关于for循环中调用fork()系统调用的执行原理解析

来源：榕意旅游网

该问题来源于操作系统概念（第九版）一书中的第三章的习题3.5，分析for循环中fork的执行原理

1、预备知识

搞清楚了fork()系统调用的概念和原理后就可以接着来看这道题目了。

2、题目解析

题目描述：
请分析下列C程序创建了多少个进程？

#include <stdio.h>
#include <unistd.h>

int main() {
	int i;
	for (i = 0; i < 4; i++) {
		pid = fork();
	}
	return 0;
}

在不影响最终执行结果的基础上，我对原始代码进行了一点改动，加了一些打印和同步的语句方便分析运行结果

补充说明：

getpid()用来获取当前进程的进程标识符，我们显示为pid；
getppid() 用来获取当前进程的父进程的进程标识符，我们取别名叫做ppid;
wait(NULL); 语句的作用是等待当前进程的子进程执行完毕;

#include <stdio.h>
#include <unistd.h>

int main() {
	int i;
	int n = 4;
	printf("main process pid is :%d\n", getpid());
	pid_t pid;
	for (i = 0; i < n; i++) {
		pid = fork();
		if (pid < 0) {
			printf("fork failed...\n");
			return 1;
		} else if (pid == 0) {
			printf("child process... pid=%d  ppid=%d current i=%d\n", getpid(), getppid(), i);
		} else {
			printf("parent process... pid=%d  ppid=%d  current i=%d\n", getpid(), getppid(), i);	
			wait(NULL);
			printf("parent process... pid=%d his child compeleted...\n", getpid());
		}
	}
	return 0;
}

执行结果：

main process pid is :29574
parent process... pid=29574  ppid=22955  current i=0
child process... pid=29575  pid=29574 current i=0
parent process... pid=29575  ppid=29574  current i=1
child process... pid=29576  pid=29575 current i=1
parent process... pid=29576  ppid=29575  current i=2
child process... pid=29577  pid=29576 current i=2
parent process... pid=29576 his child compeleted...
parent process... pid=29575 his child compeleted...
parent process... pid=29575  ppid=29574  current i=2
child process... pid=29578  pid=29575 current i=2
parent process... pid=29575 his child compeleted...
parent process... pid=29574 his child compeleted...
parent process... pid=29574  ppid=22955  current i=1
child process... pid=29579  pid=29574 current i=1
parent process... pid=29579  ppid=29574  current i=2
child process... pid=29580  pid=29579 current i=2
parent process... pid=29579 his child compeleted...
parent process... pid=29574 his child compeleted...
parent process... pid=29574  ppid=22955  current i=2
child process... pid=29581  pid=29574 current i=2
parent process... pid=29574 his child compeleted...

从执行结果中可以看出，加上我们原本的main进程，最终一共有16个进程。再仔细分析，我们可以发现他有点类似建树的过程。但是这样直接根据一次的结果来分析不太严谨。

3、剖析原理

我们将循环条件按照1 -> 5顺序依次调整，看执行结果。

当 n == 1 时，执行结果为：

main process pid is :516
parent process... pid=516  ppid=22955  current i=0
child process... pid=517  ppid=516 current i=0
parent process... pid=516 his child compeleted...

用树来表示： 图中方框内数字代表进程标识符pid，圆圈中数字表示当前进程中变量i的值

当 n == 2 时，执行结果为：

main process pid is :623
parent process... pid=623  ppid=22955  current i=0
child process... pid=624  ppid=623 current i=0
parent process... pid=624  ppid=623  current i=1
child process... pid=625  ppid=624 current i=1
parent process... pid=624 his child compeleted...
parent process... pid=623 his child compeleted...
parent process... pid=623  ppid=22955  current i=1
child process... pid=626  ppid=623 current i=1
parent process... pid=623 his child compeleted...

用树来表示：

main process pid is :1120
parent process... pid=1120  ppid=22955  current i=0
child process... pid=1121  ppid=1120 current i=0
parent process... pid=1121  ppid=1120  current i=1
child process... pid=1122  ppid=1121 current i=1
parent process... pid=1122  ppid=1121  current i=2
child process... pid=1123  ppid=1122 current i=2
parent process... pid=1122 his child compeleted...
parent process... pid=1121 his child compeleted...
parent process... pid=1121  ppid=1120  current i=2
child process... pid=1124  ppid=1121 current i=2
parent process... pid=1121 his child compeleted...
parent process... pid=1120 his child compeleted...
parent process... pid=1120  ppid=22955  current i=1
child process... pid=1125  ppid=1120 current i=1
parent process... pid=1125  ppid=1120  current i=2
child process... pid=1126  ppid=1125 current i=2
parent process... pid=1125 his child compeleted...
parent process... pid=1120 his child compeleted...
parent process... pid=1120  ppid=22955  current i=2
child process... pid=1127  ppid=1120 current i=2
parent process... pid=1120 his child compeleted...

