# lgamma man page

## Prolog

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

lgamma, lgammaf, lgammal, signgam — log gamma function

## Synopsis

```
#include <math.h>
double lgamma(double
```*x*);
float lgammaf(float *x*);
long double lgammal(long double *x*);
extern int signgam;

## Description

The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1‐2008 defers to the ISO C standard.

These functions shall compute $log\_e\text{}\│\Gamma \left(\text{}x\right)\│$ where $\Gamma \left(\text{}x\right)$ is defined as $\underset{0}{\overset{\infty}{\int}}e\text{^}\text{}-tt\text{^}\text{}x-1dt$. The argument *x* need not be a non-positive integer ($\Gamma \left(\text{}x\right)$ is defined over the reals, except the non-positive integers).

If *x* is NaN, -Inf, or a negative integer, the value of *signgam* is unspecified.

These functions need not be thread-safe.

An application wishing to check for error situations should set *errno* to zero and call *feclearexcept*(FE_ALL_EXCEPT) before calling these functions. On return, if *errno* is non-zero or *fetestexcept*(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.

## Return Value

Upon successful completion, these functions shall return the logarithmic gamma of *x*.

If *x* is a non-positive integer, a pole error shall occur and *lgamma*(), *lgammaf*(), and *lgammal*() shall return +HUGE_VAL, +HUGE_VALF, and +HUGE_VALL, respectively.

If the correct value would cause overflow, a range error shall occur and *lgamma*(), *lgammaf*(), and *lgammal*() shall return ±HUGE_VAL, ±HUGE_VALF, and ±HUGE_VALL (having the same sign as the correct value), respectively.

If *x* is NaN, a NaN shall be returned.

If *x* is 1 or 2, +0 shall be returned.

If *x* is ±Inf, +Inf shall be returned.

## Errors

These functions shall fail if:

- Pole Error
The

*x*argument is a negative integer or zero.If the integer expression (

*math_errhandling*& MATH_ERRNO) is non-zero, then*errno*shall be set to**[ERANGE]**. If the integer expression (*math_errhandling*& MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception shall be raised.- Range Error
The result overflows.

If the integer expression (

*math_errhandling*& MATH_ERRNO) is non-zero, then*errno*shall be set to**[ERANGE]**. If the integer expression (*math_errhandling*& MATH_ERREXCEPT) is non-zero, then the overflow floating-point exception shall be raised.

*The following sections are informative.*

## Application Usage

On error, the expressions (*math_errhandling* & MATH_ERRNO) and (*math_errhandling* & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.

## See Also

*exp()*, *feclearexcept()*, *fetestexcept()*, *isnan()*

The Base Definitions volume of POSIX.1‐2008, *Section 4.19*, *Treatment of Error Conditions for Mathematical Functions*, **<math.h>**

## Copyright

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .