From owner-freebsd-hackers  Fri Jun 16 12:49:26 2000
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Date: Fri, 16 Jun 2000 12:49:17 -0700 (PDT)
From: Matthew Dillon <dillon@apollo.backplane.com>
Message-Id: <200006161949.MAA60866@apollo.backplane.com>
To: Joy Ganguly <joy@niksun.com>
Cc: freebsd-hackers <freebsd-hackers@FreeBSD.ORG>
Subject: Re: mmap and file writes...???
References:  <394A3727.1FEC441E@falcon.niksun.com>
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:hi all,
:
:i have a question regarding mmap and file write. a file is mmap'ed and
:data is written into the mapped region. now the same region is written.
:(as shown below)
:
:buf = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
:.
:.  /* some data put in buf */
:.
:
:write(fd, buf, len);
:
:
:now does this lead to unnecessary copies?? more specifically  what
:happens if two virtual addresses point to the same physical address and
:data is copied from one to the other.
:
:thanx in advance
:
:joy

    You can copy data from one part of a file to another using
    this method, but only if the two areas do not overlap.    If
    the areas overlap, the result will be possible corruption of the
    file.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>


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