#include <math.h> double expm1(double x);Link with -lm.
float expm1f(float x);
long double expm1l(long double x);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
exp(x) - 1The result is computed in a way that is accurate even if the value of x is near zero---a case where exp(x) - 1 would be inaccurate due to subtraction of two numbers that are nearly equal.
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.
The following errors can occur:
|expm1(), expm1f(), expm1l()||Thread safety||MT-Safe|
For some large positive x values, expm1() raises a bogus invalid floating-point exception in addition to the expected overflow exception, and returns a NaN instead of positive infinity.