Compute the base-2 logarithm of a number
Synopsis:
#include <math.h>
double log2( double x );
float log2f( float x );
long double log2l( long double x );
Arguments:
- x
- The number that you want to compute the base-2 log of.
Library:
- 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.
Description:
The log2(), log2f(), and log2l() functions
compute the base-2 logarithm of x:
log2x
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.
Returns:
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.
Classification:
C11,
POSIX 1003.1
| Safety: |
|
|---|
| Cancellation point |
No |
| Interrupt handler |
Yes |
| Signal handler |
Yes |
| Thread |
Yes |