Why doesn't "cat a b > foo" work?

[Grant Edwards]

On 2021-01-19, Grant Edwards <grant.b.edwards at gmail.com> wrote:

> I've confirmed that with our older version of busybox (1.26?) 'cat'
> doesn't use sendfile() for the test case above (which works
> fine). AFAICT from the strace output the sendfile64() call is made
> correctly, so it must be a kernel problem and not a gcc or libc
> problem.

This is embarassing.

I discovered and fixed the same thing 4 years ago by disabling use of
sendfile() when building 1.26. Unfortunately, the layout of the
.config file had changed so much between 1.26 and 1.31 that I missed
that when I was comparing the 1.26 config with the 1.31 config.

Sorry for the noise.

--
Grant