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The signgam value is incorrect for lgamma(-0.), at least on x86 and PowerPC (and various glibc versions): it is 1, whereas it should be -1 (since tgamma(-0.) is -inf). For instance, sysdeps/ieee754/dbl-64/e_lgamma_r.c contains: if((ix|lx)==0) return one/fabs(x); i.e. it lets signgam to the value 1, even when x is -0. Other files contains the same error. Moreover, on values where tgamma is NaN, the glibc sets signgam to 1. But is this correct? The POSIX standard is silent on this point, but as the sign isn't defined, the value of signgam could be left unchanged (and I don't think that taking into account the sign of the NaN returned by tgamma doesn't really make much sense). In any case, this should be documented.
For lgamma (-0.) according to http://www.opengroup.org/onlinepubs/009695399/functions/lgamma.html If x is a non-positive integer, a pole error should occur and +HUGE_VAL be returned, the same as for lgamma (0.). That's different from tgamma http://www.opengroup.org/onlinepubs/009695399/functions/tgamma.html which has: If x is +-0, a pole error shall occur, and tgamma(), tgammaf(), and tgammal() shall return +-HUGE_VAL, +-HUGE_VALF, and +-HUGE_VALL, respectively. As for the second part, I believe the current behavior is ok, NaNs don't have a sign and thus the content of signgam after lgamma(NAN) is IMNSHO undefined.
(In reply to comment #1) > For lgamma (-0.) according to > http://www.opengroup.org/onlinepubs/009695399/functions/lgamma.html > If x is a non-positive integer, a pole error should occur and +HUGE_VAL be > returned, the same as for lgamma (0.). Yes, the return value is correct. But the signgam value (which is the sign of the Gamma function, i.e. tgamma) is not for lgamma(-0.).
FYI, Solaris for: #define _XOPEN_SOURCE 600 #include <math.h> #include <stdio.h> double zero = 0.0; double mzero; int main (void) { double d; mzero = copysign (zero, -1.0); d = lgamma (zero); printf ("%g %d\n", d, signgam); d = lgamma (mzero); printf ("%g %d\n", d, signgam); return 0; } also prints Inf 1 twice.
But both HP-UX and OSF1 return the correct sign -1 (though OSF1 has other bugs).
I've changed the code to store -1.