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table of contents

LGAMMA(3P) | POSIX Programmer's Manual | LGAMMA(3P) |

# 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.

# NAME¶

lgamma, lgammaf, lgammal, signgam — log gamma function

# SYNOPSIS¶

#include <math.h>

double lgamma(doublex); float lgammaf(floatx); long double lgammal(long doublex); 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.1‐2017 defers to the ISO C standard.

These functions shall compute
$\mathrm{log}\_e\u2502\Gamma \left(x\right)\u2502$
where
$\Gamma \left(x\right)$
is defined as
$\underset{0}{\overset{\infty}{\int}}e^-tt^x-1\mathrm{dt}$
. The argument *x* need not be a non-positive integer (
$\Gamma (x)$
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.*

# EXAMPLES¶

None.

# 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.

# RATIONALE¶

None.

# FUTURE DIRECTIONS¶

None.

# SEE ALSO¶

* exp()*,

*,*

*feclearexcept*()*,*

*fetestexcept*()

*isnan*()The Base Definitions volume of POSIX.1‐2017, *Section
4.20*, *Treatment of Error Conditions for Mathematical Functions*,
**<math.h>**

# COPYRIGHT¶

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

2017 | IEEE/The Open Group |