(logarithm of the) gamma function
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
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 computes the natural logarithm of the Gamma function.
Because of historical variations in behavior across systems, this function 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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