为什么在成为孤立进程组时没有收到SIGHUP信号

Question

在关于孤立进程组的 GNU libc手册中,它提到:

“process groups that continue running even after the session leader 
has terminated are marked as orphaned process groups. 

When a process group becomes an orphan, its processes are sent a SIGHUP 
signal. Ordinarily, this causes the processes to terminate. However, 
if a program ignores this signal or establishes a handler for it 
(see Signal Handling), it can continue running as  in the orphan process
 group even after its controlling process terminates; but it still 
cannot access the terminal any more. ”

我编写了一个测试程序,但当进程组成为一个孤儿时,它的进程没有收到SIGHUP信号.我想知道为什么？

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>


static void  
sig_hup(int signo) //**never get called ???**
{
    printf("SIGHUP received, pid = %ld\n", (long)getpid());
}

static void
pr_ids(char *name)
{
    printf("%s: pid = %ld, ppid = %ld, pgrp = %ld, tpgrp = %ld\n",
        name, (long)getpid(), (long)getppid(), (long)getpgrp(),
        (long)tcgetpgrp(STDIN_FILENO));
    fflush(stdout);
}

int
main(void)
{
    char    c;
    pid_t   pid;

    pr_ids("parent");
    pid = fork();
    if (pid > 0) {       // parent 
        sleep(5);
        exit(0);         // parent exit;
    } else {
        pr_ids("child");
        setsid();        //create new session, and "child" becomes the session leader
        pid = fork();
        if(pid>0) {
            sleep(20);
            exit(0);     // "child" exit
                         // so the process group become an orphan process group
        }
        else{
            pr_ids("grandson");
            signal(SIGHUP, sig_hup);    // establish signal handler 
            sleep(60);                  // now becoming orphan process group
            printf("end\n");
        }
    }
    exit(0);
}

Answer 1

该文档部分专门讨论了一个进程的控制终端丢失，该进程通常由于调制解调器挂断或虚拟等效物（结束 ssh 会话等）而丢失一个\xe2\x80\x94。（我认为文档中的措辞可以在这里改进）。当你使用setsid()这里时，你就放弃了对控制终端的访问setsid()，因此从那里开始不会丢失控制终端。

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你可以用open()一个 tty 设备（比如 pty 从设备）来获得一个控制终端（注意你可能还需要做一些额外的操作\xe2\x80\x94FreeBSD需要一个TIOCSCTTYioctl），然后再次失去它，然后你应该得到SIGHUP信号。

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