Why can't QSettings be written to QTemporaryFile

Pl45m4

Got (actually not me) the solution...

Try to open the tempFile one time BEFORE you create the QSettings object.
Then it should work :)

Here's the MVP :)
https://stackoverflow.com/a/57471942

fileName returns an empty string when the file is not open. Which you did in your first try (but wrong sync). In your latest approach, the file is closed when creating the settings.
It's a QTemporaryFile thing, this is why it worked with QFile even when the file is closed.

So open before settings and sync after endGroup :)

jeremy_k

This works for me:

#include <QCoreApplication>
#include <QSettings>
#include <QTemporaryFile>
#include <QTextCodec>
#include <QDebug>
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QTemporaryFile tmpFile("/tmp/tmpfile.XXXXXXX");
    tmpFile.open();
    tmpFile.close();

    QSettings settings(tmpFile.fileName(), QSettings::IniFormat);
    settings.setIniCodec(QTextCodec::codecForName("UTF-8"));
    settings.setValue("UserCode", "test value");
    settings.sync();
    
    QFile reader(tmpFile.fileName());
    reader.open(QIODevice::ReadOnly);
    qDebug() << "file" << tmpFile.fileName();
    qDebug() << "content" << reader.readAll();
}

Output:

21:19:24: Starting /tmp/build-settingsfile-Desktop_Qt_5_15_5_clang_64bit-Debug/settingsfile ...
file "/tmp/tmpfile.ZlFULTY"
content "[General]\nUserCode=test value\n"

jeremy_k

The issue seems to be caching in the QIODevice, at least on macOS with Qt 5.15.5.

#include <QCoreApplication>
#include <QSettings>
#include <QTemporaryFile>
#include <QTextCodec>
#include <QDebug>
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QTemporaryFile tmpFile("/tmp/tmpfile.XXXXXXX");
    tmpFile.open();
    tmpFile.write("some text");
    tmpFile.seek(0);
    qDebug() << "tmpfile" << tmpFile.readAll();
    tmpFile.close();

    QSettings settings(tmpFile.fileName(), QSettings::IniFormat);
    settings.setIniCodec(QTextCodec::codecForName("UTF-8"));
    settings.setValue("UserCode", "test value");
    settings.sync();

    QFile reader(tmpFile.fileName());
    reader.open(QIODevice::ReadOnly);
    qDebug() << "file" << tmpFile.fileName() << reader.fileName();
    qDebug() << "content" << reader.readAll();
    tmpFile.open();
    qDebug() << "tmpfile" << tmpFile.readAll();
}

Output:

tmpfile "some text"
file "/tmp/tmpfile.NmlyxbW" "/tmp/tmpfile.NmlyxbW"
content "[General]\nUserCode=test value\n"
tmpfile "some text"

jeremy_k

https://doc.qt.io/qt-6/qiodevice.html says:
Some subclasses, such as QFile and QTcpSocket, are implemented using a memory buffer for intermediate storing of data.

My guess is that closing and reopening the QTemporaryFile doesn't discard its buffer and fails to notice the underlying file change.

tovax

@Pl45m4
Output:
JCDemoAES::test 1 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.NYWrOp"
JCDemoAES::test 2 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.NYWrOp"
JCDemoAES::test 3 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.NYWrOp"
JCDemoAES::test 4 ""

void JCDemoAES::test()
{
    // Init temp file
    tmpFile = new QTemporaryFile(QStringLiteral("%1/%2").arg(QApplication::applicationDirPath()).arg(QApplication::applicationName()), this);
    tmpFile->open();
    qDebug() << __FUNCTION__ << 1 << tmpFile->fileName();

    // Settings
    QSettings settings(tmpFile->fileName(), QSettings::IniFormat);
    qDebug() << __FUNCTION__ << 2 << settings.fileName();
    settings.setIniCodec(QTextCodec::codecForName("UTF-8"));
    settings.beginGroup("GroupTest");

    QStringList list;
    list << QStringLiteral("0000") << QStringLiteral("1111") << QStringLiteral("2222") << QStringLiteral("3333");
    settings.setValue("UserCode", list);
    settings.setValue("UserCodeDefault", QStringLiteral("1234"));

    settings.endGroup();
    settings.sync();

