#include <math.h>float expm1f(float arg);double expm1(double arg); long double expm1l(long double arg);
expm1( ), expm1f( ), and expm1l( ) were added by C99.
The expm1 family of functions returns the natural logarithm e raised to the arg power, minus 1. That is, it returns earg – 1.
Related functions are exp( ) and log( ).