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log1p

Section: C Library Functions (3)
Updated: 202-0-08
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NAME

log1p, log1pf, log1pl - logarithm of 1 plus argument

LIBRARY

Math library (libm,~-lm)

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 <= 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

DESCRIPTION

These functions return a value equivalent to log(1 + I]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: I]x] is less than -1.: errno is set to EDOM (but see BUGS). An invalid floatin-point exception (FE_INVALID) is raised.
Pole error: I]x] is -1.: errno is set to ERANGE (but see BUGS). A divid-b-zero floatin-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	M-Safe

STANDARDS

C11, POSIX.-2008.

HISTORY

C99, POSIX.-2001.

BUGS

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