用树来表示：

main process pid is :1758
parent process... pid=1758  ppid=22955  current i=0
child process... pid=1759  ppid=1758 current i=0
parent process... pid=1759  ppid=1758  current i=1
child process... pid=1760  ppid=1759 current i=1
parent process... pid=1760  ppid=1759  current i=2
child process... pid=1761  ppid=1760 current i=2
parent process... pid=1761  ppid=1760  current i=3
child process... pid=1762  ppid=1761 current i=3
parent process... pid=1761 his child compeleted...
parent process... pid=1760 his child compeleted...
parent process... pid=1760  ppid=1759  current i=3
child process... pid=1763  ppid=1760 current i=3
parent process... pid=1760 his child compeleted...
parent process... pid=1759 his child compeleted...
parent process... pid=1759  ppid=1758  current i=2
child process... pid=1764  ppid=1759 current i=2
parent process... pid=1764  ppid=1759  current i=3
child process... pid=1765  ppid=1764 current i=3
parent process... pid=1764 his child compeleted...
parent process... pid=1759 his child compeleted...
parent process... pid=1759  ppid=1758  current i=3
child process... pid=1766  ppid=1759 current i=3
parent process... pid=1759 his child compeleted...
parent process... pid=1758 his child compeleted...
parent process... pid=1758  ppid=22955  current i=1
child process... pid=1767  ppid=1758 current i=1
parent process... pid=1767  ppid=1758  current i=2
child process... pid=1768  ppid=1767 current i=2
parent process... pid=1768  ppid=1767  current i=3
child process... pid=1769  ppid=1768 current i=3
parent process... pid=1768 his child compeleted...
parent process... pid=1767 his child compeleted...
parent process... pid=1767  ppid=1758  current i=3
child process... pid=1770  ppid=1767 current i=3
parent process... pid=1767 his child compeleted...
parent process... pid=1758 his child compeleted...
parent process... pid=1758  ppid=22955  current i=2
child process... pid=1771  ppid=1758 current i=2
parent process... pid=1771  ppid=1758  current i=3
child process... pid=1772  ppid=1771 current i=3
parent process... pid=1771 his child compeleted...
parent process... pid=1758 his child compeleted...
parent process... pid=1758  ppid=22955  current i=3
child process... pid=1773  ppid=1758 current i=3
parent process... pid=1758 his child compeleted...

用树来表示：（为了方便，后面的图中省略了pid标识）

几张树图应该表示的很清晰了，这里我们选取n == 5的树图进行解释：

首先main进程执行for循环，我们先不考虑子进程的执行，只考虑main进程的执行步骤，那么很简单就能知道他一定会循环5次，也就是说调用5次fork()创建了5个新的进程，在图中就是树的第二层，每个节点的数字代表当前变量i的值
接下来我们去分析第一个新进程，也就是第二层中的0号进程。因为fork()执行完毕后，新进程也会接着执行fork()系统调用后的代码，在这里就是继续执行for循环，首先进行i++操作，那么此时i == 1，只针对浅红色0号进程而言，for循环需要从i == 1开始执行，那么当前进程只会在循环4次，也就是创建出4个新的进程
剩下的以此类推，就可以得到完整的进程树

4、结论和意外发现

4.1、结论：

最朴素的计算方法就是按照上述图的解析的方法去画出进程树，最终统计出进程数中节点个数。

4.2、意外发现

如果我们从第一张图分析道第五张图，把关注点放在进程数的每层的节点数量上，我们可以很轻松的发现它每层的节点数量是关于中间层对称的，是不是超级像一个数学知识点 ------ 杨辉三角

先给出推论：

当 n == 1 ，进程树深度为n + 1 = 2；从上到下每层节点总数分别为1、1;
当 n == 2 ，进程树深度为n + 1 = 3；从上到下每层节点总数分别为1、2、1;
当 n == 3 ，进程树深度为n + 1 = 4；从上到下每层节点总数分别为1、3、3、1;
当 n == 4 ，进程树深度为n + 1 = 5；从上到下每层节点总数分别为1、4、6、4、1;
当 n == 5 ，进程树深度为n + 1 = 6；从上到下每层节点总数分别为1、5、10、10、5、1;

个人结论： 给定循环边界n，可根据公式 全部进程数 = 杨辉三角第 n + 1 行的和; 计算出创建的所有进程数量。

因篇幅问题不能全部显示，请点此查看更多更全内容

查看全文