Why can't QSettings be written to QTemporaryFile

[tovax]

Hi, all
I want to encrypt the contents of the INI file. My idea is to first write the contents of qsettings to the temporary file, then encrypt the contents of the temporary file, and finally write to the INI file. But I found that writing qsettings to the temporary file failed. Could you help me please?
Best regards!

void JCDemoAES::test()
{
        // Open temp file
        tmpFile = new QTemporaryFile(QStringLiteral("%1/%2").arg(QApplication::applicationDirPath()).arg(QApplication::applicationName()), this);
        tmpFile->open();
    
        // Settings
        QSettings settings(tmpFile->fileName(), QSettings::IniFormat);
        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();
    
        // Close temp file
        qDebug() << __FUNCTION__ << tmpFile->readAll();
        tmpFile->close();
}

[tovax]

Debug output: JCDemoAES::test ""

[jeremy_k]

There's no call to QSettings::sync, and the QSettings object isn't destroyed until after attempting to read. The settings may not have been flush to the file yet.

Waiting until after the sync completes to open the file for reading may also be necessary.

[Pl45m4]

@tovax

Double check what tempFile->fileName() returns and if it really matches your complete file path.

Can you confirm the creation of such file?

• https://doc.qt.io/qt-5/qtemporaryfile.html#fileName

[tovax]

@jeremy_k Hi, thank you very much! I modified the code, but I still can't write it successfully.

Debug output:
JCDemoAES::test "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.sEbGkR"
JCDemoAES::test ""

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

    // Settings
    QSettings settings(tmpFile->fileName(), QSettings::IniFormat);
    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.sync();
    settings.endGroup();

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

[tovax]

@Pl45m4 Hi, thanks!
The temporary file has indeed been successfully created. I can see it in the folder.

Debug output:
JCDemoAES::test 1 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.LBTGmC"
JCDemoAES::test 2 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.LBTGmC"
JCDemoAES::test 3 ""

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();
    tmpFile->close();

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

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

[jeremy_k]

QSettings::sync() should go after QSettings::endGroup().

[tovax]

@jeremy_k Thank you for your patience!
Debug output:
JCDemoAES::test 1 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.zJpgBl"
JCDemoAES::test 2 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.zJpgBl"
JCDemoAES::test 3 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.zJpgBl"
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();
    tmpFile->close();

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

[Pl45m4]

@tovax

There could be an issue because InitFormat may expect an *.ini extension. Your temp file seem to have ini file format, but a random file extension.
It could be possible that QSettings wont recognize this file. But this is just a guess.
The read/write access looks good now

On Unix, NativeFormat and IniFormat mean the
 same thing, except that the file extension
 is different (.conf for NativeFormat, .ini for IniFormat).

(from: https://doc.qt.io/qt-5/qsettings.html#Format-enum)

[tovax]

@Pl45m4 I tested QFile, the write failure should not be caused by the extension.

Debug output:
JCDemoAES::test 1 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.TBjIps"
JCDemoAES::test 2 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.TBjIps"
JCDemoAES::test 3 "E:/JCShared/Projects/JCDemo/JCDemoAES/build-JCDemoAES-Desktop_Qt_5_14_2_MSVC2017_64bit-Debug/debug/JCDemoAES.TBjIps"
JCDemoAES::test 4 "[GroupTest]\nUserCode=0000, 1111, 2222, 3333\nUserCodeDefault=1234\n"

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

    // 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
    tmpFile->open(QIODevice::ReadWrite | QIODevice::Text);
    qDebug() << __FUNCTION__ << 3 << tmpFile->fileName();
    qDebug() << __FUNCTION__ << 4 << tmpFile->readAll();
    tmpFile->close();
}

[Pl45m4]

@tovax

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();