lgamma(3M)

DESCRIPTION

lgamma() and gamma() return ln(|Γ(x)|) , where Γ(x) is defined as the integral, as t goes from zero to infinity, of exp(-t) times t to the power (x-1).

The sign of Γ(x) is returned in the external integer signgam. The argument x must not be zero or a negative integer. (lgamma() is defined over the reals excluding the non-positive integers.)

The following C program fragment can be used to calculate Γ(x):

where if y is greater than LN_MAXDOUBLE, as defined in the <values.h> header file, exp() returns a range error (see exp(3M)).

The log gamma function lgamma() is not reentrant because it uses the global variable signgam. The function lgamma_r() is a reentrant version of lgamma() that can be used in multi-threaded applications. The function lgamma_r() returns the sign of Γ(x) in its parameter list, through the pointer sign. The value pointed to by sign is +1 if Γ(x) is positive, -1 if it is negative.

The gamma() function is functionally equivalent to lgamma(), and in the XPG4.2 standard it is marked TO BE WITHDRAWN. A true gamma() function has been approved for inclusion in the C9X draft standard.

The ISO/ANSI C committee has approved the lgamma() function for inclusion in the C9X draft standard.

To use these functions, compile either with the default -Ae option or with the -Aa and -D_HPUX_SOURCE options. Make sure your program includes <math.h>. Link in the math library by specifying -lm on the compiler or linker command line.

RETURN VALUE

If x is a non-positive integer, lgamma() and gamma() return HUGE_VAL.

If the correct value would overflow, lgamma() and gamma() return HUGE_VAL.

If the correct value after rounding would be smaller in magnitude than MINDOUBLE, lgamma() and gamma() return zero.

If x is NaN, lgamma() and gamma() return NaN.

ERRORS

No errors are defined.

WARNINGS

lgamma() and gamma() are unsafe in multi-thread applications. lgamma_r() is MT-Safe and should be used instead.

SEE ALSO

exp(3M), math(5), values(5).

STANDARDS CONFORMANCE

lgamma(): SVID3, XPG4.2

gamma(): SVID2, XPG4.2 (TO BE WITHDRAWN)

signgam: SVID3, XPG4.2