svn: head/bin/dd

[Thomas Quinot]

* 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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