LGAMMA(3)

NAME
lgamma, lgammaf, lgammal, lgamma_r, lgammaf_r, lgammal_r, signgam - log gamma function

SYNOPSIS
#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(), lgammaf(), lgammal(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE || _ISOC99_SOURCE; or cc -std=c99
lgamma_r(), lgammaf_r(), lgammal_r(): _BSD_SOURCE || _SVID_SOURCE
signgam: _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE

DESCRIPTION
For the definition of the Gamma function, see tgamma(3).

The lgamma() function returns 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() etc. have been introduced; they return this sign via the parameter signp.

For non-positive integer values of x, lgamma() returns HUGE_VAL, sets errno to ERANGE and raises the zero divide exception. (Similarly, lgammaf() returns HUGE_VALF and lgammal() returns HUGE_VALL.)

ERRORS
In order to check for errors, set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.

A range error occurs if x is too large. A pole error occurs if x is a negative integer or zero.

CONFORMING TO
The lgamma() functions are specified in C99 and POSIX.1-2001. signgam is specified in POSIX.1-2001, but not in C99. The lgamma_r() functions are non-standard, but present on several other systems.

SEE ALSO
tgamma(3)