# Manual Reference Pages  - LOG1P (3)

### NAME

log1p, log1pf, log1pl - logarithm of 1 plus argument

Synopsis
Description
Return Value
Errors
Colophon

### SYNOPSIS

```
#include <math.h>

double log1p(double x);

float log1pf(float x);

long double log1pl(long double x);

```

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

log1p():

_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99

log1pf(), log1pl():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99

### DESCRIPTION

log1p(x) returns a value equivalent to
```
log (1 + x)

```
It is computed in a way that is accurate even if the value of x is near zero.

### RETURN VALUE

On success, these functions return the natural logarithm of (1 + x).

If x is a NaN, a NaN is returned.

If x is positive infinity, positive infinity is returned.

If x is -1, a pole error occurs, and the functions return -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.

If x is less than -1 (including negative infinity), a domain error occurs, and a NaN (not a number) is returned.

### ERRORS

See math_error(7) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:
Domain error: x is less than -1
An invalid floating-point exception (FE_INVALID) is raised.
Pole error: x is -1
A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised.
These functions do not set errno.

### CONFORMING TO

C99, POSIX.1-2001.