linux多进/线程编程（3）——wait、waitpid函数和孤儿、僵尸进程

当使用fork创建多个进程后，需要解决子进程回收的问题。wait和waitpid函数就是做这个工作的。

假设子进程没有合理的回收，可能会带来两个问题：

1.孤儿进程（父进程挂了，子进程活着），孤儿进程会被init进程回收，可以理解其没有危害，不会占用资源。

2.僵尸进程（子进程挂了，父进程活着），僵尸进程是当父进程活着时，子进程没有其他进程帮忙回收后产生的“有害进程”，所以僵尸必须要回收，否则僵尸进程太多了会占用系统资源。此时就需要用到wait或waitpid函数。

wait和waitpid的区别在于：

wait函数是阻塞的，必须等到子进程被回收才会执行wait之后的代码；

waitpid可以设置为非阻塞的，不过非阻塞可能带来新的问题，子进程还没有回收waitpid这句代码就走完了（一般会写循环，通过判断返回值解决）。

wait:

WAIT(2)                                                                                Linux Programmer's Manual                                                                               WAIT(2)

NAME
       wait, waitpid, waitid - wait for process to change state

SYNOPSIS
       #include <sys/types.h>
       #include <sys/wait.h>

       pid_t wait(int *status);

       pid_t waitpid(pid_t pid, int *status, int options);

返回值：

RETURN VALUE
       wait(): on success, returns the process ID of the terminated child; on error, -1 is returned.

       waitpid(): on success, returns the process ID of the child whose state has changed; if WNOHANG was specified and one or more child(ren) specified by pid exist, but have not yet changed state,
       then 0 is returned.  On error, -1 is returned.

waitpid的参数：

The child state changes to wait for are specified by ORing one or more of the following flags in options:

       WEXITED     Wait for children that have terminated.

       WSTOPPED    Wait for children that have been stopped by delivery of a signal.

       WCONTINUED  Wait for (previously stopped) children that have been resumed by delivery of SIGCONT.

       The following flags may additionally be ORed in options:

       WNOHANG     As for waitpid().

       WNOWAIT     Leave the child in a waitable state; a later wait call can be used to again retrieve the child status information.

wait用法示例：

pid_t pid = fork();

  if (pid == 0) {

    //while (true) {
      //sleep(1);
    //}
    printf("i am child, i will die\n");
    sleep(2);
    //return 101;
    exit(2);
  } else if (pid > 0) {
    printf("i am parent, wait for a child die\n");
    int status { 0 };

    pid_t wait_pid = wait(&status);//1，阻塞等待（子进程死亡）；2，回收子进程资源（避免僵尸进程）；3，查看状态（死亡原因）
    printf("wait ok, wait_pid=%d, pid=%d\n", wait_pid, pid);

    if (WIFEXITED(status)) {
      printf("child exit with %d\n", WEXITSTATUS(status));
    }

    if (WIFSIGNALED(status)) {
      printf("child killed by %d\n", WTERMSIG(status));
    }

    while (true) {
      sleep(1);
    }
  }

waitpid用法示例：

pid_t pid = fork();
  int ret { 0 };

  if (pid == 0) {
    printf("i am child, pid=%d\n", getpid());
    sleep(2);

    exit(101);
  } else if (pid > 0) {
    printf("i am parent, self=%d, child_pid=%d\n", getpid(), pid);

    //不阻塞可能会带来一个问题，子进程还没回收，这句代码就走完了，所以要加循环（让代码阻塞在这里。。这。。。那还不如直接用wait????）
    //WNOHANG: 不阻塞
    int status { 0 };

    while ((ret = waitpid(-1, &status, WNOHANG)) == 0) {
      sleep(1);
    }
    printf("ret=%d\n", ret);

    if (WIFEXITED(status)) {
      printf("child exit with %d\n", WEXITSTATUS(status));
    }

    if (WIFSIGNALED(status)) {
      printf("child killed by %d\n", WTERMSIG(status));
    }

    ret = waitpid(-1, nullptr, WNOHANG);

    if (ret < 0) {
      perror("wait err");
    }

    while (true) {
      sleep(1);
    }
  }

waitpid回收多个子进程（且忽略子进程状态即死亡原因）

  int i { 0 };
  int n { 5 };
  int ret { 0 };
  pid_t pid { 0 };

  for (i = 0; i < n; ++i) {
    pid = fork();
    if (pid == 0) {
      printf("i am child, pid=%d\n", getpid());

      sleep(2);
      break;
    }
  }

  if (pid > 0) {

    while ((ret = waitpid(-1, nullptr, WNOHANG)) != -1) {
      sleep(1);
      printf("ret = %d\n", ret);
    }

    printf("ret = %d\n", ret);

    while (true) {
      sleep(1);
      printf("i am parent, self=%d, child_pid=%d\n", getpid(), pid);
    }
  }