Re: cp(1) fails to copy file from /proc

[Jim Meyering]

Jim Meyering wrote:
...
> I chose /proc/kallsyms:
>
>     $ cp /proc/kallsyms /dev/shm/k
>     $ wc -c /proc/kallsyms /dev/shm/k
>     1492084 /proc/kallsyms
>     4090 /dev/shm/k
>
> I was surprised to see different results when copying to /tmp:
>
>     $ cp /proc/kallsyms /tmp/k
>     $ wc -c /proc/kallsyms /tmp/k
>     1492084 /proc/kallsyms
>     1492084 /tmp/k
>
> That's because the two devices have different fundamental block sizes:
>
>     $ stat -f --format '%S %n' /dev/shm/k /tmp/k
>     4096 /dev/shm/k
>     1024 /tmp/k
>
> and cp chooses its input buffer size based on that.
> With the smaller block size, cp happens to avoid the short read problem.

Another interesting tidbit:
cp copies that 1.5MB file using a measly 1024-byte buffer
because the kernel (via stat) reports the size as "0" bytes.

But this is /proc after all, and there are few large files there,
and even if there were many, how often do people copy them?
I don't plan to try to optimize for such a rare case.