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Subject: bug#54785: for floating point, printf should use double like in C instead of long double
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The printf command assumes that floating-point arguments are long double
values, which can yield surprising results, while most of the time the
double type is assumed by applications (for instance, this is the case
of XPath).

For instance:

$ zsh -fc '/usr/bin/printf "%a\n" $((43./2**22))'
0xa.c0000000000025cp-20

instead of

0xa.cp-20

(Note that ksh uses long double internally, but does not ensure the
round trip back to long double, so that this is incorrect anyway; see
https://unix.stackexchange.com/questions/422122/why-does-0-1-expand-to-0-10000000000000001-in-zsh
by Stephane Chazelas.)

I suppose that the issue is at the parsing level (after parsing the
value as a double, i.e. rounding to a double, the resulting binary
value can internally be stored either in a double or a long double
without changing its value).

I suggest to parse the argument as a "long double" only if the "L"
length modifier is provided, like in C.

-- 
Vincent Lef�vre <vincent@HIDDEN> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

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To: Vincent Lefevre <vincent@HIDDEN>
Subject: bug#54785: Acknowledgement (for floating point, printf should use
 double like in C instead of long double)
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Subject: bug#54785: for floating point, printf should use double like in C instead of long double
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Vincent Lefevre wrote in <https://bugs.gnu.org/54785>:

> $ zsh -fc '/usr/bin/printf "%a\n" $((43./2**22))'
> 0xa.c0000000000025cp-20
> 
> instead of
> 
> 0xa.cp-20

To summarize, this test case is:

printf '%a\n' 1.0251998901367188e-05

and the problem is that converting 1.0251998901367188e-05 to long double 
prints the too-precise "0xa.c0000000000025cp-20", whereas you want it to 
convert to double (which matches what most other programs do) and to 
print "0xa.cp-20" or equivalent.

> (Note that ksh uses long double internally, but does not ensure the
> round trip back to long double

Yes, ksh messes up here. However, it's more important for Coreutils 
printf to be compatible with the GNU shell, and Bash uses long double:

$ echo $BASH_VERSION
5.1.8(1)-release
$ /usr/bin/printf --version | head -n1
printf (GNU coreutils) 8.32
$ printf '%a\n' 1.0251998901367188e-05
0xa.c0000000000025cp-20
$ /usr/bin/printf '%a\n' 1.0251998901367188e-05
0xa.c0000000000025cp-20


> I suggest to parse the argument as a "long double" only if the "L"
> length modifier is provided, like in C.
Thanks, good idea.

I checked, and this also appears to be a POSIX conformance issue. POSIX 
  says that floating point operands "shall be evaluated as if by the 
strtod() function". This means double, not long double.

Whatever decision we make here, we should be consistent with Bash so 
I'll cc this email to bug-bash.

I propose that we change both coreutils and Bash to use 'double' rather 
than 'long double' here, unless the user specifies the L modifier (e.g., 
"printf '%La\n' ...". I've written up a patch (attached) to Bash 5.2 
alpha to do that. Assuming the Bash maintainer likes this proposal, I 
plan to implement something similar for Coreutils printf.
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Subject: bug#54785: for floating point, printf should use double like in C instead of long double
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On 4/9/22 3:31 PM, Paul Eggert wrote:

>> I suggest to parse the argument as a "long double" only if the "L"
>> length modifier is provided, like in C.
> Thanks, good idea.
> 
> I checked, and this also appears to be a POSIX conformance issue. POSIX 
>   says that floating point operands "shall be evaluated as if by the 
> strtod() function". This means double, not long double.
> 
> Whatever decision we make here, we should be consistent with Bash so I'll 
> cc this email to bug-bash.
> 
> I propose that we change both coreutils and Bash to use 'double' rather 
> than 'long double' here, unless the user specifies the L modifier (e.g., 
> "printf '%La\n' ...". I've written up a patch (attached) to Bash 5.2 alpha 
> to do that. Assuming the Bash maintainer likes this proposal, I plan to 
> implement something similar for Coreutils printf.

It sounds like there are three cases.

1. If the `L' modifier is supplied, as an extension (POSIX doesn't allow
    length modifiers for the printf utility), use long double. This would
    work in both default and posix modes.

2. In posix mode, use strtod() and double.

3. In default mode, use the existing code to get the highest possible
    precision, as the code has done for over 20 years.


