GAMMA(3) NEWLIB GAMMA(3)NAME
1.26 `gamma', `gammaf', `lgamma', `lgammaf', `gamma_r', `gammaf_r',
`lgamma_r', `lgammaf_r', `tgamma', and `tgammaf'-logarithmic and plain
gamma functions
SYNOPSIS
#include <math.h>
double gamma(double X);
float gammaf(float X);
double lgamma(double X);
float lgammaf(float X);
double gamma_r(double X, int *SIGNGAMP);
float gammaf_r(float X, int *SIGNGAMP);
double lgamma_r(double X, int *SIGNGAMP);
float lgammaf_r(float X, int *SIGNGAMP);
double tgamma(double X);
float tgammaf(float X);
DESCRIPTION
`gamma' calculates the natural logarithm of the gamma function of X.
The gamma function (`exp(gamma(X))') is a generalization of factorial,
and retains the property that `exp(gamma(N))' is equivalent to
`N*exp(gamma(N-1))'. Accordingly, the results of the gamma function
itself grow very quickly. `gamma' is defined as the natural log of the
gamma function, rather than the gamma function itself, to extend the
useful range of results representable.
The sign of the result is returned in the global variable `signgam',
which is declared in math.h.
`gammaf' performs the same calculation as `gamma', but uses and
returns `float' values.
`lgamma' and `lgammaf' are alternate names for `gamma' and `gammaf'.
The use of `lgamma' instead of `gamma' is a reminder that these func‐
tions compute the log of the gamma function, rather than the gamma
function itself.
The functions `gamma_r', `gammaf_r', `lgamma_r', and `lgammaf_r' are
just like `gamma', `gammaf', `lgamma', and `lgammaf', respectively, but
take an additional argument. This additional argument is a pointer to
an integer. This additional argument is used to return the sign of the
result, and the global variable `signgam' is not used. These functions
may be used for reentrant calls (but they will still set the global
variable `errno' if an error occurs).
`tgamma' and `tgammaf' are the "true gamma" functions, returning the
gamma function of X-without a logarithm. (They are apparently so named
because of the prior existence of the old, poorly-named `gamma' func‐
tions which returned the log of gamma up through BSD 4.2.)
RETURNS
Normally, the computed result is returned.
When X is a nonpositive integer, `gamma' returns `HUGE_VAL' and
`errno' is set to `EDOM'. If the result overflows, `gamma' returns
`HUGE_VAL' and `errno' is set to `ERANGE'.
You can modify this error treatment using `matherr'.
PORTABILITY
Neither `gamma' nor `gammaf' is ANSI C. It is better not to use either
of these; use `lgamma' or `tgamma' instead. `lgamma', `lgammaf',
`tgamma', and `tgammaf' are nominally C standard in terms of the base
return values, although the `matherr' error-handling is not standard,
nor is the SIGNGAM global for `lgamma'.
SEE ALSO
gamma is part of the library. The full documentation for is maintained
as a Texinfo manual. If info and are properly installed at your site,
the command
info
will give you access to the complete manual.
NEWLIB April 2010 GAMMA(3)
