How to kill a process with kill function in c++?

Question

I'm learning processes in Linux. I need to write a task which operates processes(Create, kill, change nicety, and suspend). I've already written that program, but kill and suspend are not working. To fix that I launched that program as a superuser, but this didn't help me. What do I need to do to fix that bug?

#include <iostream>
#include <sys/types.h>
#include <sys/wait.h>
#include <cstring>
#include <unistd.h>
#include <sys/resource.h>
using namespace std;

int main(){
    float l,r;
    //  system("gnome-terminal -e 'sh -c \" /home/neo/Desktop/OSLab8/childprocess/a.out 1 2 3\"'");
    cout<<"Enter left and right side of the loop\n";
    cin>>l>>r;
    int n;
    cout<<"Enter number of processes\n";
    cin>>n;
    //   cout<<"Process 0";
    int currentPid;
    string cmd;
    double s = l;
    double step = (r-l)/n;
    cout<<"Father id "<<getpid()<<endl;
    int mode;
    cout<<"Enter 1 to time measure mode, 2 - demo mode\n";
    cin>>mode;
    for(int i=0;i<n;i++) // loop will run n times (n=5)
    {

        switch(currentPid = fork()){
            case 0:
                cmd = "gnome-terminal -e 'sh -c \" /home/neo/Desktop/OSLab8/childprocess/a.out "+to_string(s)+" "+to_string(s+step)+" "+((mode==1)?"":"1 ")+"\"'";
                system(cmd.c_str());
                return 0;
            case -1:
                printf("Error when forking\n");
                break;
            default:
                break;
        }
        cout<<"Son process id - "<<currentPid<<endl;
        s+=step;
    }
    if(mode==1){}
    else{
        int choise;
        do{
         cout<<"1 Set priority of processes "<<endl;
         cout<<"2 Kill processes "<<endl;
         cout<<"3 Suspend:"<<endl;
         cout<<"4 Exit";
         cout<<"Your command: ";
         cin>>choise;
         switch(choise)
         {
         case 1:
                {
                    cout << "Enter ID of the process: ";
                    int id; cin >> id;
                    cout << "Enter the priority (from -20 up to 19): ";
                    int prior; cin >> prior;
                    setpriority(PRIO_PROCESS,id,prior);
                    break;
                }
          case 2: 
                {
                    cout << "Enter ID of the process: ";
                    pid_t id; cin >> id;
                    kill(id,9);
                    break;
                }
          case 3: 
                {
                    cout << "Enter ID of the process: ";
                    int id; cin >> id;
                    kill(id,SIGSTOP);
                    break;
                }
          case 4: 
                {
                    cout << "Enter ID of the process: ";
                    int id; cin >> id;
                    kill(id,SIGCONT);
                    break;
                }
        }
        }while(choise!=4);
    }
}

Answer 1

I eliminated the unrelated "stuff" and came up with this:

#include <iostream>
#include <sys/types.h>
#include <sys/wait.h>
#include <cstring>
#include <unistd.h>
#include <sys/resource.h>
#include <stdlib.h>
#include <string>

int main(int argc, char **argv) {

    if (argc != 2) { 
        std::cerr << "Usage: kil <process ID\n";
        return EXIT_FAILURE;
    }

    pid_t process = std::stoi(argv[1]);
    if (kill(process, 9)) {
        std::cerr << strerror(errno) << '\n';
        return EXIT_FAILURE;
    }
}

I did a quick test, killing a couple of processes, and it seems to have worked correctly. If you want to kill the process a bit more...gently, you might want to use kill(process, SIGINT) instead.