LOG1P

Section: Linux Programmer's Manual (3)
Updated: 2008-08-05
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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(): _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

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.

COLOPHON

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