From owner-freebsd-hackers  Sun Apr 11 18:23:51 1999
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Date: Sun, 11 Apr 1999 21:20:53 -0400 (EDT)
From: Brian Feldman <green@unixhelp.org>
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To: Jason Thorpe <thorpej@nas.nasa.gov>
Cc: jwm@CSUA.Berkeley.EDU, freebsd-hackers@FreeBSD.ORG
Subject: Re: read() and pread() syscalls 
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On Sun, 11 Apr 1999, Jason Thorpe wrote:

> On Sun, 11 Apr 1999 14:16:45 -0700 
>  John Milford <jwm@CSUA.Berkeley.EDU> wrote:
> 
>  > 	Should read() be turned into a library function now that we
>  > have pread(), and read() is a proper subset of pread()?  I am not sure
> 
> Not that I care too much, since read/pread, etc work fine in NetBSD (which
> is what I use), but...
> 
> read() is _NOT_ a subset if pread() in the common sense.  read() works
> from the current file offset, while pread() always takes an explicit offset.
> 
> There is no way to emulate read() with pread() in userspace.
> 
>  > if this is a POSIX compliance issue, but I've been digging around in
>  > this code recently, and it seems that there is a lot of overlap in the
>  > read system calls, and we might want to consider doing something
>  > similar to the approach taken with wait().  I could understand that
>  > this case is different so we may not want to be doing conversion of
>  > read() or readv() into a hypothetical preadv(), but I can see no issue
>  > with converting read()'s into pread()'s.
> 
> ...and why not implement preadv()?  I did when I implemented pread()/pwrite()
> in NetBSD.
> 
> But, see above about it not being possible to emulate read() with pread()
> in userspace.

You mean, without lseek(2) =)

> 
>         -- Jason R. Thorpe <thorpej@nas.nasa.gov>
> 
> 
> 
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