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

## PROLOG         top

```       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.

delim \$\$
```

## NAME         top

```       lgamma, lgammaf, lgammal, signgam — log gamma function
```

## SYNOPSIS         top

```       #include <math.h>

double lgamma(double x);
float lgammaf(float x);
long double lgammal(long double x);
extern int signgam;
```

## DESCRIPTION         top

```       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" " │ Γ ( ^ x ) │\$ where \$Γ ( ^
x )\$ is defined as \$int from 0 to inf e"^" " "{ - t } t"^" " "{ x - 1
} dt\$.  The argument x need not be a non-positive integer (\$Γ( ^ 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         top

```       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         top

```       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         top

```       None.
```

## APPLICATION USAGE         top

```       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         top

```       None.
```

## FUTURE DIRECTIONS         top

```       None.
```

```       exp(3p), feclearexcept(3p), fetestexcept(3p), isnan(3p)

The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment
of Error Conditions for Mathematical Functions, math.h(0p)
```

```       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 .