C++：如何控制进程号

Question

I am trying a multiprocess code with C++. I have two questions about the code below:

• Q1. It seems 'Vtmp' or 'Ntmp' did not store the info properly for each process. I don't know why.

• Q2. If Q1 fixed, My final purpose is to let each Vtmp(V[i]) run one by one. I mean I used a multi-processly way to read "Vtmp", I want to
  wait until "read[V[0]]" is done, then "read(V[1])", wait some times between
  them.

• My main purpose is Q2. If "read" is a time-consuming function and generates many subprocesses. I want it run one by one, each time with only 6 subprocesses in theback-end， not 18 subprocesses running together.

#include<iostream>
#include<vector>
#include <sys/types.h>
#include <unistd.h>
using namespace std;

void read(vector<double> Vsub){
    pid_t pid;
    double Ntmp;
    for(int i = 0; i < Vsub.size(); i++){
        Ntmp = Vsub[i];
        pid = fork();
        if(pid = 0||pid == -1) break;
    }
    if(pid == -1){
        cout<<"fail to fork!"<<endl;
        exit(1);
    }
    else if(pid == 0){
        cout<<" This is childern process Vsub, id = "<<getpid()<<endl;
        cout<<" This is childern process Vsub, id = "<<Ntmp<<endl;
        sleep(30000);
        exit(0);
    }
    else {
        cout<<"This is main process, id = "<<getpid()<<endl;
    }

}
int main() {
     vector<vector<double>> V;
    V.push_back({1,2,3,4,5,6});
    V.push_back({11,12,13,14,15,16});
    V.push_back({21,22,23,24,25,26});
    pid_t pid;
    vector<double> Vtmp;
    cout<<" *******************   "<<V.size()<<endl;
    for(int it = 0; it < V.size(); ++it){
        Vtmp = V[it];
        pid = fork();
        if(pid = 0||pid == -1) break;
    }
    if(pid == -1){
        cout<<"fail to fork!"<<endl;
        exit(1);
    }
    else if(pid == 0){
        cout<<" This is childern process V, id = "<<getpid()<<" Vtmp "<<Vtmp[0]<<endl;
       read(Vtmp);
        sleep(30000);
        exit(0);
    }
    else {
        cout<<"This is main process, id = "<<getpid()<<endl;
    }
    return 0;
}

To be compared, the following is the good code to keep tmp info for each process. But I still don't know why the above one can not.

#include<stdio.h>
#include<unistd.h>
#include <sys/types.h>
#include<vector>
#include <iostream>

using namespace std;
int main()
{
    string sMatch;
    pid_t pid;
    vector<string> provList;
    provList.push_back("100");
    provList.push_back("200");
    provList.push_back("300");
    provList.push_back("400");
    provList.push_back("500");
    cout<<"main process,id="<<getpid()<<endl;
    for (vector<string>::iterator it = provList.begin(); it != provList.end(); ++it)
    {
        sMatch=*it;
        pid = fork();
        if(pid==0||pid==-1)
        {
            break;
        }
    }
    if(pid==-1)
    {
        cout<<"fail to fork!"<<endl;
        exit(1);
    }
    else if(pid==0)
    {
      
        cout<<"this is children process,id="<<getpid()<<",start to process "<<sMatch<<endl;
        sleep(10);
        exit(0);
    }
    else
    {
    
        cout<<"this is main process,id="<<getpid()<<",end to process "<<sMatch<<endl;
        exit(0);
    }
    return 0;
}
/*-------------------------------------------------------*/
/*
main process,id=63854
this is children process,id=63855,start to process 100
this is children process,id=63856,start to process 200
this is children process,id=63857,start to process 300
this is children process,id=63858,start to process 400
this is main process,id=63854,end to process 500
this is children process,id=63859,start to process 500
*/

Answer 1

The problem is that you got a typo in your condition after forking.

if(pid = 0||pid == -1) break;

You have the same typo in your main as well as in your read method so the child process always continues the cycle and ends with the last element(vector) of your vector of vectors.