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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)