logb() vs floor(log2()) (was: Man page issues: logb, significand, cbrt, log2, log10, exp10)

logb() vs floor(log2()) (was: Man page issues: logb, significand, cbrt, log2, log10, exp10)

[Alejandro Colomar]

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Hi Morten,

On Thu, Feb 29, 2024 at 07:28:10PM -0500, Morten Welinder wrote:
> I came across some minor issues in some math man pages.
> 
> M.
> 
> 
> 
> logb:
> The formula "floor(log2(x))" should be "floor(log2(fabs(x)))".  (Or
> ...abs(...) if it's meant to be math and not C.)

Confirmed.  This is a bug in glibc too, BTW.  That text seems to be
copied from their manual.

ISO C says this function is specified in ANSI/IEEE 854, but I don't have
access to that document, so I'm not sure what's the specification of the
function.  I'm not sure if it should fail for negative values (like
log2(3)) or not; although the standard mentions the behavior for
negative infinity, so it probably is specified to work for negative
values too.

So, the behavior of the function seems to be correct, and it's just the
manual that needs to be fixed.

I've CCed glibc, in case they want to comment.  But yeah, your
suggestion seems correct.

Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.

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Re: logb() vs floor(log2())

[Adhemerval Zanella Netto]

On 04/03/24 14:52, Alejandro Colomar wrote:
> Hi Morten,
> 
> On Thu, Feb 29, 2024 at 07:28:10PM -0500, Morten Welinder wrote:
>> I came across some minor issues in some math man pages.
>>
>> M.
>>
>>
>>
>> logb:
>> The formula "floor(log2(x))" should be "floor(log2(fabs(x)))".  (Or
>> ...abs(...) if it's meant to be math and not C.)
> 
> Confirmed.  This is a bug in glibc too, BTW.  That text seems to be
> copied from their manual.
> 
> ISO C says this function is specified in ANSI/IEEE 854, but I don't have
> access to that document, so I'm not sure what's the specification of the
> function.  I'm not sure if it should fail for negative values (like
> log2(3)) or not; although the standard mentions the behavior for
> negative infinity, so it probably is specified to work for negative
> values too.
> 
> So, the behavior of the function seems to be correct, and it's just the
> manual that needs to be fixed.
> 
> I've CCed glibc, in case they want to comment.  But yeah, your
> suggestion seems correct.

It does seems to be an error on manual, could you send a patch to fix it?

Re: logb() vs floor(log2())

[Alejandro Colomar]

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Hi Adhemerval,

On Mon, Mar 04, 2024 at 03:47:26PM -0300, Adhemerval Zanella Netto wrote:
> It does seems to be an error on manual, could you send a patch to fix it?

Sure!

Have a lovely night!
Alex

-- 
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.

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Re: logb() vs floor(log2()) (was: Man page issues: logb, significand, cbrt, log2, log10, exp10)

[Alejandro Colomar]

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Hi Morten,

On Mon, Mar 04, 2024 at 06:52:10PM +0100, Alejandro Colomar wrote:
> > logb:
> > The formula "floor(log2(x))" should be "floor(log2(fabs(x)))".  (Or
> > ...abs(...) if it's meant to be math and not C.)

I've applied thge following patch:

commit 1d83ce827aac984a26430b4f6107182f4b076874 (HEAD -> contrib, alx/contrib)
Author: Alejandro Colomar <alx@kernel.org>
Date:   Tue Mar 5 00:20:09 2024 +0100

    logb.3: logb(x) is floor(log2(fabs(x)))
    
    log2(3) doesn't accept negative input, but it seems logb(3) does accept
    it.
    
    Link: <https://lore.kernel.org/linux-man/ZeYKUOKYS7G90SaV@debian/T/#u>
    Reported-by: Morten Welinder <mwelinder@gmail.com>
    Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
    Signed-off-by: Alejandro Colomar <alx@kernel.org>

diff --git a/man3/logb.3 b/man3/logb.3
index 7cbb2470a..7a1ad2f4a 100644
--- a/man3/logb.3
+++ b/man3/logb.3
@@ -58,7 +58,7 @@ .SH DESCRIPTION
 is 2,
 .BI logb( x )
 is equal to
-.BI floor(log2( x ))\fR,
+.BI floor(log2(fabs( x )))\f[R],\f[]
 except that it is probably faster.
 .P
 If



I'll push it tomorrow.

Have a lovely night!
Alex

-- 
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.

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