On Mon, 2005-02-21 at 00:21 -0500, Deron Meranda wrote: > On Sun, 20 Feb 2005 23:46:56 -0500, Scot L. Harris <webid@xxxxxxxxxx> wrote: > > So if the parent process crashes all of the child processes will be > > cleared? > > When a parent process dies, all of it's children (whether currently > running or if they're zombies), are "inherited" by the main init > process, always process ID 1. And init will always immediately reap > any zombies. (Init is really a fake process which serves as the > process-face to the kernel itself.) So in effect, if the parent process > of a zombie dies, then the zombies will immediately be reaped and > disappear almost immediately. > > NOT always. Some processes leave zombies that last forever (until the next reboot). > > I thought the parent process had to issue an _exit() (or > > something similar) to get the child process status so it would exit > > cleanly. > > The parent needs to call one of the wait() system calls [which > includes wait(), waitpid(), wait3(), wait4()]. That will return > to the parent the exit code that the kernel has been remembering; > and which also "reaps" the zombie and makes it disappear. > [Or, instead, the parent can tell the kernel to do auto-reaping > via the SA_NOCLDWAIT signal action flag.] > > The exit() call is performed by a child process when it wishes > to pass back a specific exit code. This is done automatically > in C/C++ when a return is made from main(). But, even if a > child doesn't call exit() because it terminated some other way > (like a core dump), the parent must still call wait(). > > It can be a little trickier on Linux, especially if you start > talking about advanced subjects like multithreaded processes > and the clone() system call. But the most common case > is the child calls exit() and the parent calls wait(). The > time in between the two calls is when a zombie exists. > > > > If it did not do that then the child process sits around > > waiting for the parent which is no longer running so it stays forever. > > When a child dies, it dies. It doesn't need to wait around on > any other process. Of course a zombie may result, which only > means that the parent hasn't yet recognized that the child has > died. The only exception is when the child process is currently > being traced (such as being debugged)..in which case the > tracing program must detach before the child actually dies. > > -- > Deron Meranda >
