wait" call that allow a process to wait for any of its child processes to exit, or to wait for a single specific child process (identified by its process ID) to exit.Some operating systems issue a signal (SIGCHLD) to the parent process when a child process terminates, notifying the parent process and allowing it to retrieve the child process's exit status.