c - 为什么在成为孤立进程组时未收到 SIGHUP 信号

Question

在 GNU libc 手册中关于 orphaned process groups ，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. ”

我写了一个测试程序，但是当进程组成为孤儿时，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

如果孤立的进程组在成为孤立进程时被停止，则会得到 SIGHUP 后跟 SIGCONT。

sleep 还不够，你需要:

kill(getpid(), SIGSTOP); //or raise(SIGSTOP);

除此之外，如果孤立是由 setsid() 或setprgrp()，因为它不是由无意识地不知道作业控制的退出进程引起的(参见 http://pubs.opengroup.org/onlinepubs/9699919799/functions/_exit.html)。

但是，使用 kill(getpid(), SIGSTOP) 而不是子进程中的 sleep(60)，您的程序将得到一个停止的孤儿，即使您不调用 setsid()。

#define _GNU_SOURCE
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <signal.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)
{
    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(2);
            exit(0);     // "child" exit
                         // so the process group become an orphan process group
        }
        else{
            pr_ids("grandson");
            signal(SIGHUP, sig_hup);    // establish signal handler 
            kill(getpid(), SIGSTOP);
            printf("end\n");
        }
    }
    exit(0);
}

应该在 parent 去世后(5 秒)让您在 child 中得到一个SIGHUP。