Question

我有三个子进程和一个父进程。

我希望程序以该顺序运行[Child1 then child2 then child3 then parent]。

我一直在尝试使用以下代码，但它没有给我正确的序列！

代码：

#include<iostream>
#include<string.h>
#include<fstream>
#include<cstdlib>
#include <sys/types.h>
#include <sys/wait.h>
using namespace std;
int main()
{
    pid_t ch11;
    pid_t ch22;
    pid_t ch33;
    int ch1 = fork();
    int ch2 = fork();
    int ch3 = fork();


    if (ch1==0) //child1
    {    cout<<"this is child 1\nGoing from 1 to child 2 ...\n\n";
        exit(0);
    }
    else if ( ch2==0)
    {    waitpid(ch11,0,0);
        cout<<"This is child 2\nGoing from 2 To Child 3 ...\n\n";
        exit(0);
    }
    else if (ch3==0)
    {    waitpid(ch22,0,0);
        cout<<"This is child 3\nFineshed !! going from 3 to parent\n\n";
        exit(0);
    }
    else
    {    waitpid(ch33,0,0);
        cout<<"This is parent , waited the whole childes to finish !!\n\n";
        exit(0);
    }
    return 0 ;
}

输出：

ubuntu@ubuntu-desktop:~$ c++ a.cpp -o p1
ubuntu@ubuntu-desktop:~$ ./p1
This is child 2
Going from 2 To Child 3 ...

this is child 1
Going from 1 to child 2 ...

this is child 1
Going from 1 to child 2 ...

this is child 1
Going from 1 to child 2 ...

This is child 2
Going from 2 To Child 3 ...

This is child 3
Fineshed !! going from 3 to parent

this is child 1
Going from 1 to child 2 ...

This is parent , waited the whole childs to finish.

我知道它会通过使用waitpid()函数来解决，我认为即时使用waitpid()错误。

Answer 1

尽管您的评论“代码正在运行，但它给了我错误的执行顺序”但整个事情仍然完全不正确，显然你不明白其他人试图说什么，因此我会尝试这里。

int main()
{
    pid_t ch11;
    pid_t ch22;
    pid_t ch33;

这些变量的目的是什么？

    int ch1 = fork();

您是否阅读过fork手册页？它清楚地说明了何时创建子进程，它在子进程中返回0，在父进程中返回pid。它也可能失败，所以你应该检查一下。

考虑到这一点......

    int ch2 = fork();

孩子和父母都会到达此行。因此，之前创建的子分叉。

    int ch3 = fork();

猜猜是什么。

    if (ch1==0) //child1
    {    cout<<"this is child 1\nGoing from 1 to child 2 ...\n\n";

除非你不是（见下文）。

        exit(0);

应该是_Exit。

    }
    else if ( ch2==0)
    {    waitpid(ch11,0,0);

什么？ ch11甚至没有初始化，那么这应该完成什么呢？您是否在启用警告的情况下编译代码？

一般情况下，我不知道您是否正在尝试创建 children fork，从而创建一个包含父＆lt; - ＆gt;子关系的进程链，或者您想要3个相同的子进程父进程。如果后者这只是坏事，如果前者更加错误，因为你无法等待这样的过程。

[snip the rest]

最后，让我们来谈谈

执行错误的序列

什么？

对流程执行顺序无任何保证。实际上，任何数量的它们都可以执行任何时间，并且在其他任何数量的机会运行之前可以多次安排。

鉴于到目前为止所显示的内容，我倾向于猜测你要么误解了你的家庭作业，要么非常“非常规”地接近它。无论哪种方式，我强烈建议您说明应该用您的代码解决的实际问题。

Answer 2

这是代码的更好的检测版本;它包括输出中的PID，输出各为一行。

#include <iostream>
#include <cstdlib>
#include <sys/wait.h>
#include <unistd.h>

using namespace std;

int main()
{
    int ch1 = fork();
    int ch2 = fork();
    int ch3 = fork();

    if (ch1 == 0) // child1
    {
        cout << (int)getpid() << ": This is child 1 - Finished\n";
        exit(0);
    }
    else if (ch2 == 0)
    {
        waitpid(ch1, 0, 0);
        cout << (int)getpid() << ": This is child 2 - Finished\n";
        exit(0);
    }
    else if (ch3 == 0)
    {
        waitpid(ch2, 0, 0);
        cout << (int)getpid() << ": This is child 3 - Finished!\n";
        exit(0);
    }
    else
    {
        waitpid(ch3, 0, 0);
        cout << (int)getpid() << ": This is parent - waited for all children to finish!\n";
        exit(0);
    }
    return 0;
}

示例输出：

$ ./3kids
40287: This is child 3 - Finished!
40285: This is child 1 - Finished
40286: This is child 2 - Finished
40290: This is child 1 - Finished
40289: This is child 2 - Finished
40288: This is child 1 - Finished
40284: This is parent - waited for all children to finish!
40291: This is child 1 - Finished
$

正如您所看到的，有一个过程认为自己是孩子3，两个过程认为自己是孩子2，四个过程认为自己是孩子1，一个认为自己是父母。这与创建8个过程的无约束分叉是一致的。

只要有3个孩子，并依次等待每个孩子，你需要的代码更像是：

#include <iostream>
#include <cstdlib>
#include <sys/wait.h>
#include <unistd.h>

using namespace std;

void child(int n)
{
    flush(cout);        // No pending output
    int pid = fork();
    if (pid < 0)
        cerr << (int)getpid() << ": failed to fork\n";
    else if (pid == 0)
    {
        cout << (int)getpid() << ": This is child " << n << " - Finished\n";
        exit(0);
    }
    else
    {
        int corpse;
        int status;
        while ((corpse = wait(&status)) != -1)
            cout << (int)getpid() << ": PID " << corpse << " exited with status "
                 << status << "\n";
    }
}

int main()
{
    child(1);
    child(2);
    child(3);
    cout << (int)getpid() << ": This is parent - waited for all children to finish!\n";
    return 0;
}

示例输出：

$ ./3kids
40336: This is child 1 - Finished
40335: PID 40336 exited with status 0
40337: This is child 2 - Finished
40335: PID 40337 exited with status 0
40338: This is child 3 - Finished
40335: PID 40338 exited with status 0
40335: This is parent - waited for all children to finish!
$

运行3个子进程

2 个答案: