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)):
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600
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).
The libm in 4.4BSD and some versions of FreeBSD had a gamma() function
that computes the Gamma function, as one would expect.
Glibc has a gamma() function that is equivalent to lgamma(3) and com-
putes the natural logarithm of the Gamma function.
Because of historical variations in behavior across systems, this func-
tion is not specified in any standard.
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.
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