Unix环境下C++二叉fork算法生成9个子进程的问题排查

我正在学习类Unix操作系统中fork的工作机制，希望通过二叉fork算法（每个父进程仅能创建2个子进程）生成恰好9个子进程。但我无法在第三轮递归调用中正确限制子进程的创建数量：当前代码第一轮生成2个子进程，第二轮生成4个，第三轮生成了4个，而预期应为3个。以下是子进程创建流程图、现有代码及运行输出：

[子进程创建流程图]

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

using namespace std;

static int CHILD_SIZE = 9;
static int LEVEL = 1;
static int MAX_LEVEL = 3;

void createChildren(int remaining, int level) {

    int CURRENT_REMAINING = remaining;
    
    if(CURRENT_REMAINING <= 0 || level > MAX_LEVEL){
        return;
    }
    
    pid_t firstChild,secondChild;
    
    // Fork the first child
    firstChild = fork();
    
    if(firstChild == -1){
        cerr << "Fork failed!" << endl;
    }
    else if (firstChild == 0 && CURRENT_REMAINING > 1)
    {
        // first child
        cout << "First child process (PID: " << getpid() << ") with parent process (PID: " << getppid() << ") at level: " << level << endl;
        
        // calculate how many child needs after this iteration
        CURRENT_REMAINING -= (2 * level);
        if(CURRENT_REMAINING < 0){
            CURRENT_REMAINING = 0;
        }
    }
    else if (firstChild > 0 && CURRENT_REMAINING > 1)
    {
        // Fork the second child
        secondChild = fork();
    
        if(firstChild == -1){
            cerr << "Fork failed!" << endl;
        }
        else if (secondChild == 0) {
            // second child
            waitpid(firstChild, NULL, 0);
            cout << "Second child process (PID: " << getpid() << ") with parent process (PID: " << getppid() << ") at level: " << level << endl;
    
            // calculate how many child needs after this iteration
            CURRENT_REMAINING -= (2 * level);
            if(CURRENT_REMAINING < 0){
                CURRENT_REMAINING = 0;
            }
        } else if (secondChild > 0){
            // Wait for both child processes to complete
            waitpid(firstChild, NULL, 0);
            waitpid(secondChild, NULL, 0);
        }
    }
    
    
    //---------- recursion call-----------//
    
    //  upper child recursion call
    if(firstChild == 0){
    
        cout << "current remaining in first child is: " << CURRENT_REMAINING << " at level " << level << endl;
    
        createChildren(CURRENT_REMAINING, level + 1);
    }
    
    // lower child recursion call
    if(secondChild == 0){
    
        cout << "current remaining in second child is: " << CURRENT_REMAINING << " at level " << level << endl;
    
        if(CURRENT_REMAINING > 3){
            //cout << "Create two more processes" << endl;
            createChildren(CURRENT_REMAINING, level + 1);
        }
    }

}

int main() {

    std::cout << "Parent process " << getpid() << std::endl;
    createChildren(CHILD_SIZE, LEVEL);
    
    return 0;

}

运行输出 #

Parent process 8571
First child process (PID: 8573) with parent process (PID: 8571) at level: 1
current remaining in first child is: 7 at level 1
Second child process (PID: 8574) with parent process (PID: 8571) at level: 1
current remaining in second child is: 7 at level 1
First child process (PID: 8575) with parent process (PID: 8573) at level: 2
current remaining in first child is: 3 at level 2
First child process (PID: 8576) with parent process (PID: 8574) at level: 2
current remaining in first child is: 3 at level 2
Second child process (PID: 8577) with parent process (PID: 8573) at level: 2
current remaining in second child is: 3 at level 2
Second child process (PID: 8578) with parent process (PID: 8574) at level: 2
current remaining in second child is: 3 at level 2
First child process (PID: 8579) with parent process (PID: 8575) at level: 3
current remaining in first child is: 0 at level 3
Second child process (PID: 8581) with parent process (PID: 8575) at level: 3
current remaining in second child is: 0 at level 3
First child process (PID: 8580) with parent process (PID: 8576) at level: 3
current remaining in first child is: 0 at level 3
Second child process (PID: 8582) with parent process (PID: 8576) at level: 3
current remaining in second child is: 0 at level 3

内容的提问来源于stack exchange，提问作者djflvv233