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[Bug math/13629] New: clog() returns wrong values around z=1
- From: "fxcoudert at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sources dot redhat dot com
- Date: Fri, 27 Jan 2012 14:11:58 +0000
- Subject: [Bug math/13629] New: clog() returns wrong values around z=1
- Auto-submitted: auto-generated
http://sourceware.org/bugzilla/show_bug.cgi?id=13629
Bug #: 13629
Summary: clog() returns wrong values around z=1
Product: glibc
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: math
AssignedTo: aj@suse.de
ReportedBy: fxcoudert@gcc.gnu.org
Classification: Unclassified
On the example below, clog() is called on value 1+epsilon*I. The result (first
output line) is pretty different from the expected result, calculated in the
second output line as a second-order series expansion.
I could reproduce it with glibc versions 2.5, 2.11.2, and 2.13, on both i686
and x86_64 systems (and many compilers).
$ cat a.c
#include <math.h>
#include <complex.h>
#include <stdio.h>
int main ()
{
complex double z, fz,gz;
z = 1.0f + 0.00000001f*I;
fz = clog(z);
gz = (z-1) - (z-1)*(z-1)/2;
printf (" %.15e + %.15ei\n", creal(fz), cimag(fz));
printf (" %.15e + %.15ei\n", creal(gz), cimag(gz));
return 0;
}
$ gcc a.c -lm -O0 -W -Wall -std=c99 && ./a.out
0.000000000000000e+00 + 1.000000000000000e-08i
5.000000000000001e-17 + 1.000000000000000e-08i
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