c - SIGCONT 和 SIGHUP 命令发送到孤立的 linux 进程组

Question

APUE 说

Since the process group is orphaned when the parentterminates, POSIX.1 requires that every process in the newly orphaned process group that is stopped (as our child is) be sent the hang-up signal (SIGHUP) followed by the continue signal (SIGCONT)

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <errno.h>
#define errexit(msg) do{ perror(msg); exit(EXIT_FAILURE); } while(0)
static void sig_hup(int signo)
{
    printf("SIGHUP received, pid = %d\n", getpid());
}
static void sig_cont(int signo)
{
    printf("SIGCONT received, pid = %d\n", getpid());
}
static void sig_ttin(int signo)
{
    printf("SIGTTIN received, pid = %d\n", getpid());
}
static void pr_ids(char *name)
{
    printf("%s: pid = %d, ppid = %d, pgrp = %d, tpgrp = %d\n",
           name, getpid(), getppid(), getpgrp(), tcgetpgrp(STDIN_FILENO));
}
int main(int argc, char *argv[])
{
    char c;
    pid_t pid;
    setbuf(stdout, NULL);
    pr_ids("parent");
    if ((pid = fork()) < 0) {
            errexit("fork error");
    } else if (pid > 0) { /* parent */
            sleep(5);
            printf("parent exit\n");
            exit(0);
    } else { /* child */
            pr_ids("child...1");
            signal(SIGCONT, sig_cont);
            signal(SIGHUP, sig_hup);
            signal(SIGTTIN, sig_ttin);
            kill(getpid(), SIGTSTP);
            //sleep(10);
            pr_ids("child...2");
            if (read(STDIN_FILENO, &c, 1) != 1) {
                    printf("read error from controlling TTY, errno = %d\n", 
                                                             errno);
            }
            printf("child exit\n");
    }
    exit(0);
  }

程序输出:

parent: pid = 2036, ppid = 1959, pgrp = 2036, tpgrp = 2036
child...1: pid = 2037, ppid = 2036, pgrp = 2036, tpgrp = 2036
parent exit
xiejingfeng@xiejingfeng-desktop:/codes/apue$ SIGCONT received, pid = 2037
SIGHUP received, pid = 2037
child...2: pid = 2037, ppid = 1, pgrp = 2036, tpgrp = 1959
read error from controlling TTY, errno = 5
child exit

输出并不像书上说的那样，因为程序首先接收到 SIGCONT 然后是 SIGHUP，这让我很困惑，你们能帮帮我吗？

提前致谢。

Answer 1

SIGHUP 在 child 的执行恢复之前无法传递。当进程停止时，除了 SIGCONT 和 SIGKILL 之外的所有信号传递都将暂停。

因此，SIGHUP确实先到了，但是直到SIGCONT唤醒进程执行后才能处理。