sendfile(2) fails with -EINVAL every time I try to read from a device
file.
This sounds like a bug... is it? (the man page doesn't mention such a
restriction)
I am using kernel 2.4.0-test11-pre2. All other tests with sendfile(2)
succeed: file->file, file->STDOUT, STDIN->file...
--
Jeff Garzik |
Building 1024 | Would you like a Twinkie?
MandrakeSoft |
In article <[email protected]>,
Jeff Garzik <[email protected]> wrote:
>sendfile(2) fails with -EINVAL every time I try to read from a device
>file.
>
>This sounds like a bug... is it? (the man page doesn't mention such a
>restriction)
sendfile() on purpose only works on things that use the page cache.
EINVAL is basically sendfiles way of saying "I would fall back on doing
a read+write, so you might as well do it yourself in user space because
it might actually be more efficient that way".
>I am using kernel 2.4.0-test11-pre2. All other tests with sendfile(2)
>succeed: file->file, file->STDOUT, STDIN->file...
Yes, as long as STDIN is a file ;)
sendfile() wants the source to be in the page cache, because the whole
point of sendfile() was to avoid a copy.
The current device model does _not_ use the page cache. Now, arguably
that's a bug - it also means that you cannot mmap() a block device - but
as it could be easily documented (maybe it is, somewhere), I'll call it
a bad feature for now.
Now, if you want to add the code to do address spaces for block devices,
I wouldn't be all that unhappy. I've wanted to see it for a while. I'm
not likely to apply it for 2.4.x any more, but I'd love to have it early
for 2.5.x.
Linus