Messages in this thread | | | From | (H. Peter Anvin) | Subject | Re: dev=[0000] | Date | 25 Aug 1998 23:24:24 GMT |
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Followup to: <199808252058.QAA26064@red.seas.upenn.edu> By author: "Vladimir Dergachev" <vladimid@seas.upenn.edu> In newsgroup: linux.dev.kernel > > > I know I asked this question before, but apparently there is more to it than > I thought.. > > Here goes= > in struct bufferhead { } there are two kind of fields for device: > b_dev and b_rdev.. b_rdev is (in comments) said to be the "real" device > (whatever this means) b_dev has a comment saying that when it's equal to > B_FREE( current value 0xffff) the bufferhead is free. However when I list > the current buffer contents I get a lot of entries with b_rdev equal to 0 > and b_dev!=B_FREE. So what is this device 0 ? One of previous answers > suggested that the 0 device is kernel's equivalent of NULL pointer, but > now I think it is not.. Here is an example of such : > > anua:/proc/103/fd# ll > total 0 > l-wx------ 1 anya users 64 Aug 25 16:52 1 -> [0304]:244877 > l-wx------ 1 anya users 64 Aug 25 16:52 2 -> [0304]:244877 > lr-x------ 1 anya users 64 Aug 25 16:52 0 -> [0304]:236684 > lrwx------ 1 anya users 64 Aug 25 16:52 5 -> [0000]:785 > > (103 is kaudioserver) > > You see that the last file descriptor points to inode on device 0. > You can also find a lot of such inodes if you look in the entry corresponding > to Xserver.. > > So what are they ? Are they sockets ? Are they buffered ? >
Major 0 is usually used for anonymous mounts (filesystems with "nodev" in /proc/filesystems), but major 0 minor 0 is reserved as "no such device". If it's used for other things, then that is a bug. However, a bug such that anonymous mounts are allocated minors starting at 0 instead of 1 isn't really particularly far-fetched.
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