GNU bug report logs - #45648
`dd` seek/skip which way is up?

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Bela Lubkin <bela.lubkin <at> gmail.com>
Subject: bug#45648: closed (Re: bug#45648: `dd` seek/skip which way is up?)
Date: Tue, 22 Feb 2022 17:13:01 +0000

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Your bug report

#45648: `dd` seek/skip which way is up?

which was filed against the coreutils package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 45648 <at> debbugs.gnu.org.

-- 
45648: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=45648
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Bela Lubkin <bela.lubkin <at> gmail.com>
Cc: 45648-done <at> debbugs.gnu.org
Subject: Re: bug#45648: `dd` seek/skip which way is up?
Date: Tue, 22 Feb 2022 09:12:42 -0800

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On 1/4/21 20:08, Paul Eggert wrote:
> On 1/4/21 7:44 PM, Bela Lubkin wrote:
>> TLDR: *huge* existing presence of 'iseek' and 'oseek'; most OSes document
>> them as pure synonyms for 'skip' and 'seek'.
> 
> Thanks for doing all that research. It's compelling, and I think your 
> patch (or something like it) should go in. I'll wait for a bit to hear 
> other opinions.

After thinking about the patch a bit more, let's omit the part about 
adding new conversions iseek_bytes etc., as I think there's a better way 
to address that issue. I proposed something in <https://bugs.gnu.org/54112>.

So instead of your patch, I installed the attached patches. The first 
one adds the iseek and oseek operands that you suggested; the second one 
clarifies dd documentation, as I found several things were confusing 
when rereading it carefully. Something like these patches should appear 
in the next coreutils release.

[0001-dd-support-iseek-and-oseek.patch (text/x-patch, attachment)]

[0002-dd-improve-doc-relative-to-POSIX.patch (text/x-patch, attachment)]

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From: Bela Lubkin <bela.lubkin <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: `dd` seek/skip which way is up?
Date: Sun, 3 Jan 2021 19:03:17 -0800

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Hello --

I constantly confuse 'seek=N' and 'skip=N'.  The two words have no natural
affinity to one I/O direction or the other.

I previously encountered a `dd` implementation which also accepted
'oseek=N' and 'iseek=N', which I found far more natural and easy to
remember.

Here is a small patch implementing the same for coreutils `dd`.  Patch is
against just-gotten git tree; `dd --version` reports 'dd (coreutils)
8.32.101-ebf2c-dirty'.  (I probably got the .texi formatting wrong; please
repair as needed.)

While in the area, I slightly improved some of the help (and therefore man
page).

>Bela<

========================================================================

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index e9dd21c4e..417857c5e 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -9100,6 +9100,15 @@ Skip @var{n} @samp{obs}-byte blocks in the output
file before copying.
 if @samp{oflag=seek_bytes} is specified, @var{n} is interpreted
 as a byte count rather than a block count.

+@item oseek
+@item iseek
+@opindex oseek
+@opindex iseek
+As the distinction between @samp{seek} and @samp{skip}
+is easily confused, @samp{oseek} is accepted as an alias
+for @samp{seek}; @samp{iseek} for @samp{skip}.
+Do not use these in scripts, as this reduces compatibility.
+
 @item count=@var{n}
 @opindex count
 Copy @var{n} @samp{ibs}-byte blocks from the input file, instead
@@ -9457,6 +9466,15 @@ rather than a block count, which allows specifying
 an offset that is not a multiple of the I/O block size.
 This flag can be used only with @code{oflag}.

+@item oseek_bytes
+@item iseek_bytes
+@opindex oseek_bytes
+@opindex iseek_bytes
+As the distinction between @samp{seek_bytes} and @samp{skip_bytes}
+is easily confused, @samp{oseek_bytes} is accepted as an alias
+for @samp{seek_bytes}; @samp{iseek_bytes} for @samp{skip_bytes}.
+Do not use these in scripts, as this reduces compatibility.
+
 @end table

 These flags are not supported on all systems, and @samp{dd} rejects
diff --git a/src/dd.c b/src/dd.c
index 9152a2550..a187522c2 100644
--- a/src/dd.c
+++ b/src/dd.c
@@ -381,7 +381,9 @@ static struct symbol_value const flags[] =
   {"fullblock",   O_FULLBLOCK}, /* Accumulate full blocks from input.  */
   {"count_bytes", O_COUNT_BYTES},
   {"skip_bytes",  O_SKIP_BYTES},
+  {"iseek_bytes", O_SKIP_BYTES},
   {"seek_bytes",  O_SEEK_BYTES},
+  {"oseek_bytes", O_SEEK_BYTES},
   {"", 0}
 };

@@ -571,7 +573,7 @@ Copy a file, converting and formatting according to the
operands.\n\
                   overrides ibs and obs\n\
   cbs=BYTES       convert BYTES bytes at a time\n\
   conv=CONVS      convert the file as per the comma separated symbol
list\n\
-  count=N         copy only N input blocks\n\
+  count=N         copy only N input blocks (bytes if iflag=count_bytes)\n\
   ibs=BYTES       read up to BYTES bytes at a time (default: 512)\n\
 "), stdout);
       fputs (_("\
@@ -580,8 +582,8 @@ Copy a file, converting and formatting according to the
operands.\n\
   obs=BYTES       write BYTES bytes at a time (default: 512)\n\
   of=FILE         write to FILE instead of stdout\n\
   oflag=FLAGS     write as per the comma separated symbol list\n\
-  seek=N          skip N obs-sized blocks at start of output\n\
-  skip=N          skip N ibs-sized blocks at start of input\n\
+  seek=N (or oseek=N)  skip N obs-sized blocks at start of output (bytes
if oflag=seek_bytes)\n\
+  skip=N (or iseek=N)  skip N ibs-sized blocks at start of input (bytes if
iflag=skip_bytes)\n\
   status=LEVEL    The LEVEL of information to print to stderr;\n\
                   'none' suppresses everything but error messages,\n\
                   'noxfer' suppresses the final transfer statistics,\n\
@@ -660,10 +662,10 @@ Each FLAG symbol may be:\n\
         fputs (_("  count_bytes  treat 'count=N' as a byte count (iflag
only)\n\
 "), stdout);
       if (O_SKIP_BYTES)
-        fputs (_("  skip_bytes  treat 'skip=N' as a byte count (iflag
only)\n\
+        fputs (_("  skip_bytes (or iseek_bytes)  treat 'skip=N' as a byte
count (iflag only)\n\
 "), stdout);
       if (O_SEEK_BYTES)
-        fputs (_("  seek_bytes  treat 'seek=N' as a byte count (oflag
only)\n\
+        fputs (_("  seek_bytes (or oseek_bytes)  treat 'seek=N' as a byte
count (oflag only)\n\
 "), stdout);

       {
@@ -1554,9 +1556,11 @@ scanargs (int argc, char *const *argv)
               n_max = SIZE_MAX;
               conversion_blocksize = n;
             }
-          else if (operand_is (name, "skip"))
+          else if (operand_is (name, "skip") ||
+                    operand_is (name, "iseek"))
             skip = n;
-          else if (operand_is (name, "seek"))
+          else if (operand_is (name, "seek") ||
+                   operand_is (name, "oseek"))
             seek = n;
           else if (operand_is (name, "count"))
             count = n;

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