    // Read temp file
    qDebug() << __FUNCTION__ << 3 << tmpFile->fileName();
    qDebug() << __FUNCTION__ << 4 << tmpFile->readAll();
    tmpFile->close();
}

tovax

@jeremy_k
PC:
Win10 64bit
Qt 5.14.2

    QTemporaryFile tmpFile(QStringLiteral("%1/%2").arg(QApplication::applicationDirPath()).arg(QApplication::applicationName()));
    tmpFile.open();
    tmpFile.close();

    QSettings settings(tmpFile.fileName(), QSettings::IniFormat);
    settings.setIniCodec(QTextCodec::codecForName("UTF-8"));
    settings.setValue("UserCode", "test value");
    settings.sync();

    QFile reader(tmpFile.fileName());
    reader.open(QIODevice::ReadOnly);
    qDebug() << "file" << tmpFile.fileName();
    qDebug() << "content" << reader.readAll();

Output:
file "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.IwayTr"
content ""

tovax

@jeremy_k said in Why can't QSettings be written to QTemporaryFile:

e seems to be caching in the QIODev

    QTemporaryFile tmpFile(QStringLiteral("%1/%2").arg(QApplication::applicationDirPath()).arg(QApplication::applicationName()));
    tmpFile.open();
    tmpFile.write("some text");
    tmpFile.seek(0);
    qDebug() << "tmpfile" << tmpFile.readAll();
    tmpFile.close();

    QSettings settings(tmpFile.fileName(), QSettings::IniFormat);
    settings.setIniCodec(QTextCodec::codecForName("UTF-8"));
    settings.setValue("UserCode", "test value");
    settings.sync();

    QFile reader(tmpFile.fileName());
    reader.open(QIODevice::ReadOnly);
    qDebug() << "file" << tmpFile.fileName() << reader.fileName();
    qDebug() << "content" << reader.readAll();
    tmpFile.open();
    qDebug() << "tmpfile" << tmpFile.readAll();

Output:
tmpfile "some text"
file "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.bveDgi" "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.bveDgi"
content "some text"
tmpfile "some text"

tovax

I tested qt5.15.2 and couldn't work.
PC: Win10 64-bit
Compiler: MSVC2019

Christian Ehrlicher

You should check QSettings::status() and will notice that you get a QSettings::FormatError since you try to load an invalid ini file.

qDebug() << "Settings status: " <<  settings.status();

tovax

@Christian-Ehrlicher Hi, thanks.
It's "QSettings::NoError".
Output:
JCDemoAES::test 1 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_15_2_MSVC2019_64bit-Debug/debug/JCDemoAES.tSHxeC"
JCDemoAES::test 2 QSettings::NoError
JCDemoAES::test file: "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_15_2_MSVC2019_64bit-Debug/debug/JCDemoAES.tSHxeC"
JCDemoAES::test content: ""

    // Init temp file
    QTemporaryFile tmpFile(QStringLiteral("%1/%2").arg(QApplication::applicationDirPath(), QApplication::applicationName()), this);
    tmpFile.open();
    tmpFile.close();
    qDebug() << __FUNCTION__ << 1 << tmpFile.fileName();

    // Settings
    QSettings settings(tmpFile.fileName(), QSettings::IniFormat);
    qDebug() << __FUNCTION__ << 2 << settings.status();
    settings.setIniCodec(QTextCodec::codecForName("UTF-8"));
    settings.beginGroup("GroupTest");

    QStringList list;
    list << QStringLiteral("0000") << QStringLiteral("1111") << QStringLiteral("2222") << QStringLiteral("3333");
    settings.setValue("UserCode", list);
    settings.setValue("UserCodeDefault", QStringLiteral("1234"));

    settings.endGroup();
    settings.sync();

    // Read temp file
    QFile reader(tmpFile.fileName());
    reader.open(QIODevice::ReadOnly);
    qDebug() << __FUNCTION__ << "file:" << tmpFile.fileName();
    qDebug() << __FUNCTION__ << "content:" << reader.readAll();

tovax

@Christian-Ehrlicher
QSettings::status() is "AccessError" after settings.sync();
Output:
JCDemoAES::test 3 QSettings::AccessError

    settings.endGroup();
    settings.sync();
    qDebug() << __FUNCTION__ << 3 << settings.status();

tovax

I don't know why access error occurred.

AccessError
public static final QSettings.Status AccessError

An access error occurred (e.g. trying to write to a read-only file).

