Linux manual pages Section 3  

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);
 
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
log1p():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE;
or cc -std=c99
log1pf(), log1pl():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE;
or cc -std=c99
[Note] Note

Link with −lm.

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.

SEE ALSO

exp(3), expm1(3), log(3)

COLOPHON

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