-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
		 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@HIDDEN    http://tiswww.cwru.edu/~chet/

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On 2022-04-11 14:52:50 -0400, Chet Ramey wrote:
> It sounds like there are three cases.
> 
> 1. If the `L' modifier is supplied, as an extension (POSIX doesn't allow
>    length modifiers for the printf utility), use long double. This would
>    work in both default and posix modes.
> 
> 2. In posix mode, use strtod() and double.
> 
> 3. In default mode, use the existing code to get the highest possible
>    precision, as the code has done for over 20 years.

Do users really need more precision than double in the context of
bash (or Coreutils) printf?

Moreover, if the argument comes from a double written in decimal
(as often done), using more precision will actually show garbage
that was not present initially. This was how I found this issue
with Coreutils printf (as I expected parsing as a double, this
was very disturbing):

cventin% /usr/bin/printf "%a\n" $((12196067*2**(-22)))
0xb.a18e300000000d1p-2

Note also that the "long double" precision depends on the architecture,
so that this may confuse users who work with different architectures.

-- 
Vincent Lef�vre <vincent@HIDDEN> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

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On 4/11/22 11:52, Chet Ramey wrote:
> On 4/9/22 3:31 PM, Paul Eggert wrote:

> It sounds like there are three cases.
>=20
> 1. If the `L' modifier is supplied, as an extension (POSIX doesn't allo=
w
>  =C2=A0=C2=A0 length modifiers for the printf utility), use long double=
. This would
>  =C2=A0=C2=A0 work in both default and posix modes.
>=20
> 2. In posix mode, use strtod() and double.
>=20
> 3. In default mode, use the existing code to get the highest possible
>  =C2=A0=C2=A0 precision, as the code has done for over 20 years.

That'll fix the POSIX compatibility bug. However, it may be better for=20
Bash to just do (1) if 'L' is supplied and (2) otherwise, even if this=20
is less precise than (3). Doing it this simpler way will likely be more=20
useful for the small number of people who care whether 'printf' uses=20
'double' or 'long double' internally (and nobody else will care).

Doing it this way is what BSD sh does, and it's good to be compatible=20
with BSD sh. Similarly for dash and for other shells.

It's also what other GNU programs do. For example, on x86-64:

   $ awk 'BEGIN {printf "%.100g\n", 0.1}'
   0.1000000000000000055511151231257827021181583404541015625
   $ emacs -batch -eval '(message "%.100g" 0.1)'
   0.1000000000000000055511151231257827021181583404541015625
   $ printf "%.100g\n" 0.1
   0.1000000000000000000013552527156068805425093160010874271392822265625

printf is the outlier here, and although its answer is closer to the=20
mathematical value, that's not as useful as being closer to what most=20
other apps do.

Perhaps it was OK for sh printf to use long double 20 years ago. I even=20
had a hand in implementing that.[1] But nowadays it feels like a=20
misfire. The overwhelming majority of apps that have developed over the=20
past 20 years that use newer languages like JavaScript and Java, do not=20
support 'long double'; when interoperating with these apps, using 'long=20
double' in Bash printf likely causes more trouble than it cures.

[1]:=20
https://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=3D830de2708207=
b7e48464e4778b55e582bac49832

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On 4/25/22 11:03 AM, Paul Eggert wrote:
> On 4/11/22 11:52, Chet Ramey wrote:
>> On 4/9/22 3:31 PM, Paul Eggert wrote:
> 
>> It sounds like there are three cases.
>>
>> 1. If the `L' modifier is supplied, as an extension (POSIX doesn't allow
>>     length modifiers for the printf utility), use long double. This would
>>     work in both default and posix modes.
>>
>> 2. In posix mode, use strtod() and double.
>>
>> 3. In default mode, use the existing code to get the highest possible
>>     precision, as the code has done for over 20 years.
> 
> That'll fix the POSIX compatibility bug. However, it may be better for Bash 
> to just do (1) if 'L' is supplied and (2) otherwise, even if this is less 
> precise than (3). Doing it this simpler way will likely be more useful for 
> the small number of people who care whether 'printf' uses 'double' or 'long 
> double' internally (and nobody else will care).

Thanks for the input.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
		 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@HIDDEN    http://tiswww.cwru.edu/~chet/

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On 4/25/22 11:22, Chet Ramey wrote:
> Thanks for the input.

