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Forum Topic - libm error?:

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Thomas Buschmann

06/16/2008 11:09 AM

post9192

Hi,

I believe there might be a bug in libm. "pow(x,2.0)" gives incorrect results for very small x.
The following program gives incorrect results for double pow(double,double), but correct 
results for pow(double,int). 

#include <cmath>
#include <iostream>
using namespace std;

int main(int argc,char**argv)
{
   double dz=+1e-170;
   cout<<"dz= "<<dz<<endl;
   cout<<"pow(dz,2.0)= "<<pow(dz,2.0)<<endl;
   cout<<"pow(dz,2)= "<<pow(dz,2)<<endl;
   return EXIT_SUCCESS;
}

If compiled with " QCC -o foo foo.cpp -lm"  the output is:
dz= 1e-170
pow(dz,2.0)= inf
pow(dz,2)= 0

Am I doing something wrong and if not, is there a known workaround?

Thanks,

Thomas Buschmann

Ryan Mansfield(deleted)

06/16/2008 11:21 AM

post9197

Re: libm error?

I believe this problem has already been fixed in the 6.4 libm. You can download an updated libm from the OS project at:

http://community.qnx.com/sf/frs/do/viewRelease/projects.core_os/frs.qnx_neutrino_core_os_0.6_4_m7

Regards,

Ryan Mansfield

Thomas Buschmann

06/18/2008 2:15 PM

post9403

Re: libm error?

Thanks a lot, I'll try that!
Sincerely,
Thomas Buschmann

Thomas Allen

08/12/2008 5:42 PM

post11710

libm error- very serious

Hi,

I just spent the last 3 days tracking down an issue which turned out to be this exact problem.  pow(x,2.0f) was 
returning inf.

Downloading the new libm solved this problem.  I have a few questions

1) how do I identify the version of the libm libraries to find out if this issue is lingering elsewhere.

2) which versions of the libm library have this issue.

3) does QNX not have some mechanism to notify clients when something as critical as the math library being broken is 
identified.  We are building mission critical applications and I only ran into this issue by fluke. I would have 
expected something like a RECALL being generated for something this severe.

4) do you have a list of other critical issues such as this which exist so I know what else to look out for instead of 
wasting the last 3 days like I just did.

Regards
Thom Allen

Ryan Mansfield(deleted)

08/12/2008 6:26 PM

post11711

Re: libm error- very serious

Thomas Allen wrote:
> Hi,
> 
> I just spent the last 3 days tracking down an issue which turned out to be this exact problem.  pow(x,2.0f) was 
returning inf.
> 
> Downloading the new libm solved this problem.  I have a few questions

Someone in the OS forum should be able to provide you with the 
information about which versions are affected and if there are any other 
known issues with libm.

Regards,

Ryan Mansfield

Rafael Antonio Longo(deleted)

09/05/2008 1:35 PM

post12956

Re: libm error- very serious

Seems to only works for me if I remove the 

libm.so, 
libm.so.2 and
libmS.a

and keep the libm.a of release 6.4.
Anybody knows why? The feeling I had was that it was actually first trying link to libm.so which is would then be still 
faulty and if not found link to the archive file (libm.a)
Does it make sense to anybody?

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