linux 父进程退出时，激活了子进程的情况说明

今天在此记录一下几种情况。
情况一：杀掉父进程，子进程依然运行。
情况二：先暂停子进程，然后杀掉父进程，则子进程退出
这两种情况下的代码如下所示。

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

typedef void (*SIGFUNCTYPE)(int);

SIGFUNCTYPE Signal_(int sig, void (*handler)(int))
{
    struct sigaction act, oact;

    act.sa_handler = handler;
    act.sa_flags = 0;
    sigemptyset(&act.sa_mask);
    sigaddset(&act.sa_mask, sig);
    if(sigaction(sig, &act, &oact) == -1){
        oact.sa_handler = (void (*)())SIG_ERR;
    }
    return(oact.sa_handler);
}

void
term_handler(int signum)
{
    exit(0);
}

void hup_handler(int signum)
{
int pid = getpid();
printf("receive hup sig in pid %d\n", pid);
}

void cont_handler(int signum)
{
int pid = getpid();
printf("receive cont sig in pid %d\n", pid);
}

int main()
{
	//Signal_(SIGTERM, (SIGFUNCTYPE) term_handler);
	//Signal_(SIGHUP,  hup_handler);
	//Signal_(SIGCONT,  cont_handler);
	int pid = fork();
	if(pid == 0)
	{
		/*
		int pid2=getpid();
		int sid2 = getsid(pid2);
		printf("in child process, session id is %d\n", sid2);
		
		setsid();
		sid2 = getsid(pid2);
		printf("in child process, after setsid, session id is %d\n", sid2);
		*/
		while(1);
	}
	int sid1 = getsid(getpid());
	printf("in parent process, session id is %d\n", sid1);
	sleep(60);

	//while(1);
	return 0;
}

针对第二种情况，为何子进程会退出，这里做简单说明，杀掉父进程后，子进程会接收到SIGHUP信号，而此信号，子进程没有处理，则默认结束子进程；但是这里有一点本人没明白，杀掉父进程，子进程为何会收到SIGHUP信号，这一点没弄明白，因为本人发现子进程若是活动状态，则其不会收到SIGHUP信号，只有kill -SIGSTOP暂停子进程后，杀掉父进程，子进程才会收到SIGHUP信号，哪位大神若是明白，请评论区中给予指导，谢谢。

第三种情况：针对SIGHUP，子进程进行处理。此时暂停子进程，然后杀掉父进程，则子进程被激活，并且不会退出。
代码如下：

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

typedef void (*SIGFUNCTYPE)(int);

SIGFUNCTYPE Signal_(int sig, void (*handler)(int))
{
    struct sigaction act, oact;

    act.sa_handler = handler;
    act.sa_flags = 0;
    sigemptyset(&act.sa_mask);
    sigaddset(&act.sa_mask, sig);
    if(sigaction(sig, &act, &oact) == -1){
        oact.sa_handler = (void (*)())SIG_ERR;
    }
    return(oact.sa_handler);
}

void
term_handler(int signum)
{
    exit(0);
}

void hup_handler(int signum)
{
int pid = getpid();
printf("receive hup sig in pid %d\n", pid);
}

void cont_handler(int signum)
{
int pid = getpid();
printf("receive cont sig in pid %d\n", pid);
}

int main()
{
	//Signal_(SIGTERM, (SIGFUNCTYPE) term_handler);
	Signal_(SIGHUP,  hup_handler);
	//Signal_(SIGCONT,  cont_handler);
	int pid = fork();
	if(pid == 0)
	{
		/*
		int pid2=getpid();
		int sid2 = getsid(pid2);
		printf("in child process, session id is %d\n", sid2);
		
		setsid();
		sid2 = getsid(pid2);
		printf("in child process, after setsid, session id is %d\n", sid2);
		*/
		while(1);
	}
	int sid1 = getsid(getpid());
	printf("in parent process, session id is %d\n", sid1);
	sleep(60);

	//while(1);
	return 0;
}

这里针对第三种情况解释下，为何杀掉父进程后，子进程还被激活了，这里是由于操作系统的设计，当操作系统需要给进程发送SIGHUP信号时，若该进程被暂停了，则首先发送SIGCONT给该进程，然后发送SIGHUP给该进程，所以该进程必须对SIGHUP进行处理，否则其会退出。

第四种情况：暂停子进程后，如果杀掉父进程，依然想让子进程呈暂停状态怎么办（不死掉，也不激活），这个可以给子进程设置独立的session id来解决，代码如下：

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

typedef void (*SIGFUNCTYPE)(int);

SIGFUNCTYPE Signal_(int sig, void (*handler)(int))
{
    struct sigaction act, oact;

    act.sa_handler = handler;
    act.sa_flags = 0;
    sigemptyset(&act.sa_mask);
    sigaddset(&act.sa_mask, sig);
    if(sigaction(sig, &act, &oact) == -1){
        oact.sa_handler = (void (*)())SIG_ERR;
    }
    return(oact.sa_handler);
}

void
term_handler(int signum)
{
    exit(0);
}

void hup_handler(int signum)
{
int pid = getpid();
printf("receive hup sig in pid %d\n", pid);
}

void cont_handler(int signum)
{
int pid = getpid();
printf("receive cont sig in pid %d\n", pid);
}

int main()
{
	//Signal_(SIGTERM, (SIGFUNCTYPE) term_handler);
	Signal_(SIGHUP,  hup_handler);
	//Signal_(SIGCONT,  cont_handler);
	int pid = fork();
	if(pid == 0)
	{
		
		int pid2=getpid();
		int sid2 = getsid(pid2);
		printf("in child process, session id is %d\n", sid2);
		
		setsid();
		sid2 = getsid(pid2);
		printf("in child process, after setsid, session id is %d\n", sid2);
		
		while(1);
	}
	int sid1 = getsid(getpid());
	printf("in parent process, session id is %d\n", sid1);
	sleep(60);

	//while(1);
	return 0;
}