lgamma(3M)
NAME
lgamma, lgamma_r, gamma, gamma_r − log gamma function
SYNOPSIS
cc [ flag ... ] file ... −lm [ library ... ]
#include <math.h>
extern int signgam;
double lgamma(double x);
double lgamma_r(double x, int ∗signgamp);
MT-LEVEL
See the NOTES section of this page.
DESCRIPTION
Both lgamma() and lgamma_r() return
ln|Γ(x)|
where
Γ(x) = ∫0s10∞tx−1e−tdt
for x > 0 and
Γ(x) = π/(Γ(1−x)sin(πx))
for x < 1.
lgamma() uses the external integer signgam to return the sign of Γ(x) while lgamma_r() uses the user-allocated space addressed by signgamp.
IDIOSYNCRASIES
In the case of lgamma(), do not use the expression signgam∗exp(lgamma(x)) to compute ‘g := Γ(x)’. Instead compute lgamma() first:
lg = lgamma(x); g = signgam∗exp(lg);
only after lgamma() has returned can signgam be correct. Note that Γ(x) must overflow when x is large enough, underflow when −x is large enough, and generate a division by 0 exception at the singularities x a nonpositive integer.
RETURN VALUES
For exceptional cases, matherr(3M) tabulates the values to be returned as dictated by various Standards.
SEE ALSO
NOTES
Although lgamma_r() is not mentioned by POSIX.4a Draft 6, it was added to complete the functionality provided by similar thread-safe functions. This interface is subject to change to be compatible with the "spirit" of POSIX.4a when it is approved as a standard.
When compiling multi-thread applications, the _REENTRANT flag must be defined on the compile line. This flag should only be used in multi-thread applications.
lgamma() is unsafe in multithreaded applications. lgamma_r() should be used instead. should be used instead.
SunOS 5.5 — Last change: 18 Jul 1994