僵尸进程

产生僵尸进程的情况是：子进程在父进程调用wait()函数之前结束。

以下代码可以产生僵尸进程：

#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int main ()
{
  pid_t child_pid;
  /* Create a child process. */
  child_pid = fork ();
  if (child_pid > 0) {
    /* This is the parent process. Sleep for a minute. */
    sleep (60);
  }
  else {
    /* This is the child process. Exit immediately. */
    exit (0);
  }
  return 0;
}

僵尸进程的父进程结束后会自动继承为父进程是init的子进程，init具有自动清除僵尸进程的能力。因此，当僵尸进程的亲夫进程终结后，僵尸进程也会立即被清除。

僵尸进程的消除，可以利用SIGCHLD信号，因为当一个子进程终止的时候，Linux会向它的父进程发送SIGCHLD信号。

以下代码掩饰如何利用此方法防止僵尸进程：

#include <signal.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
sig_atomic_t child_exit_status;
void clean_up_child_process (int signal_number)
{
  /* Clean up the child process. */
  int status;
  wait (&status);
  /* Store its exit status in a global variable.  */
  child_exit_status = status;
}
int main ()
{
  /* Handle SIGCHLD by calling clean_up_child_process. */
  struct sigaction sigchld_action;
  memset (&sigchld_action, 0, sizeof (sigchld_action));
  sigchld_action.sa_handler = &clean_up_child_process;
  sigaction (SIGCHLD, &sigchld_action, NULL);
  /* Now do things, including forking a child process.  */
  /* ... */
  return 0;
}