In version 2.5 of glibc, on SuSE Linux 10.2, it appears that the libm functions atanh, log and log1p (and possibly others) are inconsistent in their handling of singularities: log(0.0) returns -Infinity and sets errno = ERANGE log1p(-1.0) returns -Infinity and doesn't set errno atanh(-1.0) returns -Infinity and sets errno = EDOM. According to the C99 standard (appendix F), and IEEE-754r, all of these operations should behave in the same way---they should all return -Infinity and raise the "divide-by-zero" exception. This makes sense mathematically: all three function calls correspond to singularities of log type. Admittedly, the C99 standard has little to say about errno, but I don't understand why these three calls behave differently with respect to setting of errno.
atanh was fixed by: 2009-04-26 Ulrich Drepper <drepper@redhat.com> * sysdeps/ieee754/k_standard.c (__kernel_standard): Pole errors in atanh should set ERANGE. All functions also raise divide-by-zero. So the only bug left here is for log1p, which should set errno to ERANGE. That's one particular case of the problems for log1p detailed in bug 6792. *** This bug has been marked as a duplicate of bug 6792 ***