trouble writing a text file

I am writing a text file which has the number of records in it at the start and then each record is stored at a 400 boundary - or rather should be!

In the following snippet the number of records is being updated and recorded at the start of the file OK but the string I am writing should be at 400, 800, 1200 etc but this part is not working, when I inspect the file there is only the one string and it's position varies only a little.

I am sure it must be an incorrect setting but I have tried all the ios::beg etc but none seem to make a difference

Code:

        jcin>>numrecords;
        jcin.close ();

        cout<<endl<<endl<<endl<<"There are currently "<<numrecords<<" records in this file.";

        ofstream jcout;
        jcout.open(filename, ios::out);

        long int pos;
        pos=(numrecords+1)*400;

        jcout.seekp (pos);
        jcout<<"hello sailor";//will be replace by a record
        cout<<endl<<"position is ; "<<pos<<endl;

        jcout.seekp (0);
        jcout<<numrecords++;//this is updating correctly

        jcout.close ();

I believe that this

Code:

ofstream jcout;
jcout.open(filename, ios::out);

will essentially truncate the file. Try an fstream instead of an ofstream. I believe it defaults to in/out mode, so it won't truncate.

Code:

    fstream jcout(filename);

I gave up long ago trying to understand what the default behavior of the objects were. You can just mask together features you want with the pipe operator. A list of features is here: ios_base::openmode - C++ Reference.

Code:

ofstream jcout;
jcout.open (filename, ios::out | ios::app);

I still think it's good to use ofstream and ifstream though, unless you want to read and write from the same stream.