Re: wc -c /proc /sys handling

[Jim Meyering]

On Mon, Mar 21, 2016 at 8:57 AM, Pádraig Brady <address@hidden> wrote:
> I was wondering if we should not penalize all normal files,
> to handle the /proc and /sys approximate st_size edge case?
> I.E. only do the seek to last block of file + read() when st_size %
> PAGE_SIZE == 0 ?
>
> I did a quick check of /proc and /sys sizes:
>
> $ find /proc/ /sys/ -type f -printf "%p\t%s\n" 2>/dev/null |
>   tr ' ' _ | sort -k2,2n | uniq -f1 -c
>
>  121494 /proc/100/attr/current  0
...
>   19610 /proc/bus/pci/00/1b.0   4096
...
> In all cases where st_size % PAGE_SIZE != 0, it would be the same or better
> to take the st_size.
> In most cases it was the same. There are a couple of cases with larger sizes
> where it would
> have been better to take the st_size rather than reading, I.E.:
>
> /sys/devices/pci0000:00/0000:00:1a.0/usb3/3-1/3-1.4/descriptors
>   st_size = 65553, available bytes = 195, wc -c = 65552
> /sys/devices/pci0000:00/0000:00:02.0/rom
>   st_size = 131072, available bytes = 0, wc -c = 127007 (EINVAL)
> /sys/devices/pci0000:00/0000:00:02.0/resource0
>   st_size = 4194304, available bytes = 0, wc -c = 4191231 (EIO)
>
> Also related to this is that we currently need read access to any file
> we want to wc -c, whereas with this we wouldn't. Well we would for %
> PAGE_SIZE files,
> but we probably should fall back to st_size on EPERM in any case?

Good ideas. I agree and like the proposal.
Not actually reading makes the results more conceptually consistent
with the usual (non-special) case of files listed on the command line.