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c99 [ flag... ] file... -lm [ library... ] #include <math.h> double expm1(double x);
float expm1f(float x);
long double expm1l(long double x);
These functions compute e^x-1.0.
Upon successful completion, these functions return e^x-1.0.
If x is NaN, a NaN is returned.
If x is \(+-0, \(+-0 is returned.
If x is -Inf, -1 is returned.
If x is +Inf, x is returned.
These functions will fail if:
Range Error
The result overflows. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the overflow floating-point exception is raised.
The value of expm1(x) can be more accurate than exp(x)-1.0 for small values of x.
The expm1() and log1p(3M) functions are useful for financial calculations of ((1+x)^n-1)/x, namely:
expm1(n * log1p(x))\|/\|x
when x is very small (for example, when performing calculations with a small daily interest rate). These functions also simplify writing accurate inverse hyperbolic functions.
An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Standard |
| MT-Level | MT-Safe |
exp(3M), feclearexcept(3M), fetestexcept(3M), ilogb(3M), log1p(3M), math.h(3HEAD), attributes(5), standards(5)