RFC: Bug 60749 - Do not need to link against -lm for fabs*()

RFC: Bug 60749 - Do not need to link against -lm for fabs*()

[walter harms]

hi list,
i was investigating this and the good news it that you still need a libm.
On the other side the reporter is right he does not need to specify that.
Obviously this depends on the linker script used.

The math man pages say: "Link with -lm".
Perhaps it is better to say 'You may need to Link with -lm".

any comments on that ?

re,
 wh
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Re: RFC: Bug 60749 - Do not need to link against -lm for fabs*()

[Michael Kerrisk (man-pages)]

Walter,

On Fri, Sep 13, 2013 at 6:46 PM, walter harms <wharms-fPG8STNUNVg@public.gmane.org> wrote:
> hi list,
> i was investigating this and the good news it that you still need a libm.
> On the other side the reporter is right he does not need to specify that.
> Obviously this depends on the linker script used.
>
> The math man pages say: "Link with -lm".
> Perhaps it is better to say 'You may need to Link with -lm".
>
> any comments on that ?


Thanks for looking at this bug. Could you elaborate a little more on
what it/sin't in the linker script that makes the difference?

Thanks,

Michael
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Re: RFC: Bug 60749 - Do not need to link against -lm for fabs*()

[walter harms]

Am 13.09.2013 20:20, schrieb Michael Kerrisk (man-pages):
> Walter,
> 
> On Fri, Sep 13, 2013 at 6:46 PM, walter harms <wharms-fPG8STNUNVg@public.gmane.org> wrote:
>> hi list,
>> i was investigating this and the good news it that you still need a libm.
>> On the other side the reporter is right he does not need to specify that.
>> Obviously this depends on the linker script used.
>>
>> The math man pages say: "Link with -lm".
>> Perhaps it is better to say 'You may need to Link with -lm".
>>
>> any comments on that ?
> 
> 
> Thanks for looking at this bug. Could you elaborate a little more on
> what it/sin't in the linker script that makes the difference?
> 

I have no clue what linker magic is needed.

re,
 wh

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Re: RFC: Bug 60749 - Do not need to link against -lm for fabs*()

[walter harms]

Am 14.09.2013 14:05, schrieb walter harms:
> 
> 
> Am 13.09.2013 20:20, schrieb Michael Kerrisk (man-pages):
>> Walter,
>>
>> On Fri, Sep 13, 2013 at 6:46 PM, walter harms <wharms-fPG8STNUNVg@public.gmane.org> wrote:
>>> hi list,
>>> i was investigating this and the good news it that you still need a libm.
>>> On the other side the reporter is right he does not need to specify that.
>>> Obviously this depends on the linker script used.
>>>
>>> The math man pages say: "Link with -lm".
>>> Perhaps it is better to say 'You may need to Link with -lm".
>>>
>>> any comments on that ?
>>
>>
>> Thanks for looking at this bug. Could you elaborate a little more on
>> what it/sin't in the linker script that makes the difference?
>>
> 
> I have no clue what linker magic is needed.
> 
I know what it going on. the is no linker magic required.
The point is gcc build-ins. Some functions are replaced with
build-ins. That happens e.g. with fabs() what the original poster noted.
Disabling the build-ins with gcc -fno-builtin shows that the -lm is still
a requirement.

next problem, who will do the bugzilla-entry ?

re,
 wh


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Re: RFC: Bug 60749 - Do not need to link against -lm for fabs*()

[Michael Kerrisk (man-pages)]

Walter,

On 09/16/2013 04:57 PM, walter harms wrote:
> 
> 
> Am 14.09.2013 14:05, schrieb walter harms:
>>
>>
>> Am 13.09.2013 20:20, schrieb Michael Kerrisk (man-pages):
>>> Walter,
>>>
>>> On Fri, Sep 13, 2013 at 6:46 PM, walter harms <wharms-fPG8STNUNVg@public.gmane.org> wrote:
>>>> hi list,
>>>> i was investigating this and the good news it that you still need a libm.
>>>> On the other side the reporter is right he does not need to specify that.
>>>> Obviously this depends on the linker script used.
>>>>
>>>> The math man pages say: "Link with -lm".
>>>> Perhaps it is better to say 'You may need to Link with -lm".
>>>>
>>>> any comments on that ?
>>>
>>>
>>> Thanks for looking at this bug. Could you elaborate a little more on
>>> what it/sin't in the linker script that makes the difference?
>>>
>>
>> I have no clue what linker magic is needed.
>>
> I know what it going on. the is no linker magic required.
> The point is gcc build-ins. Some functions are replaced with
> build-ins. That happens e.g. with fabs() what the original poster noted.
> Disabling the build-ins with gcc -fno-builtin shows that the -lm is still
> a requirement.

Thanks! I hadn't thought of that. I see there is a list available in
the gcc documentation at http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

> next problem, who will do the bugzilla-entry ?

I will add something to https://bugzilla.kernel.org/show_bug.cgi?id=60749
Fundamentally, this is a gcc feature, and I don't think there's much that
can be sensibly said about it in man-pages.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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