log1p (3)  Linux Manuals
log1p: logarithm of 1 plus argument
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);Link with lm.
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 floatingpoint exception (FE_INVALID) is raised.
 Pole error: x is 1
 errno is set to ERANGE (but see BUGS). A dividebyzero floatingpoint exception (FE_DIVBYZERO) is raised.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).Interface  Attribute  Value 
log1p(), log1pf(), log1pl()  Thread safety  MTSafe 
CONFORMING TO
C99, POSIX.12001, POSIX.12008.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.
COLOPHON
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