Messages in this thread |  | | From | (Linus Torvalds) | Subject | Re: sendfile(2) fails for devices? | Date | 11 Nov 2000 16:57:29 -0800 |
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In article <3A0DE0C8.C700F33D@mandrakesoft.com>, Jeff Garzik <jgarzik@mandrakesoft.com> 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.
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