Updated: October 26, 2022 |

*Compute the radix-independent exponent*

#include <math.h> double logb ( doublex); float logbf ( floatx); long double logbl ( long doublex);

`x`- The number that you want to compute the radix-independent exponent of.

- 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 logb(), logbf(), and logbl() functions
compute the exponent part of `x`, which is the
integral part of:

log_{r} |`x`|

as a signed floating point value, for nonzero finite `x`, where
`r` is the radix of the machine's floating point arithmetic.

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

- Call
`feclearexcept(FE_ALL_EXCEPT)`before calling logb(), logbf(), or logbl(). - On return, if
`fetestexcept(FE_ALL_EXCEPT)`is nonzero, then an error has occurred.

The binary exponent of `x`, a signed integer converted to double-precision
floating-point.

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

x is ±0.0 |
-Inf | FE_DIVBYZERO |

±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 <inttypes.h> #include <math.h> #include <fenv.h> #include <stdlib.h> int main( void ) { int except_flags; double a, b; a = 0.5; feclearexcept(FE_ALL_EXCEPT); b = logb(a); except_flags = fetestexcept(FE_ALL_EXCEPT); if(except_flags) { /* An error occurred; handle it appropriately. */ } printf("logb(%f) = %f (%f = 2^%f) \n", a, b, a, b); return EXIT_SUCCESS; }

produces the output:

logb(0.500000) = -1.000000 (0.500000 = 2^-1.000000)

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

Cancellation point | No |

Interrupt handler | Yes |

Signal handler | Yes |

Thread | Yes |