1.18 expm1, expm1f—exponential minus 1

Synopsis

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

Description
expm1 and expm1f calculate the exponential of x and subtract 1, that is, e raised to the power x minus 1 (where e is the base of the natural system of logarithms, approximately 2.71828). The result is accurate even for small values of x, where using exp(x)-1 would lose many significant digits.

Returns
e raised to the power x, minus 1.

Portability
Neither expm1 nor expm1f is required by ANSI C or by the System V Interface Definition (Issue 2).