#include <iostream>
#include <stdio.h>   // perror
#include <stdlib.h>  // for getenv, putenv, setenv, unsetenv, ...
#include <errno.h>   // errno
#include <string.h>  // for strncmp

using namespace std;

void PrintAVar(string varName) {
    char * result;

    cout << "Checking the value of $" << varName << endl;
    result = getenv(varName.c_str());
    cout << "\t";
    if (result == nullptr) {
       cout << varName << " is not defined" << endl;
    } else {
       cout << varName << " is set to " << result << endl;
    }

    cout << endl;
    return;
}

void SetButNoMod(string name, string value) {
   
    cout << "Attempting to set (but not modify) " << name 
         << " to be " << value << endl;
    if (-1 == setenv(name.c_str(), value.c_str(), 0) ) {
        perror("\t\tERROR  setenv");
    }
    cout << endl;
    return;
}

void SetOrMod(string name, string value) {
   
    cout << "Attempting to set or modify " << name 
         << " to be " << value << endl;
    if (-1 == setenv(name.c_str(), value.c_str(), true) ) {
        perror("\t\tERROR  setenv");
    }
    cout << endl;
    return;
}

void FindName(string name, char ** envp, string where ) {
   int i;
   string search = name+"=";

   cout << "Searching " << where << " for " << name << endl;
   for (i=0;envp[i] != nullptr; i++) {
      if (0==strncmp(envp[i],search.c_str(),search.size())) {
          cout <<"\tFound $" << name << " at envp[ " << i << "] it is \"" 
	       << envp[i] << '"' << endl;
      }
   }
   cout << endl;

   return;
}

void Remove(string name) {
    cout << "Attempting to remove " << name << endl;
    if (0 != unsetenv(name.c_str())){
       perror("unsetenv");
    }

    cout << endl;

    return;
}


int main(int, char ** , char ** envp) {
     extern char ** environ;

     // show how to loop through the environment pointer
     FindName("HOME", envp, "envp");

     // print a variable that probably exists.
     PrintAVar("HOME");

     // print one that probably does not exist.
     PrintAVar("DAN");

     // set a variable.
     SetButNoMod("DAN", "IS KING of KODE");
     PrintAVar("DAN");

     // try to reset with a flag of no mod
     SetButNoMod("DAN", "IS CING of CODE");
     PrintAVar("DAN");

     // reset with a mod flag = true
     SetOrMod("DAN", "IS CING of CODE");
     PrintAVar("DAN");

     // can the value contain an =, why yes, yes it can.
     SetOrMod("DAN", "DAN=The Best");
     PrintAVar("DAN");
     // but note the new value is not in envp
     FindName("DAN", envp, "envp");
     // but it is in environ
     FindName("DAN", environ, "environ");

     // can the variable name have an =, not it can't 
     SetOrMod("DAN=", "DAN is the Best");

     // can the variable name be "", no again!
     SetOrMod("", "DAN is the Best");

     // Can I remove a variable?
     Remove("DAN");
     PrintAVar("DAN");
     FindName("DAN", environ, "environ");

     // can I remove it again?, well, probably not, but no error.
     Remove("DAN");

     return 0;
}