sendfile(2) fails for devices?

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?
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In article <[email protected]>,

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".

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
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