fork() Primer

• A fork refers to the way Unix creates new processes.
  • Processes can refer to terminal commands, programs, games, etc.
• How it works is the parent process (already existing) fork()’s a child process (new one) and the child process receives a copy of the parents data.
  • We end up with 2 processes when there was initially only one.

Process “Death”

• After a **child process “**dies” or finishes execution, it actually does not go away completely but it is stopped and no longer running i.e. dead / zombie.
• However, the kernel keeps a small remnant of this process in case the parent process picks up or starts again.
  • Usually waits until the parent process calls wait() or waitpid() to read the exit status i.e. wait for the child process to exit.
  • When this happens, the remnant of the child process vanishes (from the process table).
  • It also does not mean that once a child process dies, a parent process dies.
• But there are exceptions to this as well.

Normal Flow

• A child process that dies becomes a zombie until the parent process calls wait() or waitpid() to read its exit status
• If the parent never calls wait(), the zombie sticks around (shows as <defunct> in ps).

Exception #1 - Ignoring SIGCHLD

• SIGCHLD is sent to a parent whenever one of its children dies.
• On some systems, if the parent tells the kernel "I’m ignoring SIGCHLD", the kernel automatically reaps the child
  • This means that there is no zombie process left and no wait() is needed.

// Ignore the signal
signal(SIGCHLD, SIG_IGN)

Exception #2 — Parent dies first

• If the parent dies before the child finishes:
  1. The child gets re-parented to init (PID 1) — a special system process whose job includes cleaning up orphans (child processes with no parents).
  2. If the child later dies:
     • On most systems, init will automatically wait() for it → no zombie.
     • On some systems, init might refuse to adopt already-defunct children and just kill them instantly.

Best Practices

• Call wait() for every child you fork()
• Explicitly ignore SIGCHLD if you don’t care about the exit status of the child process

The Full Process