A QTemporaryFile will always be opened in QIODevice::ReadWrite mode, this allows easy access to the data in the file. This function will return true upon success and will set the fileName() to the unique filename used.

tovax

Output:
JCDemoAES::test 1 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_15_2_MSVC2019_64bit-Debug/debug/JCDemoAES.lwKvTB"
JCDemoAES::test 2 QSettings::NoError
JCDemoAES::test 3 QSettings::AccessError
JCDemoAES::test file: "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_15_2_MSVC2019_64bit-Debug/debug/JCDemoAES.lwKvTB"
JCDemoAES::test content: ""

    // Init temp file
    tmpFile = new QTemporaryFile(QStringLiteral("%1/%2").arg(QApplication::applicationDirPath(), QApplication::applicationName()), this);
    tmpFile->open();
    qDebug() << __FUNCTION__ << 1 << tmpFile->fileName();

    // Settings
    QSettings settings(tmpFile->fileName(), QSettings::IniFormat);
    qDebug() << __FUNCTION__ << 2 << settings.status();
    settings.setIniCodec(QTextCodec::codecForName("UTF-8"));
    settings.beginGroup("GroupTest");

    QStringList list;
    list << QStringLiteral("0000") << QStringLiteral("1111") << QStringLiteral("2222") << QStringLiteral("3333");
    settings.setValue("UserCode", list);
    settings.setValue("UserCodeDefault", QStringLiteral("1234"));

    settings.endGroup();
    settings.sync();
    qDebug() << __FUNCTION__ << 3 << settings.status();

    // Read temp file
    QFile reader(tmpFile->fileName());
    reader.open(QIODevice::ReadOnly);
    qDebug() << __FUNCTION__ << "file:" << tmpFile->fileName();
    qDebug() << __FUNCTION__ << "content:" << reader.readAll();

    // Close temp file
    tmpFile->close();

JonB

@tovax
https://doc.qt.io/qt-5/qtemporaryfile.html#details

Reopening a QTemporaryFile after calling close() is safe. For as long as the QTemporaryFile object itself is not destroyed, the unique temporary file will exist and be kept open internally by QTemporaryFile.

It is possible that the fact the file is kept open internally interferes with QSettings ability to write to the file. Create or hand QSettings your own filename in the same directory as QTemporaryFile would (QDir::tempPath()) and verify that succeeds.

Reduce your code to minimum. Remove the setIniCodec(), and get rid of the beginGroup() stuff and the QStringList, just write a single simple value while you test.

Christian Ehrlicher

You can't open a file twice for read/write as @JonB correctly told you.

JonB

@tovax
I suspect you will find there is indeed a problem trying to use a QTemporaryFile for QSettings, for whatever reason. I refer you to https://www.mail-archive.com/interest@qt-project.org/msg32865.html

I decided to look at writing the settings out to a QTemporaryFile,

then just reading that data back in as a string, and then let the

QTemporaryFile go out of scope and clean itself up. But according to the

following test code, QSettings does not seem to play nicely with QTemporaryFile:

I suggest you read that thread and look at his code. Read through all the responses (I did not.) I think you will find the final advice is to use your own temporary file instead.

Christian Ehrlicher

@JonB I don't see why it's needed - all what is needs to be done is written here. Create a QTemporaryFile, open it, close it again, pass filename to QSettings and go ahead.

JonB

@Christian-Ehrlicher said in Why can't QSettings be written to QTemporaryFile:

Create a QTemporaryFile, open it, close it again, pass filename to QSettings and go ahead.

I believe the OP here tried that earlier (his post before last above) but got the same "access error", did he not? See his earlier code above:

    QTemporaryFile tmpFile(QStringLiteral("%1/%2").arg(QApplication::applicationDirPath(), QApplication::applicationName()), this);
    tmpFile.open();
    tmpFile.close();

And probably that reference tried to. QTemporaryFile::close() does not close the file, reference the docs I quoted earlier, and I looked at the source and all it does it seek(0). Doubtless to do with keeping a handle into it until the QTemporaryFile goes out of scope, whereupon it will be closed and deleted. Hence if the fact that it is kept open (for all I know with exclusive access) is what stops QSettings working cannot be avoided.

Christian Ehrlicher

@JonB You're right. It can't work since completely closing a QTemporaryFile will delete it (as this is the usecase for this class).
I also don't see a reason to use a QTemporaryFile here at all.

jeremy_k

@tovax might have better luck using QTemporaryDir, and then creating a QFile with a known name within.