Don’t Forget to Reap your Zombies
Unix unix
Published: 2008-03-07
Don't Forget to Reap your Zombies

I recently received a bug report for my quick-and-dirty TCP debugging tool tcpconndbg where it was creating a large number of zombie processes. The person who filed the bug, Peter Viskup, was even kind enough to send a patch. While this is old news to anyone with extensive Unix programming experience, always remember the following:

If you create a child process using fork(), you must either:

  1. Explicitly retrieve the child process’s exit code using one of the wait() functions (e.g. waitpid())
  2. Tell the system that you aren’t interested in the child process’s exit code by using either:
    1. sigaction() with the SA_NOCLDWAIT parameter (preferred)
    2. signal(SIGCHILD, SIG_IGN); (for systems which do not support sigaction())

As I fixed this bug, I realized I hadn’t looked at tcpconndbg in 5 years. My how programming style changes…