On Mon, 12 Nov 2001, Linus Torvalds wrote:
> On Mon, 12 Nov 2001, Lionel Bouton wrote:
> >
> > Seems not the case with gnu tar : write isn't even called once on the fd
> > returned by open("/dev/null",...). In fact a "grep write" on the strace
> > output is empty in the "tar cf /dev/null" case. Every file in the tar-ed
> > tree is stat-ed but no-one is read-ed.
>
> Wow. What a sleazy optimization - it can't be anything but a special case.
>
> How do they do it anyway? By matching on the name Or by knowing what the
> minor/major numbers of /dev/null are supposed to be on that particular
> operating system?
>From src/buffer.c in tar-1.13.19 in open_archive():
[ ... ]
#if !MSDOS
/* Detect if outputting to "/dev/null". */
{
static char const dev_null[] = "/dev/null";
struct stat dev_null_stat;
dev_null_output =
(strcmp (archive_name_array[0], dev_null) == 0
|| (! _isrmt (archive)
&& S_ISCHR (archive_stat.st_mode)
&& stat (dev_null, &dev_null_stat) == 0
&& archive_stat.st_dev == dev_null_stat.st_dev
&& archive_stat.st_ino == dev_null_stat.st_ino));
}
[ ... ]
Regards,
Grant
-- Grant Erickson University of Minnesota Alumni o mail:erick205@umn.edu 1996 BSEE o http://www.umn.edu/~erick205 1998 MSEE- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Thu Nov 15 2001 - 21:00:31 EST