lgamma, lgammaf, lgammal, lgamma_r, lgammaf_r, lgammal_r, signgam - log gamma function
#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;Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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 thread-safe,
the functions lgamma_r(), lgammaf_r(),
and lgammal_r() have been introduced; they return the
sign via the argument signp.
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.
tgamma(3)