Updated: April 19, 2023 |

*Compute the exponential of a number, then subtract 1*

#include <math.h> double expm1 ( doublex); float expm1f ( floatx); long double expm1l ( long doublex);

`x`- The number for which you want to calculate the exponential minus one.

- libm
- The general-purpose math library.
- libm-sve
- (QNX Neutrino 7.1 or later) A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

- If you want only selected processes to run with the SVE version, you can include both libraries in your OS image and use the -l m or -l m-sve option to qcc to link explicitly against the appropriate one.
- If you want all processes to use the SVE version, include libm-sve.so in your OS image and set up a symbolic link from libm.so to libm-sve.so. Use the -l m option to qcc to link against the library.

Note:
Compile your program with the -fno-builtin option to prevent the compiler from using a
built-in version of the function.

The expm1(), expm1f(), and expm1l()
functions compute the exponential of `x`, minus 1 (e^{x} - 1).

The value of expm1( `x` ) may be more accurate than
exp( `x` ) - `1.0` for small values of `x`.

The expm1() and
log1p() 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.

To check for error situations, use feclearexcept() and fetestexcept(). For example:

- Call
`feclearexcept(FE_ALL_EXCEPT)`before calling expm1(), expm1f(), or expm1l(). - On return, if
`fetestexcept(FE_ALL_EXCEPT)`is nonzero, then an error has occurred.

The exponential value of `x`, minus 1.

If x is: |
These functions return: | Errors: |
---|---|---|

±0.0 | 0.0, with the same sign as x |
— |

A value that would cause overflow | Inf | FE_OVERFLOW |

-Inf | -1 | — |

Inf | Inf | — |

NaN | NaN | — |

These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.

#include <stdio.h> #include <math.h> #include <fenv.h> #include <stdlib.h> int main( void ) { int except_flags; double a, b; feclearexcept(FE_ALL_EXCEPT); a = 2; b = expm1(a); printf("(e ^ %f) -1 is %f \n", a, b); except_flags = fetestexcept(FE_ALL_EXCEPT); if(except_flags) { /* An error occurred; handle it appropriately. */ } return EXIT_SUCCESS; }

produces the output:

(e ^ 2.000000) -1 is 6.389056

Safety: | |
---|---|

Cancellation point | No |

Interrupt handler | Yes |

Signal handler | Yes |

Thread | Yes |