lgamma_r (3)
Leading comments
Copyright 2002 Walter Harms (walter.harms@informatik.unioldenburg.de) and Copyright 2008, Linux Foundation, written by Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(GPL_NOVERSION_ONELINE) Distributed under GPL %%%LICENSE_END based on glibc infopages
NAME
lgamma, lgammaf, lgammal, lgamma_r, lgammaf_r, lgammal_r, signgam  log gamma functionSYNOPSIS
#include <math.h> double lgamma(double x);
float lgammaf(float x);
long double lgammal(long double x); double lgamma_r(double x, int *signp);
float lgammaf_r(float x, int *signp);
long double lgammal_r(long double x, int *signp); extern int signgam;
Link with lm.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
lgamma():

_BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE  _ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or cc std=c99
lgammaf(), lgammal():

_BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE >= 600  _ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or cc std=c99
 _BSD_SOURCE  _SVID_SOURCE
 _BSD_SOURCE  _SVID_SOURCE  _XOPEN_SOURCE
DESCRIPTION
For the definition of the Gamma function, see tgamma(3).The lgamma(), lgammaf(), and lgammal() functions return the natural logarithm of the absolute value of the Gamma function. The sign of the Gamma function is returned in the external integer signgam declared in <math.h>. It is 1 when the Gamma function is positive or zero, 1 when it is negative.
Since using a constant location signgam is not threadsafe, the functions lgamma_r(), lgammaf_r(), and lgammal_r() have been introduced; they return the sign via the argument signp.
RETURN VALUE
On success, these functions return the natural logarithm of Gamma(x).If x is a NaN, a NaN is returned.
If x is 1 or 2, +0 is returned.
If x is positive infinity or negative infinity, positive infinity is returned.
If x is a nonpositive integer, a pole error occurs, and the functions return +HUGE_VAL, +HUGE_VALF, or +HUGE_VALL, respectively.
If the result overflows, a range error occurs, and the functions return HUGE_VAL, HUGE_VALF, or HUGE_VALL, respectively, with the correct mathematical sign.
ERRORS
See math_error(7) for information on how to determine whether an error has occurred when calling these functions.The following errors can occur:
 Pole error: x is a nonpositive integer
 errno is set to ERANGE (but see BUGS). A dividebyzero floatingpoint exception (FE_DIVBYZERO) is raised.
 Range error: result overflow
 errno is set to ERANGE. An overflow floatingpoint exception (FE_OVERFLOW) is raised.