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comp / comp.unix.questions / File I/O and copying

SubjectAuthor
o File I/O and copyingSteve Keller

1
Subject: File I/O and copying
From: Steve Keller
Newsgroups: comp.unix.questions
Organization: Aioe.org NNTP Server
Date: Fri, 8 Feb 2019 11:39 UTC
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From: keller@no.invalid (Steve Keller)
Newsgroups: comp.unix.questions
Subject: File I/O and copying
Date: Fri, 08 Feb 2019 12:39:48 +0100
Organization: Aioe.org NNTP Server
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I'd like to understand how file I/O is typically handled on todays PCs
or servers running any modern Unix (*BSD, Linux, etc.).

How much copying is involved when I read from a file using the read
system call? And does this depend on alignment of my user space
buffer that I read into?

Can the I/O hardware and block device driver (maybe using DMA) copy
directly to user space, and if so to any address or are there
alignment constraints? Or is data copied to a kernel buffer first and
then copied to user space?

Steve

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