关于signal程序的一个思考——进程排队问题

linux中的信号是一种异步通信，所谓“异步”，就是指这个事件发生的时间是无法确定的，他有可能在任何时刻发生。

之所以把标题写成“进程排队问题”，是我对我要提到的问题的思考而已，如果有高人能否决我的观点，我诚心接受。

请看如下c程序：

#include <stdio.h>
#include <stdlib.h> //exit() is used
#include <string.h>
#include <signal.h>
#include <unistd.h>

int wait_mark = 1; //if == 0, exit pause and return

void stop(int signo) { 
	wait_mark = 0;
}

void waiting() { //to pause if wait_mark != 0
	while (wait_mark)
		sleep(1);
}

int main(int argc, char *argv[])
{
	pid_t pid1, pid2;

//	signal(SIGINT, stop);
	if (-1 == (pid1 = fork() ) ) {
		perror("fork error:");
		exit(-1);
	}

	if (pid1 > 0) {
		if (-1 == (pid2 = fork() ) ) {
			perror("fork error:");
			exit(-1);
		}
		else if (pid2 > 0) {
			signal(SIGINT, stop);
			waiting();
			kill(pid1, SIGUSR1);
			kill(pid2, SIGUSR2);
			wait(0);
			wait(0);
			lockf(1, 1, 0);
			printf("parent is now exiting...\n");
			lockf(1, 0, 0);
		}
		else if (pid2 == 0) {
			signal(SIGUSR2, stop);
			signal(SIGINT, SIG_IGN);
			waiting();
			lockf(1, 1, 0);
			printf("child 2 is now exiting ...\n");
			lockf(1, 0, 0);
			exit(0);
		}
	}
	else if (pid1 == 0) {
		signal(SIGUSR1, stop);
		signal(SIGINT, SIG_IGN);
		waiting();
		lockf(1, 1, 0);
		printf("child 1 is now exiting ...\n");
		lockf(1, 0, 0);
		exit(0);
	}

	return 0;
}

运行情况大多数是先打印child 1退出的信息，但极少数情况会出现先打印child 2退出的信息。请注意，父进程发信号的时候是先给child 1发出的，见相邻两句kill的顺序。
下面是我从虚拟机vmware6.4安装的ubuntu9.04上运行的结果（注意粗体字部分）：

引用

canlynet@canlynet-desktop:~/vm_share$ gcc -o signal1 signal1.c
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 2 is now exiting ...
child 1 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...
canlynet@canlynet-desktop:~/vm_share$ ./signal1
^Cchild 1 is now exiting ...
child 2 is now exiting ...
parent is now exiting...

关于这个特殊出现的情况，我估计是父进程执行kill的时候速度是很快的，所以万一某一次child 2在系统调度时排列在child 1的后面了，就可能出现child 2先退出的打印语句。
也就是说：大多数情况下父进程执行kill的两句话对子进程1和子进程2来说是对等的，虽然接收到信号有时间先后，但逻辑上没有先后关系，除非父进程执行完一个kill后正好时间片到了或者被打断了。