You're welcome. Whenever you decide what to do about this, could you 
please let us know? I'd like coreutils printf to stay compatible with 
Bash printf. Thanks.

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On Mon, Apr 25, 2022, at 13:06, Paul Eggert wrote:
>
> I'd like coreutils printf to stay compatible with Bash printf. Thanks.
>

Is there any interest/motivation for consistentizing {coreutils printf, bash printf} with glibc printf? There's a minor but notable inconsistency between them for %a format. See

    https://lists.gnu.org/archive/html/coreutils/2022-04/msg00020.html

I asked about this on the coreutils list, but no response.

Thanks,
Glenn

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On 4/25/22 16:50, Glenn Golden wrote:
> 
> 
> On Mon, Apr 25, 2022, at 13:06, Paul Eggert wrote:
>>
>> I'd like coreutils printf to stay compatible with Bash printf. Thanks.
>>
> 
> Is there any interest/motivation for consistentizing {coreutils printf, bash printf} with glibc printf? There's a minor but notable inconsistency between them for %a format. See
> 
>      https://lists.gnu.org/archive/html/coreutils/2022-04/msg00020.html
> 
> I asked about this on the coreutils list, but no response.

To some extent it's the same problem. If Bash and coreutils printf 
change to use 'double', they'll output the same thing that C printf outputs.

But to some extent it's a different problem, as the Bash and coreutils 
printf use glibc printf with long double, and the latter isn't working 
consistently with double. I suppose filing a glibc bug report might 
address this different problem.

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On Mon, Apr 25, 2022, at 19:33, Paul Eggert wrote:
> On 4/25/22 16:50, Glenn Golden wrote:
>> 
>> 
>> On Mon, Apr 25, 2022, at 13:06, Paul Eggert wrote:
>>>
>>> I'd like coreutils printf to stay compatible with Bash printf. Thanks.
>>>
>> 
>> Is there any interest/motivation for consistentizing {coreutils printf, bash printf} with glibc printf? There's a minor but notable inconsistency between them for %a format. See
>> 
>>      https://lists.gnu.org/archive/html/coreutils/2022-04/msg00020.html
>> 
>> I asked about this on the coreutils list, but no response.
>
> To some extent it's the same problem. If Bash and coreutils printf 
> change to use 'double', they'll output the same thing that C printf outputs.
>
> But to some extent it's a different problem, as the Bash and coreutils 
> printf use glibc printf with long double, and the latter isn't working 
> consistently with double. I suppose filing a glibc bug report might 
> address this different problem.
>

Ok, I see what you mean, thanks for the explanation. I'll pose the question (or maybe file a bug report) on the glibc list.

Thanks,
Glenn

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On 4/27/22 05:10, Glenn Golden wrote:
> Ok, I see what you mean, thanks for the explanation. I'll pose the question (or maybe file a bug report) on the glibc list.

By the way I now think I see a reason for why glibc does things the way 
it does: it minimizes output size.

'double' has 53 bits counting the hidden bit, and with 53/4 you have 13 
hex digits plus one leading digit that is either 0 (unnormalized) or 1 
(normalized).

'long double' has 64 bits and with 64/4 you have 16 hex digits, where 
the leading digit is 0-7 (unnormalized), 8-f (normalized).

Any proposal to change 'long double' to always output leading 0 or 1 
needs to deal with the fact that this'd lengthen the output string.

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From: Andreas Schwab <schwab@HIDDEN>
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X-Yow: I have seen these EGG EXTENDERS in my Supermarket..
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Date: Wed, 27 Apr 2022 20:52:26 +0200
In-Reply-To: <89b811d1-8956-e4d4-14d6-3c096c5e5728@HIDDEN> (Paul Eggert's
 message of "Wed, 27 Apr 2022 10:25:23 -0700")
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On Apr 27 2022, Paul Eggert wrote:

> Any proposal to change 'long double' to always output leading 0 or 1 needs
> to deal with the fact that this'd lengthen the output string.

The real reason for not using that format is that 64 bits can be
conveniently formatted without having to shift around the mantissa, and
having to handle the shifted out bits specially.  For IEEE quad with its
112 bits of mantissa and the hidden bit, this format is again more
convenient.

-- 
Andreas Schwab, schwab@HIDDEN
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

Last modified: Wed, 27 Apr 2022 19:00:02 UTC

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