如果父母去世，孤儿的PPID变为1 -你只需要检查你自己的PPID。 在某种程度上，这就是上面提到的轮询。 这是它的外壳部分:

check_parent () {
      parent=`ps -f|awk '$2=='$PID'{print $3 }'`
      echo "parent:$parent"
      let parent=$parent+0
      if [[ $parent -eq 1 ]]; then
        echo "parent is dead, exiting"
        exit;
      fi
}


PID=$$
cnt=0
while [[ 1 = 1 ]]; do
  check_parent
  ... something
done

通过滥用终端控制和会话，我设法用3个进程实现了一个可移植的、非轮询的解决方案。

诀窍在于:

process A is started process A creates a pipe P (and never reads from it) process A forks into process B process B creates a new session process B allocates a virtual terminal for that new session process B installs SIGCHLD handler to die when the child exits process B sets a SIGPIPE handler process B forks into process C process C does whatever it needs (e.g. exec()s the unmodified binary or runs whatever logic) process B writes to pipe P (and blocks that way) process A wait()s on process B and exits when it dies

这种方式:

如果进程A死亡:进程B得到一个SIGPIPE并死亡 如果进程B死亡:进程A的wait()返回并死亡，进程C将得到一个SIGHUP(因为当一个连接终端的会话的会话领导者死亡时，前台进程组中的所有进程都会得到一个SIGHUP) 如果进程C死亡:进程B得到一个SIGCHLD并死亡，那么进程a也会死亡

缺点:

进程C不能处理SIGHUP 进程C将在不同的会话中运行 进程C不能使用会话/进程组API，因为这会破坏脆弱的设置 为每一个这样的操作创建一个终端并不是最好的主意

我在过去通过在“子”中运行“原始”代码和在“父”中运行“衍生”代码来实现这一点(也就是说:你在fork()之后反转了通常意义上的测试)。然后在“衍生”代码中捕获SIGCHLD…

对你来说可能不行，但管用的时候很可爱。

Historically, from UNIX v7, the process system has detected orphanity of processes by checking a process' parent id. As I say, historically, the init(8) system process is a special process by only one reason: It cannot die. It cannot die because the kernel algorithm to deal with assigning a new parent process id, depends on this fact. when a process executes its exit(2) call (by means of a process system call or by external task as sending it a signal or the like) the kernel reassigns all children of this process the id of the init process as their parent process id. This leads to the most easy test, and most portable way of knowing if a process has got orphan. Just check the result of the getppid(2) system call and if it is the process id of the init(2) process then the process got orphan before the system call.

这种方法会产生两个问题:

first, we have the possibility of changing the init process to any user process, so How can we assure that the init process will always be parent of all orphan processes? Well, in the exit system call code there's a explicit check to see if the process executing the call is the init process (the process with pid equal to 1) and if that's the case, the kernel panics (It should not be able anymore to maintain the process hierarchy) so it is not permitted for the init process to do an exit(2) call. second, there's a race condition in the basic test exposed above. Init process' id is assumed historically to be 1, but that's not warranted by the POSIX approach, that states (as exposed in other response) that only a system's process id is reserved for that purpose. Almost no posix implementation does this, and you can assume in original unix derived systems that having 1 as response of getppid(2) system call is enough to assume the process is orphan. Another way to check is to make a getppid(2) just after the fork and compare that value with the result of a new call. This simply doesn't work in all cases, as both call are not atomic together, and the parent process can die after the fork(2) and before the first getppid(2) system call. The processparent id only changes once, when its parent does anexit(2)call, so this should be enough to check if thegetppid(2)result changed between calls to see that parent process has exit. This test is not valid for the actual children of the init process, because they are always children ofinit(8)`, but you can assume safely these processes as having no parent either (except when you substitute in a system the init process)

一些海报已经提到了管道和kqueue。事实上，你也可以通过socketpair()调用创建一对连接的Unix域套接字。套接字类型应该是SOCK_STREAM。

Let us suppose you have the two socket file descriptors fd1, fd2. Now fork() to create the child process, which will inherit the fds. In the parent you close fd2 and in the child you close fd1. Now each process can poll() the remaining open fd on its own end for the POLLIN event. As long as each side doesn't explicitly close() its fd during normal lifetime, you can be fairly sure that a POLLHUP flag should indicate the other's termination (no matter clean or not). Upon notified of this event, the child can decide what to do (e.g. to die).

