DESCRIPTION

For the definition of the Gamma function, see tgamma(3).

The lgamma() function returns the natural logarithm of the absolute value of the Gamma function. The sign of the Gamma function is returned in the external integer signgam declared in <math.h> It is 1 when the Gamma function is positive or zero, −1 when it is negative.

Since using a constant location signgam is not thread-safe, the functions lgamma_r() etc. have been introduced; they return this sign via the parameter signp.

For non-positive integer values of x, lgamma() returns HUGE_VAL, sets errno to ERANGE and raises the zero divide exception. (Similarly, lgammaf() returns HUGE_VALF and lgammal() returns HUGE_VALL.)

ERRORS

In order to check for errors, set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is nonzero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is nonzero, an error has occurred.

A range error occurs if x is too large. A pole error occurs if x is a negative integer or zero.

CONFORMING TO

The lgamma() functions are specified in C99 and POSIX.1-2001. signgam is specified in POSIX.1-2001, but not in C99. The lgamma_r() functions are non-standard, but present on several other systems.

SEE ALSO

tgamma(3)