how to use QFile().flash?

nishiokas

We have this code.
Please tell us these question.
1.This code do "flash()" after "close()", is this collect?
2.If "flash()" return false, this code can't break out of the infinite loop. Is it possible?
if return false, it will return true in near future absolutely?

    QFile file(filename);
    if (file.open(QIODevice::WriteOnly|QIODevice::Append))
    {
        QTextStream out(&file);
        for(int i = 0; i < 128; i++) {
            out << log.sData[i];
            out << ",";
        }
        file.close();
        for( ; ; ) {
            if( file.flush() ) {
                break;
            }
        }
    }

Thanks.

jsulm

@nishiokas

Yes. Explained here: https://doc.qt.io/qt-5/qfiledevice.html#close
You should have a second escape condition for that loop. I don't think it is guaranteed that flush() will return true.

jeremy_k

@jsulm I think there's some confusion here.

Calling close() followed by flush(), or any file operation that expects an open file doesn't make sense. For better or worse, that resource is gone. The QIODevice::openMode() is set to make flush() a successful no-op.

Call flush() on its own if the program benefits from increasing the chance that a user or file system sees the data before proceeding, or if knowing of a failure is useful. flush() may never succeed. The device could be full, or physically removed.

QFile::close() calls flush() before closing the file. As noted in the documentation, it ignores a flush() failure. If you can't do anything anyway, this is the way to go.

Getting back to the original 2 questions:

No. Don't close(); ... flush();. Use flush();... close(), or just close().
while (!flush()) on a closed file will not lead to an infinite loop, but is pointless. while (!flush()) before close() may lead to an infinite loop. Based on https://code.woboq.org/qt5/qtbase/src/corelib/io/qfsfileengine.cpp.html#472, if flush() returns false once, it will continue to do so until the file is closed.

jsulm

@jeremy_k said in how to use QFile().flash?:

Calling close() followed by flush(), or any file operation that expects an open file doesn't make sense.

You're right of course I did not notice that there was a close() before flush(). @nishiokas this indeed makes no sense.