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(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE ||
       _ISOC99_SOURCE; or cc -std=c99
       lgammaf(), lgammal(): _BSD_SOURCE || _SVID_SOURCE ||
       _XOPEN_SOURCE >= 600 || _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 func-
       tions lgamma_r() etc. 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
       returned.

       If x is a non-positive 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 math-
       ematical sign.
              errno  is  set  to ERANGE.  An overflow floating-point exception
              (FE_OVERFLOW) is raised.

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.

BUGS
       For a pole error, errno is set to EDOM; POSIX.1 says it should  be  set
       to ERANGE.

SEE ALSO
       tgamma(3)

COLOPHON
       This  page  is  part of release 3.23 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.