# 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

Use the -l m option to qcc to link against this library.

## Description:

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

logr |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:

Safety:
Cancellation point No
Interrupt handler No
Signal handler No