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():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 500 || /* Since glibc 2.19:
*/ _DEFAULT_SOURCE || /* Glibc <= 2.19:
*/ _BSD_SOURCE || _SVID_SOURCE
log1pf(), log1pl():
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19:
*/ _DEFAULT_SOURCE || /* Glibc <= 2.19:
*/ _BSD_SOURCE || _SVID_SOURCE
[Note] Note

Link with −lm.

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
log1p(), log1pf(), log1pl() 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.

SEE ALSO

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

COLOPHON

This page is part of release 5.11 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.


  Copyright 1995 Jim Van Zandt <jrvvanzandt.mv.com>
and Copyright 2008, Linux Foundation, written by Michael Kerrisk
    <mtk.manpagesgmail.com>

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Modified 2002-07-27 by Walter Harms
(walter.harmsinformatik.uni-oldenburg.de)