expm1(3) -- Linux man page
expm1, expm1f, expm1l - exponential minus 1
double expm1(double x);
float expm1f(float x);
long double expm1l(long double x);
returns a value equivalent to `exp (x) - 1'. It 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.
The float and the long double variants are C99 requirements.