const QSettings file1("file1.txt", QSettings::IniFormat);
    qDebug() << "file1 keys" << file1.allKeys();

    const QSettings file2("file2.txt", QSettings::IniFormat);
    qDebug() << "file2 keys" << file2.allKeys();

    foreach (const QString &key, file1.allKeys()) {
        if (!file2.contains(key))
            qDebug() << "in file1 only:" << key;
    }

    foreach (const QString &key, file2.allKeys()) {
        if (!file1.contains(key))
            qDebug() << "in file2 only:" << key;
    }

    foreach (const QString &key, file1.allKeys()) {
        if ((file2.contains(key)) && (file1.value(key) != file2.value(key)))
            qDebug() << "file1 and file2 differ:" << key << file1.value(key) << file2.value(key);
    }

Output (with your sample files above):

file1 keys ("s1", "s2", "s3")
file2 keys ("s1", "s2")
in file1 only: "s3"
file1 and file2 differ: "s1" QVariant(QString, "ABCDE") QVariant(QString, "ABCD23")
file1 and file2 differ: "s2" QVariant(QString, "ERTY 1") QVariant(QString, "EEEE")

Cheers.

If your files are not compatible with INI format, just implement your own parsing method. I'd store each key=value entry in a QMap. Then it's simply a matter of checking the value of "s1" key in both QMap instances. Note that this method also applies to real INI files.