# logb(), logbf(), logbl()

QNX SDP8.0C Library ReferenceAPIDeveloper

*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
- 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:

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

Cancellation point | No |

Signal handler | Yes |

Thread | Yes |

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