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. > 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
