1.6 How do I get rid of zombie processes?

1.6.1 What is a zombie?

When a program forks and the child finishes before the parent, the kernel still keeps some of its information about the child in case the parent might need it – for example, the parent may need to check the child's exit status. To be able to get this information, the parent calls wait(); when this happens, the kernel can discard the information.

In the interval between the child terminating and the parent calling wait(), the child is said to be a `zombie'. (If you do `ps', the child will have a `Z' in its status field to indicate this.) Even though it's not running, it's still taking up an entry in the process table. (It consumes no other resources, but some utilities may show bogus figures for e.g. CPU usage; this is because some parts of the process table entry have been overlaid by accounting info to save space.)

This is not good, as the process table has a fixed number of entries and it is possible for the system to run out of them. Even if the system doesn't run out, there is a limit on the number of processes each user can run, which is usually smaller than the system's limit. This is one of the reasons why you should always check if fork() failed, by the way!

If the parent terminates without calling wait(), the child is `adopted' by init, which handles the work necessary to cleanup after the child. (This is a special system program with process ID 1 – it's actually the first program to run after the system boots up).

1.6.2 How do I prevent them from occuring?

You need to ensure that your parent process calls wait() (or waitpid(), wait3(), etc.) for every child process that terminates; or, on some systems, you can instruct the system that you are uninterested in child exit states.

Another approach is to fork() twice, and have the immediate child process exit straight away. This causes the grandchild process to be orphaned, so the init process is responsible for cleaning it up. For code to do this, see the function fork2() in the examples section.

To ignore child exit states, you need to do the following (check your system's manpages to see if this works):

    struct sigaction sa;
    sa.sa_handler = SIG_IGN;
#ifdef SA_NOCLDWAIT
    sa.sa_flags = SA_NOCLDWAIT;
#else
    sa.sa_flags = 0;
#endif
    sigemptyset(&sa.sa_mask);
    sigaction(SIGCHLD, &sa, NULL);

If this is successful, then the wait() functions are prevented from working; if any of them are called, they will wait until all child processes have terminated, then return failure with errno == ECHILD.

The other technique is to catch the SIGCHLD signal, and have the signal handler call waitpid() or wait3(). See the examples section for a complete program.

This document was generated on September, 10 2007 using texi2html 1.77.