System() starting and killing a process within a thread

Hi everybody,

I was wondering how I can kill a process started with system(cmd) within a thread.

For example I start a new process (another c++ program fi) from a thread like this:

class SystemThread: public ofThread{

 public:
   string cmd;

   void setup(string _cmd){
       this->cmd = _cmd;
   }

 void threadedFunction(){
    if (isThreadRunning()){
       system(cmd.c_str());
    }
};

in main.h
#include "SystemThread.h"
...
SystemThread systhread;

in main.cpp
void ofApp::setup(){

    string execpath = ofToDataPath("ThisIsAnotherProgram_FFMPEG_FI");
    string cmd="bash --login -c '";
    cmd += execpath;
    cmd += "'";

    systhread.setup(cmd);
    systhread.startThread();

    //continue with other stuff here
}

What I have now is a simple program that is running in the shell. (this could be ffmpeg fi coverting some files)

What I would like to know is, how to kill this running process when I stop the thread. I have tried to use popen en pclose but using pclose freezes everything...

I know it works when I look at the multiple ffmpeg video recorders out there (fi https://github.com/timscaffidi/ofxVideoRecorder), but I don't understand quite well how they manage to start/stop the ffmpeg process.
Maybe it has something to do with opening/closing the pipe that makes ffmpeg stop? I am not sure what is going on exactly.

I am looking for a general solution to do this, not only ffmpeg related, but I am a bit in the dark here.

Thank you for your help in advance,