Updated: April 19, 2023 |

*Compute the base-2 logarithm of a number*

#include <math.h> double log2( doublex); float log2f( floatx); long double log2l( long doublex);

`x`- The number that you want to compute the base-2 log 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 log2(), log2f(), and log2l() functions
compute the base-2 logarithm of `x`:

log_{2}`x`

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

- Call
`feclearexcept(FE_ALL_EXCEPT)`before calling log2(), log2f(), or log2l(). - On return, if
`fetestexcept(FE_ALL_EXCEPT)`is nonzero, then an error has occurred.

The base-2 logarithm of `x`.

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

x is ±0.0 |
-Inf | FE_DIVBYZERO |

1.0 | 0.0 | — |

Less than 0.0 and finite, or -Inf | NaN | FE_INVALID |

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.

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

Cancellation point | No |

Interrupt handler | Yes |

Signal handler | Yes |

Thread | Yes |