Messages in this thread |  | | Date | Mon, 12 Nov 2001 16:36:30 -0600 (CST) | From | Grant Erickson <> | Subject | Re: File System Performance |
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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
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