gamma (3)  Linux Manuals
gamma: (logarithm of the) gamma function
NAME
gamma, gammaf, gammal  (logarithm of the) gamma function
SYNOPSIS
#include <math.h>
double gamma(double x);
float gammaf(float x);
long double gammal(long double x);
Link with lm.
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
gamma():

_XOPEN_SOURCE
 /* Since glibc 2.19: */ _DEFAULT_SOURCE
 /* Glibc versions <= 2.19: */ _BSD_SOURCE  _SVID_SOURCE

_XOPEN_SOURCE >= 600  (_XOPEN_SOURCE && _ISOC99_SOURCE)
 /* Since glibc 2.19: */ _DEFAULT_SOURCE
 /* Glibc versions <= 2.19: */ _BSD_SOURCE  _SVID_SOURCE
DESCRIPTION
These functions are deprecated: instead, use either the tgamma(3) or the lgamma(3) functions, as appropriate.For the definition of the Gamma function, see tgamma(3).
*BSD version
The libm in 4.4BSD and some versions of FreeBSD had a gamma() function that computes the Gamma function, as one would expect.glibc version
Glibc has a gamma() function that is equivalent to lgamma(3) and computes the natural logarithm of the Gamma function.RETURN VALUE
See lgamma(3).ERRORS
See lgamma(3).ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).Interface  Attribute  Value 
gamma(), gammaf(), gammal()  Thread safety  MTUnsafe race:signgam 
CONFORMING TO
Because of historical variations in behavior across systems, this function is not specified in any recent standard. It was documented in SVID 2.NOTES
History
4.2BSD had a gamma() that computed ln(Gamma(x)), leaving the sign of Gamma(x) in the external integer signgam. In 4.3BSD the name was changed to lgamma(3), and the man page promises"At some time in the future the name gamma will be rehabilitated and used for the Gamma function"
This did indeed happen in 4.4BSD, where gamma() computes the Gamma function (with no effect on signgam). However, this came too late, and we now have tgamma(3), the "true gamma" function.
COLOPHON
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