EXPM1(3M) Mathematical Library Functions EXPM1(3M)

# NAME

expm1, expm1f, expm1l - compute exponential function

# SYNOPSIS

```c99 [ flag... ] file... -lm [ library... ]
#include <math.h>
double expm1(double x);
```

```float expm1f(float x);
```

```long double expm1l(long double x);
```

# DESCRIPTION

These functions compute e^x−1.0.

# RETURN VALUES

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.

# ERRORS

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.

# USAGE

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.

# ATTRIBUTES

See attributes(7) for descriptions of the following attributes:

 ATTRIBUTE TYPE ATTRIBUTE VALUE Interface Stability Standard MT-Level MT-Safe