Monitoring Child Processes

wait() and waitpid() can monitoring if the child process is terminated and their status when exit() in the MACRO: WEXITSTATUS (and other MACROs like WIFEXITED, WIFSTOPPED)

Orphan and Zombie

The lifetimes of parent and child processes are usually not the same—either the
parent outlives the child or vice versa.

Zombie Process

子进程结束快，父进程还在跑也没wait(),虽然子进程已经结束，但是还占着进程表, 这个时候子进程的状态变为EXIT_ZOMBIE, 并且the process’s parent被kernel通过SIGCHLD signal告知"你孩子死了". 然后父进程应该去执行wait()然后给孩子收尸，在wait()以后, zombie process completely removed.

This normally happens very quickly, so you won’t see zombie processes accumulating on your system. However, if a parent process isn’t programmed properly and never calls wait(), its zombie children will stick around in memory until they’re cleaned up.

A process which has finished the execution but still has entry in the process table to report to its parent process is known as a zombie process. A child process always first becomes a zombie before being removed from the process table. The parent process reads the exit status of the child process which reaps off the child process entry from the process table.

int main()
{
    // Fork returns process id
    // in parent process
    pid_t child_pid = fork();

    // Parent process 
    if (child_pid > 0)
        sleep(50);

    // Child process
    else       
        exit(0);

    return 0;
}

Orphan Process

父进程太快，没了以后子进程没了爹娘被 init process 收养
Parent finishes execution and exits while the child process is still executing and is called an orphan process now

int main()
{
    // Create a child process      
    int pid = fork();

    if (pid > 0)
        printf("in parent process");

    // Note that pid is 0 in child process
    // and negative if fork() fails
    else if (pid == 0)
    {
        sleep(30);
        printf("in child process");
    }

    return 0;
}