How are PIDs generated on Ubuntu?

I've just wrote a program that forks one process. The child process just displays "HI" 200 times. The father process just says he's the father. I've printed out both pids. When I run my program multiple times, I see that the parent's pid stays the same, which is normal. What I don't understand is why the child's pid keeps getting incremented by 2, and exactly 2.开发者_JS百科 My question: Is this the standard method of pid generation in Ubuntu? Incrementing by 2?

PIDs happen to be handed out monotonically increasing in Linux 2.6, but why does it matter which you get? Don't rely on any specific behavior. If there is a skip of +2 it might simply be because another process happened to spawn a child. Or because +1 would have reached a PID that is already in use.

Found a reference here saying that vfork() consumes a pid as a byproduct of its operation. As well, in some cases, if you're forking from a shell script, the fork might spawn a new shell before your actual script gets involved, which would also consume a pid.

I'd suggest suspending your program between a couple forks, and see if there's another process occupying those "missing" pids.