Re: How to avoid data copies in a driver ?

From: Jeremy Fitzhardinge
Date: Fri May 16 2008 - 04:10:51 EST

Francis Moreau wrote:
I'm suprised because what I need doens't seem so uncommon, usually
devices send or
receive data to/from files. So a helper (system call ?) to achieve
that other than the basic
read/write seems needed, no ?

It's fairly rare to have an application which requires moving data to file with absolutely no processing; normally there's at least a bit of massaging/parsing/etc. If that's really what you want to do, maybe you can do it with splice? I haven't looked at it at all, but the intention is that you can splice file descriptors together, so you can splice your device fd to a file fd and have it all just work...

Alternatively you could read() from your device into a mmaped file. That's a single copy from device to file, which is about the best you can do without going to heroic lengths.

Also, it really depends on your application. Is it a high-bandwidth thing in which the copy is a huge cost? Or do you want to eliminate the copies because it seems like a nice thing to do?

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