Inter-process synchronization methods

You want to synchronize the execution of a parent and child process on Linux, so that the parent waits for the child to do step 1, then the child waits for the parent to do step 2, and so on. How do you do it?

(Motivating example: The test program I wrote for "POSIX semaphores do not auto-release on exit")

• waitpid is the simplest method, but it only lets one process wait for the other to exit.
• It's natural to reach for signals: process A calls pause, process B calls kill. Except pause suffers from the missed-wakeup problem, so you need to mask the signal first with sigprocmask, then unblock the process, and then call sigwait. Also, make sure to remember to use sigwait instead of sigsuspend, as the former swallows terminating signals while the latter doesn't, and SIGUSR1 is a terminating signal. And definitely don't try to set SIGUSR1 to SIG_IGN to prevent that, because sigsuspend doesn't wake up for ignored signals even if they are in the signal mask you pass to it.
• If you've lost your appetite for signals, then consider taking advantage of the blocking nature of reading from pipes as a simple synchronization mechanism. Process A calls read to wait, process B calls write to unblock. You can just write an arbitrary byte since it is the receipt of the data and not its contents that matters. What makes this slightly awkward is that bidirectional communication requires two pipes and thus four file descriptors.
• A little more lightweight than pipes is eventfd, which is very similar except that there is only one file descriptor that you can both read and write (or use the glibc wrapper functions eventfd_read and eventfd_write), and it holds a single 8-byte integer instead of a large buffer. Just remember that you have to write something other than 0 (unlike for pipes) because 0 is the special empty value.
• A little more heavyweight than pipes is socketpair to create an anonymous full-duplex connection with two sockets.
• Of course you can use POSIX semaphores.

I find eventfd the cleanest, but (unlike pipes and socketpair) it can't detect when the other process has exited abnormally.