#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <poll.h>
#include <stdio.h>

int main(int argc, char ** argv)
{
    int sv[2];        /* sv[0] for parent, sv[1] for child */
    socketpair(AF_UNIX, SOCK_STREAM, 0, sv);

    pid_t pid = fork();

    if ( pid > 0 ) {  /* parent */
        close(sv[1]);
        fprintf(stderr, "parent: pid = %d\n", getpid());
        sleep(100);
        exit(0);

    } else {          /* child */
        close(sv[0]);
        fprintf(stderr, "child: pid = %d\n", getpid());

        struct pollfd mon;
        mon.fd = sv[1];
        mon.events = POLLIN;

        poll(&mon, 1, -1);
        if ( mon.revents & POLLHUP )
            fprintf(stderr, "child: parent hung up\n");
        exit(0);
    }
}

您可以尝试编译上面的概念验证代码，并在./a这样的终端中运行它。&。你有大约100秒的时间来尝试通过各种信号杀死父PID，否则它就会退出。在任何一种情况下，您都应该看到消息“child: parent hung up”。

与使用SIGPIPE处理程序的方法相比，该方法不需要尝试write()调用。

这种方法也是对称的，即进程可以使用相同的通道来监视彼此的存在。

这个解决方案只调用POSIX函数。我在Linux和FreeBSD中尝试了这个方法。我认为它应该在其他unix上工作，但我还没有真正测试过。

参见:

Linux手册中的unix(7)， FreeBSD的unix(4)， poll(2)， socketpair(2)， Linux上的socket(7)。

为了完整起见。在macOS上你可以使用kqueue:

void noteProcDeath(
    CFFileDescriptorRef fdref, 
    CFOptionFlags callBackTypes, 
    void* info) 
{
    // LOG_DEBUG(@"noteProcDeath... ");

    struct kevent kev;
    int fd = CFFileDescriptorGetNativeDescriptor(fdref);
    kevent(fd, NULL, 0, &kev, 1, NULL);
    // take action on death of process here
    unsigned int dead_pid = (unsigned int)kev.ident;

    CFFileDescriptorInvalidate(fdref);
    CFRelease(fdref); // the CFFileDescriptorRef is no longer of any use in this example

    int our_pid = getpid();
    // when our parent dies we die as well.. 
    LOG_INFO(@"exit! parent process (pid %u) died. no need for us (pid %i) to stick around", dead_pid, our_pid);
    exit(EXIT_SUCCESS);
}


void suicide_if_we_become_a_zombie(int parent_pid) {
    // int parent_pid = getppid();
    // int our_pid = getpid();
    // LOG_ERROR(@"suicide_if_we_become_a_zombie(). parent process (pid %u) that we monitor. our pid %i", parent_pid, our_pid);

    int fd = kqueue();
    struct kevent kev;
    EV_SET(&kev, parent_pid, EVFILT_PROC, EV_ADD|EV_ENABLE, NOTE_EXIT, 0, NULL);
    kevent(fd, &kev, 1, NULL, 0, NULL);
    CFFileDescriptorRef fdref = CFFileDescriptorCreate(kCFAllocatorDefault, fd, true, noteProcDeath, NULL);
    CFFileDescriptorEnableCallBacks(fdref, kCFFileDescriptorReadCallBack);
    CFRunLoopSourceRef source = CFFileDescriptorCreateRunLoopSource(kCFAllocatorDefault, fdref, 0);
    CFRunLoopAddSource(CFRunLoopGetMain(), source, kCFRunLoopDefaultMode);
    CFRelease(source);
}