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expm1

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

expm1, expm1f, expm1l - exponential minus 1

LIBRARY

Math library (libm,~-lm)

SYNOPSIS

#include <math.h>
double expm1(double x);
float expm1f(float x);
long double expm1l(long double x);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)): expm1():

    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
        || _XOPEN_SOURCE >= 500
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

expm1f(), expm1l():

    _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

    exp(x) - 1

The result is computed in a way that is accurate even if the value of x is near zero[em]a case where exp(x) - 1 would be inaccurate due to subtraction of two numbers that are nearly equal.

RETURN VALUE

On success, these functions return exp(x) - 1. If x is a NaN, a NaN is returned. If x is +0 (-0), +0 (-0) is returned. If x is positive infinity, positive infinity is returned. If x is negative infinity, -1 is returned. If the result overflows, a range error occurs, and the functions return -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.

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:

Range error, overflow: errno is set to ERANGE (but see BUGS). An overflow floatin-point exception (FE_OVERFLOW) is raised.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
expm1(), expm1f(), expm1l()	Thread safety	M-Safe

STANDARDS

C11, POSIX.-2008.

HISTORY

C99, POSIX.-2001. BSD.

BUGS

Before glibc 2.17, on certain architectures (e.g., x86, but not x86_64) expm1() raised a bogus underflow floatin-point exception for some large negative x values (where the function result approaches -1). Before approximately glibc 2.11, expm1() raised a bogus invalid floatin-point exception in addition to the expected overflow exception, and returned a NaN instead of positive infinity, for some large positive x values. Before glibc 2.11, the glibc implementation did not set errno to ERANGE when a range error occurred.