Why can't QSettings be written to QTemporaryFile

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();

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.