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);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
or cc -std=c99
lgammaf(), lgammal():
or cc -std=c99
lgamma_r(), lgammaf_r(), lgammal_r():
extern int signgam;
Link with −lm.


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 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.


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 divide-by-zero floating-point exception (FE_DIVBYZERO) is raised.

Range error: result overflow

errno is set to ERANGE. An overflow floating-point exception (FE_OVERFLOW) is raised.


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 nonstandard, but present on several other systems.


In glibc 2.9 and earlier, when a pole error occurs, errno is set to EDOM; instead of the POSIX-mandated ERANGE. Since version 2.10, glibc does the right thing.




This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright 2002 Walter Harms (
and Copyright 2008, Linux Foundation, written by Michael Kerrisk

Distributed under GPL

based on glibc infopages