gamma(3) -- Linux man page
NAMEgamma, gammaf, gammal - (logarithm of the) gamma function
double gamma(double x);
float gammaf(float x);
long double gammal(long double x);
DESCRIPTIONFor the definition of the Gamma function, see tgamma(3).
*BSD version4.4BSD and FreeBSD libm have a gamma() function that computes the Gamma function, as one would expect.
glibc versionGlibc has a gamma() function that is equivalent to lgamma() and computes the natural logarithm of the Gamma function. (This is for compatibility reasons only. Don't use this function.)
HISTORY4.2BSD 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(), and the man page promises
"At some time in the future the name gamma will be rehabilitated and used for the Gamma function"
CONFORMING TO4.2BSD. Compatible with previous mistakes.
SEE ALSOlgamma(3), signgam(3), tgamma(3)