Date: Wed, 10 Feb 2016 21:38:23 +0100 From: Thomas Quinot <thomas@FreeBSD.ORG> To: Maxim Sobolev <sobomax@FreeBSD.org> Cc: kib@freebsd.org, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn: head/bin/dd Message-ID: <20160210203823.GA74615@melamine.cuivre.fr.eu.org> In-Reply-To: <CAH7qZfs=1vebh1jJVF2Gz1VViAaMNTip0p_oKq7Sb4yAHefwJw@mail.gmail.com> References: <201405071933.s47JXUx0046697@svn.freebsd.org> <CAH7qZfs=1vebh1jJVF2Gz1VViAaMNTip0p_oKq7Sb4yAHefwJw@mail.gmail.com>
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* Maxim Sobolev, 2016-02-10 :
> Looking at the code in question I don't see how could it have worked. Look
> at the following piece of code in your diff for example:
>
> + if (force && cnt == 0) {
> + pending -= last_sp;
> + assert(outp == out.db);
> + memset(outp, 0, cnt);
> + }
>
> When the branch is taken, cnt is 0, so at the very least memset(x, y, 0) is
> NOP. Later on, write(2) is conditional on cnt != 0, so that it's never
> taken. As a result, lseek is the last operation the file sees.
Correct :-(
In the context of the current logic, I think the simple fix is:
Index: dd.c
===================================================================
--- dd.c (révision 265593)
+++ dd.c (copie de travail)
@@ -470,6 +470,7 @@
*/
if (force && cnt == 0) {
pending -= last_sp;
+ cnt += last_sp;
assert(outp == out.db);
memset(outp, 0, cnt);
}
> Also, for what it's worth, you can use ftruncate(2) instead of write() for
> regular sparse files to ensure correct size. That would write just as much
> data as needed to the end. I've made a quick and dirty patch, that seems to
> be working better than current code at least:
>
> http://sobomax.sippysoft.com/dd.diff
>
> Please fix.
Leveraging ftruncate certainly seems attractive, if the destination is a
regular file. I'll look into this option.
Thanks!
Thomas.
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