>Submitter-Id: net
>Originator: Uwe Strempel
>Organization:
>Confidential: no
>Synopsis: casting int to double with got wrong result
>Severity: critical
>Priority: medium
>Category: c
>Category: c++
>Class: sw-bug
>Release: 3.3.5 (Debian 1:3.3.5-13) (Debian testing/unstable)
>Environment:
System: Linux lux 2.6.9 #2 Tue Dec 28 22:27:56 CET 2004 i686 GNU/Linux
Architecture: i686
host: i486-pc-linux-gnu
build: i486-pc-linux-gnu
target: i486-pc-linux-gnu
configured with: ../src/configure -v
--enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared
--enable-__cxa_atexit --with-system-zlib --enable-nls
--without-included-gettext --enable-clocale=gnu --enable-debug
--enable-java-gc=boehm --enable-java-awt=xlib --enable-objc-gc i486-linux
>Description:
I've written an macro for log2 calculation to calculate the exponent of
one value.
Than I cast this value to from double to int.
The 2 values double and int differs.
example:
log2(8) should be 3 but after casting to int its 2.
This bug could be reproduced on 2 different linux distributions (Debian
GNU/Linux and Fedora Core 2) with different compiler versions.
>How-To-Repeat:
use this code to reproduce it
--- code ---
#include <math.h>
#include <stdlib.h>
#define log2(val) (log((val))/log(2.0))
int main()
{
int n1=log2(8);
double d1=log2(8);
printf("%d %f\n",n1,d1);
return 0;
}
--- end ---
>Fix:
For workaround I implemented a function for log2.
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