Redirect console output to the end of a file C++ using ofstream

Question

I want to add new out put from a buff to the end of a file rst.txt using ofstream. My problem is that every new entry erases the file containt.

 #include <iostream>
    #include <sstream>
    #include <stdio.h>
    #include <fstream>

    using namespace std;

    int main()
    {
        FILE *in;


        char buff[512];




        while(1)
        {
            //if(!(in = popen("tcpdump -i wlan0 -e -c 1 -vvv /home/pi/wifi", "r")))
            if(!(in = popen(" sudo tcpdump -i wlan0 -e -c 1 'type mgt subtype probe-req' -vvv", "r")))
                return 1;

            fgets(buff, sizeof(buff), in);

            pclose(in);

            std::istringstream iss;

            iss.str(buff);

            std::string mac_address;
            std::string signal_strength;

            std::string info;
            while(iss >> info)
            {
        if( info.find("SA") != std::string::npos )
                    mac_address = info.substr(3);

                if( info.find("dB") != std::string::npos )
                    signal_strength = info;


            }
            ofstream file;
            file.open("rst.txt");
            streambuf* sbuf=cout.rdbuf();
            cout.rdbuf(file.rdbuf());
            cout<<"file"<< endl;

            cout << "      ADRESSE MAC     : " << mac_address << "     " ;
            cout << "      SIGNAL STRENGTH : " << signal_strength << "  " << endl;
        }

         return 0;
    } 

Is there a way to redirect the output to the end of a file ? Is there any way better than ofstream ?

Answer 1

To append from http://www.cplusplus.com/reference/fstream/ofstream/open/

ofstream file;
file.open ("rst.txt", std::ofstream::out | std::ofstream::app);

Update
To fulfill the request in the comments.

Add a variable previous_mac_address

 [...]
 string previous_mac_address = "";
 while(1)
 {
 [...]

Then before printing compare the previous_mac_address and mac_address

 [...]
 if ( previous_mac_address != mac_address )
 {
     cout << "      ADRESSE MAC     : " << mac_address << "     " ;
     cout << "      SIGNAL STRENGTH : " << signal_strength << "  " << endl;
    previous_mac_address = mac_address; 
 }
 else
    cout << ", " << signal_strength << "  " << endl;

 [...]