how can i read a specific string from file?

[sajastu]

Hi,
i've written a code for writing data on binary file:

QByteArray ar=ui->User_signUp->text().toUtf8()+"%"+ ui->Pass_singUp->text().toUtf8()+"$";
        QFile file("users.dat");
        file.open(QIODevice::WriteOnly | QIODevice::Append );
        file.write(ar);

QByteArray ar=ui->User_signUp->text().toUtf8()+"%"+ ui->Pass_singUp->text().toUtf8()+"$";
        QFile file("users.dat");
        file.open(QIODevice::WriteOnly | QIODevice::Append );
        file.write(ar);

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pattern in binary file:

username0%password0&username1%password1& ...

username0%password0&username1%password1& ...

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Now assume that i want to read a specific Stirng (Like username1 - i mean search a string) from file, i don't want to read whole file,because file may have huge data, for this i'm using "%" and "&" to know where an username or a password ends.
do we have a solution for this?!

BR