man pages section 3: Extended Library Functions

## logl(3M)

#### Name

log, logf, logl– natural logarithm function

#### Synopsis

```c99 [ flag... ] file... -lm [ library... ]
#include <math.h>

double log(double x);```
`float logf(float x);`
`long double logl(long double x);`

#### Description

These functions compute the natural logarithm of their argument x, loge(x).

#### Return Values

Upon successful completion, log() returns the natural logarithm of x.

If x is ±0, a pole error occurs and log(), logf(), and logl() return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.

For finite values of x that are less than 0, or if x is -Inf, a domain error occurs and a NaN is returned.

If x is NaN, a NaN is returned.

If x is 1, +0 is returned.

If x is +Inf, x is returned.

For exceptional cases, matherr(3M) tabulates the values to be returned by log() as specified by SVID3 and XPG3.

#### Errors

These functions will fail if:

Domain Error

The finite value of x is negative, or x is -Inf.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the invalid floating-point exception is raised.

The log() function sets errno to EDOM if the value of x is negative.

Pole Error

The value of x is 0.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the divide-by-zero floating-point exception is raised.

#### Usage

An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.

An application can also set errno to 0 before calling log(). On return, if errno is non-zero, an error has occurred. The logf() and logl() functions do not set errno.

#### Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE

ATTRIBUTE VALUE

Interface Stability

Standard

MT-Level

MT-Safe