Re: updating target at the end of recipe, after updating prerequisites, might not work?

[Paul Smith]

On Mon, 2011-11-14 at 08:39 -0500, Eli Zaretskii wrote:
> > Date: Mon, 14 Nov 2011 07:25:47 -0500
> > From: David Boyce <address@hidden>
> > Cc: "address@hidden" <address@hidden>
> > 
> > On a side note, I've often wondered why, after all these years and
> > discussions like this one, nobody's modified at least the GNU version
> > of touch to do its work by opening the file for write and writing 0
> > bytes to it, or appending one byte and then removing it, or whatever
> > it takes to force an mtime update, rather than using the flawed utimes
> > syscall.

I think any change like this would be too much of a change in behavior.
The timestamp on a file is a feature of the directory, not the file
itself (at least in POSIX).  In particular this means that you CAN touch
a read-only file and change its mod time:

        ~$ ls -al foo
        -r--r--r-- 1 psmith psmith 0 Sep  7 23:59 foo
        ~$ touch foo
        ~$ ls -al foo
        -r--r--r-- 1 psmith psmith 0 Nov 14 09:18 foo

But you can't even open a read-only file for writing, much less actually
write to it.  Also, writing 0 bytes to a file does not (at least in my
experiment) change its modtime.

> Did you miss this portion of Coreutils' NEWS?
> 
>   * Noteworthy changes in release 6.12 (2008-05-31) [stable]
> 
>   ** New features
> 
>     cp, install, mv, and touch now preserve nanosecond resolution on
>     file timestamps, on platforms that have the 'utimensat' and
>     'futimens' system calls.

Yes, this actually works, too:

~$ touch --version
touch (GNU coreutils) 8.5

        ~$ ls -l --full-time foo
        -rw-r--r-- 1 psmith psmith 0 2011-11-14 09:18:16.937969078 -0500 foo
        ~$ touch foo
        ~$ ls -l --full-time foo
        -rw-r--r-- 1 psmith psmith 0 2011-11-14 09:23:59.807379653 -0500 foo


-- 
-------------------------------------------------------------------------------
 Paul D. Smith <address@hidden>          Find some GNU make tips at:
 http://www.gnu.org                      http://make.mad-scientist.net
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist