logb(), logbf(), logbl()

Compute the radix-independent exponent

Synopsis:

#include <math.h>

double logb ( double x );

float logbf ( float x );

long double logbl ( long double x );

Arguments:

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

Returns:

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.

Examples:

#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)

Classification:

C11, POSIX 1003.1

Safety:
Cancellation point	No
Interrupt handler	Yes
Signal handler	Yes
Thread	Yes