QFileInfo::birthTime() returns nothing (on Ubuntu)

[Pawel_Kleczek]

I'm using Qt 5.15.2 on Ubuntu 21.10
When I call

QFileInfo fileInfo(PATH_TO_A_FILE);
LOG() << fileInfo.birthTime().toString("dd/MM/yy HH:mm:ss");
LOG() << fileInfo.created().toString("dd/MM/yy HH:mm:ss");

then birthTime()'s result is an empty string, whereas created() returns the correct time. However, according to the docs created() is deprecated and birthTime() should be used instead...

Any ideas why do I observe such behavior?

[JonB]

@Pawel_Kleczek
For code browser for Qt5, https://codebrowser.dev/qt5/qtbase/src/corelib/io/qfileinfo.cpp.html

QDateTime QFileInfo::created() const
{
    QDateTime d = fileTime(QFile::FileBirthTime);
    if (d.isValid())
        return d;
    return fileTime(QFile::FileMetadataChangeTime);
}

QDateTime QFileInfo::birthTime() const
{
    return fileTime(QFile::FileBirthTime);
}

So at some level fileTime(QFile::FileBirthTime) is returning invalid QDateTime while for created() the fileTime(QFile::FileMetadataChangeTime) is returning the result you get.

You would have to follow https://codebrowser.dev/qt5/qtbase/src/corelib/io/qfileinfo.cpp.html#_ZNK9QFileInfo8fileTimeEN11QFileDevice8FileTimeE, QDateTime QFileInfo::fileTime(QFile::FileTime time) const, to find out why.

I believe Qt caches QFileInfo stuff, that could be an issue somehow?

Additionally per https://forum.qt.io/topic/133954/either-qfileinfo-caching-does-not-work-or-just-its-methods-are-unreasonable-very-slow/19 I believe at Qt 5.15 birthTime() (created() too?) may be "inefficient", though that is a separate matter, unless it interferes with the result here.

It may be that Qt/OS does not regard Linux fs as actually storing a "creation" time, hence the call to QFile::FileMetadataChangeTime. Google for e.g. linux file created, read what is said, how it depends on the filing system in use, etc. The c flag on a Linux file is not named "created" time, it is named "Change - the last time meta data of the file was changed (e.g. permissions)", e.g. https://unix.stackexchange.com/a/2465/104736. So you're not going to get a Linux "created/born" time unless you use a native Linux call and appropriate to the file system.

You may also use QDateTime QFileInfo::metadataChangeTime() const directly.

[ChrisW67]

Storage of actual file creation time will also depend on the file system involved. For example, EXT4 has provision for a create time while EXT2/3 do not.

Even then, the standard stat() call has no place to return it in struct stat, so you need the statx() call. Not sure what Qt ultimately uses.

[kkoehne]

@ChrisW67 said in QFileInfo::birthTime() returns nothing (on Ubuntu):

Not sure what Qt ultimately uses.

Qt uses statx (see e.g. https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/io/qfilesystemengine_unix.cpp).

But yeah, not all file systems support birth time. @Pawel_Kleczek , you should be able to check this yourself on the command line with

stat PATH_TO_A_FILE

does it print a 'Birth: ' row?

[JonB]

OP's Ubuntu should support stat file for Birth. See e.g. How to Find Out When a File Was Created in Linux for a recent discussion of the issues.