#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)):

           _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
       lgamma_r(), lgammaf_r(), lgammal_r():
           _BSD_SOURCE || _SVID_SOURCE

       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  func-
       tions  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

       If  x  is a nonpositive integer, a pole error occurs, and the functions
       Pole error: x is a nonpositive integer
              errno  is set to ERANGE (but see BUGS).  A divide-by-zero float-
              ing-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,  POSIX.1-2001,  and
       POSIX.1-2008.  signgam is specified in POSIX.1-2001  and  POSIX.1-2008,
       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.


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                                  2015-04-19                         LGAMMA(3)
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