man pages section 3: Extended Library Functions

## log1p(3M)

#### Name

log1p, log1pf, log1pl– compute natural logarithm

#### Synopsis

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

double log1p(double x);```
`float log1pf(float x);`
`long double log1pl(long double x);`

#### Description

These functions compute loge(1.0 + x).

#### Return Values

Upon successful completion, these functions return the natural logarithm of 1.0 + x.

If x is -1, a pole error occurs and log1p(), log1pf(), and log1pl() return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.

For finite values of x that are less than -1, 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 ±0 or +Inf, x is returned.

For exceptional cases, matherr(3M) tabulates the values to be returned by log1p() as specified by SVID3 and XPG3.

#### Errors

These functions will fail if:

Domain Error

The finite value of x is less than -1, or x is -Inf.

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

The log1p() function sets errno to EDOM if the value of x is less than -1.

Pole Error

The value of x is -1.

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 log1p(). On return, if errno is non-zero, an error has occurred. The log1pf() and log1pl() 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