General Question: ctime, mtime, and xattrs
So I was reading the wiki on the internal layout. The INODE description says "st_ctime. Also updated when xattrs change."
Why isn't changing the xattrs a modification (st_mtime) event?
It just seems odd to me...
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Ah...
I've been thinking "ctime" is/was (still) "create time". It seems that somewhere in the last couple decades it became "change time"; Or that I picked up that incorrect "create time" idea back in the UNIX Sys V R 3 days and just never had cause to think about it again...
Never mind. /sigh...
What a maroon. 8-)
Quoted 8 lines · cwillu
xattrs are commonly used to implement acls, which wouldn't typically be considered a content modification.
Quoted 4 lines · Robert White
So I was reading the wiki on the internal layout. The INODE description says "
st_ctime. Also updated when xattrs change."Why isn't changing the xattrs a modification (
st_mtime) event?
Quoted 4 lines · Robert White
I've been thinking "ctime" is/was (still) "create time". It seems that somewhere in the last couple decades it became "change time"; Or that I picked up that incorrect "create time" idea back in the UNIX Sys V R 3 days and just never had cause to think about it again...
Sadly there's never been a creation time in Linux that you can get with a standard system call, there was an attempt 4-5 years ago to get xstat merged that would include creation time from filesystems that support it (like ext4) but it never went anywhere (for a variety of reasons).
LWN article on the patch set:
https://lwn.net/Articles/394298/
Linus knocking it back:
https://lkml.org/lkml/2010/7/22/249
FreeBSD has:
st_birthtim Time when the inode was created.
No idea when that was added!
All the best,
Chris
Signature · 1 line
Chris Samuel : http://www.csamuel.org/ : Melbourne, VIC
Quoted 7 lines · Robert White
Ah...
I've been thinking "ctime" is/was (still) "create time". It seems that somewhere in the last couple decades it became "change time"; Or that I picked up that incorrect "create time" idea back in the UNIX Sys V R 3 days and just never had cause to think about it again...
v7 is the point where the third timestamp has first appeared (v6 has only two - access and update). And their stat(2) says this:
st_atime is the file was last read. For reasons of efficiency, it is not set when a directory is searched, although this would be more logi‐ cal. st_mtime is the time the file was last written or created. It is not set by changes of owner, group, link count, or mode. st_ctime is set both both by writing and changing the i-node.
FWIW, their /usr/sys/sys/sys4.c haschmod()
{
register struct inode *ip;
register struct a {
char *fname;
int fmode;
} *uap;
uap = (struct a *)u.u_ap;
if ((ip = owner()) == NULL)
return;
ip->i_mode &= ~07777;
if (u.u_uid)
uap->fmode &= ~ISVTX;
ip->i_mode |= uap->fmode&07777;
ip->i_flag |= ICHG;
if (ip->i_flag&ITEXT && (ip->i_mode&ISVTX)==0)
xrele(ip);
iput(ip);
}
and /usr/sys/sys/iget.c has, in the end of iupdat() (called on the final
iput(), as well as on stat(2) and several other paths), this:
if(ip->i_flag&IACC)
dp->di_atime = *ta;
if(ip->i_flag&IUPD)
dp->di_mtime = *tm;
if(ip->i_flag&ICHG)
dp->di_ctime = time;
ip->i_flag &= ~(IUPD|IACC|ICHG);(ta and tm are both equal to &time in that call chain). IOW, chmod(2) definitely sets ctime.
IOW, ctime has never been "create time"; it's "change time" and it had been that way since its introduction.