logbd32(), logbd64(), logbd128() — Unbiased exponent

Standards

Standards / Extensions	C or C++	Dependencies
C/C++ DFP	both	z/OS® V1.8

Format

#define __STDC_WANT_DEC_FP__
#include <math.h>

_Decimal32  logbd32(_Decimal32 x); 
_Decimal64  logbd64(_Decimal64 x);
_Decimal128 logbd128(_Decimal128 x);

_Decimal32  logb(_Decimal32 x);     /* C++ only */
_Decimal64  logb(_Decimal64 x);     /* C++ only */
_Decimal128 logb(_Decimal128 x);    /* C++ only */

General description

Returns the unbiased exponent of its argument x as a signed integer value in decimal floating-point mode. For typical numbers, the value returned is the logarithm of |x| rounded down (toward -INF) to the nearest integer value.

Notes:

To use IEEE decimal floating-point, the hardware must have the Decimal Floating-Point Facility installed.
These functions work in IEEE decimal floating-point format. See "IEEE Decimal Floating-Point" for more information.

Returned value

If successful, these functions return the unbiased exponent of x as a signed integer value in decimal floating-point mode.

These functions will fail under the following condition: If x is equal to 0.0, -HUGE_VAL_D32, -HUGE_VAL_D64, or -HUGE_VAL_D128 is returned and errno is set to EDOM.

Example

⁄* CELEBL24

   This program illustrates the use of logbd32() function

*⁄

#define  __STDC_WANT_DEC_FP__
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

void main()
{
  _Decimal32 x, logbx;

  printf("Illustrates the logbd32() function\n");


  ⁄* Generate the smallest possible positive _Decimal32 number       *⁄

  x = strtod32("0000001.E-101", NULL);

  logbx = logbd32(x);

  printf("x       = %Hg\n"  , x    );
  printf("logb(x) = %Hf\n\n", logbx);

  printf("powd32(10.0, logb32(x)) should equal x\n");
  printf("powd32(%Hf, %Hf) = %Hg\n",
         10.0DF, logbx, powd32(10.0DF, logbx));
}