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# log1p(3)

2017-09-15
Aliases: log1pf(3), log1pf(3), log1pf(3), log1pf(3), log1pf(3), log1pf(3), log1pf(3), log1pf(3), log1pl(3), log1pl(3), log1pl(3), log1pl(3), log1pl(3), log1pl(3), log1pl(3), log1pl(3)

### manpages-dev

Manual pages about using GNU/Linux for development

### man-pages

Linux kernel and C library user-space interface documentation

## NAME

log1p, log1pf, log1pl - logarithm of 1 plus argument

## 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():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

log1pf(), log1pl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

## DESCRIPTION

These functions return a value equivalent to
```    log (1 + x)
```
The result 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 errno is set to EDOM (but see BUGS). An invalid floating-point exception (FE_INVALID) is raised. Pole error: x is -1 errno is set to ERANGE (but see BUGS). A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised.

## ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).
 Interface Attribute Value Thread safety MT-Safe

## CONFORMING TO

C99, POSIX.1-2001, POSIX.1-2008.

## BUGS

Before version 2.22, the glibc implementation did not set errno to EDOM when a domain error occurred.
Before version 2.22, the glibc implementation did not set errno to ERANGE when a range error occurred.