lgamma(3)		    Library Functions Manual		       lgamma(3)

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

LIBRARY
     Math library (libm, -lm)

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;

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     lgamma():
	 _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE
	     || /* Since glibc 2.19: */ _DEFAULT_SOURCE
	     || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

     lgammaf(), lgammal():
	 _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
	     || /* Since glibc 2.19: */ _DEFAULT_SOURCE
	     || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

     lgamma_r(), lgammaf_r(), lgammal_r():
	 /* Since glibc 2.19: */ _DEFAULT_SOURCE
	     || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

     signgam:
	 _XOPEN_SOURCE
	     || /* Since glibc 2.19: */ _DEFAULT_SOURCE
	     || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

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

     The lgamma(), lgammaf(), and lgammal() functions return the  natural  loga-
     rithm  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 nega-
     tive.

     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.

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  re-
     turned.

     If	 x  is a nonpositive integer, a pole error occurs, and the functions re-
     turn +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 mathemat-
     ical 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  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.

STANDARDS
     lgamma()
     lgammaf()
     lgammal()
	    C11, POSIX.1-2008.

     signgam
	    POSIX.1-2008.

     lgamma_r()
     lgammaf_r()
     lgammal_r()
	    None.

HISTORY
     lgamma()
     lgammaf()
     lgammal()
	    C99, POSIX.1-2001.

     signgam
	    POSIX.1-2001.

     lgamma_r()
     lgammaf_r()
     lgammal_r()
	    None.

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

SEE ALSO
     tgamma(3)

Linux man-pages 6.18		   2026-02-08			       lgamma(3)