[Python-Dev] ctime: I don't think that word means what you think it means.

Zooko Wilcox-O'Hearn zooko at zooko.com
Sat Jun 13 18:58:16 CEST 2009

The stat module uses the "st_ctime" slot to hold two kinds of values
which are semantically different and which are frequently
confused with one another.  It chooses which kind of value to put in
there based on platform -- Windows gets the file creation time and all
other platforms get the "ctime".  The only sane way to use this API is
then to switch on platform:

if platform.system() == "Windows":
     metadata["creation time"] = s.st_ctime
     metadata["unix ctime"] = s.st_ctime

(That is an actual code snippet from the Allmydata-Tahoe project.)

Many or even most programmers incorrectly think that unix ctime is file
creation time, so instead of using the sane idiom above, they write the

metadata["ctime"] = s.st_ctime

thus passing on the confusion to the users of their metadata, who may
not be able to tell on which platform this metadata was created.   
This is
the situation we have found ourselves in for the Allmydata-Tahoe
project -- we now have a bunch of "ctime" values stored in our
filesystem and no way to tell which kind they were.

More and more filesystems such as ZFS and Mac HFS+ apparently offer
creation time nowadays.

I propose the following changes:

1.  Add a "st_crtime" field which gets populated on filesystems
(Windows, ZFS, Mac) which can do so.

That is hopefully not too controversial and we could proceed to do so
even if the next proposal gets bogged down:

2.  Add a "st_unixctime" field which gets populated *only* by the unix
ctime and never by any other value (even on Windows, where the unix
ctime *is* available even though nobody cares about it), and deprecate
the hopelessly ambiguous "st_ctime" field.

You may be interested in http://allmydata.org/trac/tahoe/ticket/628
("mtime" and "ctime": I don't think that word means what you think it
means.) where the Allmydata-Tahoe project is carefully unpicking the
mess we made for ourselves by confusing ctime with file-creation time.

This is ticket http://bugs.python.org/issue5720 .



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