logb, logbf, logbl - cppreference.com (original) (raw)

Defined in header <math.h>
float logbf( float arg ); (1) (since C99)
double logb( double arg ); (2) (since C99)
long double logbl( long double arg ); (3) (since C99)
Defined in header <tgmath.h>
#define logb( arg ) (4) (since C99)

1-3) Extracts the value of the unbiased radix-independent exponent from the floating-point argument arg, and returns it as a floating-point value.

  1. Type-generic macros: If arg has type long double, logbl is called. Otherwise, if arg has integer type or the type double, logb is called. Otherwise, logbf is called.

Formally, the unbiased exponent is the signed integral part of logr|arg| (returned by this function as a floating-point value), for non-zero arg, where r is FLT_RADIX. If arg is subnormal, it is treated as though it was normalized.

Contents

[edit] Parameters

arg - floating-point value

[edit] Return value

If no errors occur, the unbiased exponent of arg is returned as a signed floating-point value.

If a domain error occurs, an implementation-defined value is returned.

If a pole error occurs, -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL is returned.

[edit] Error handling

Errors are reported as specified in math_errhandling.

Domain or range error may occur if arg is zero.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

[edit] Notes

POSIX requires that a pole error occurs if arg is ±0.

The value of the exponent returned by logb is always 1 less than the exponent returned by frexp because of the different normalization requirements: for the exponent e returned by logb, |arg*r-e
| is between 1 and r (typically between 1 and 2), but for the exponent e returned by frexp, |arg*2-e
| is between 0.5 and 1.

[edit] Example

Compares different floating-point decomposition functions.

#include <fenv.h> #include <float.h> #include <math.h> #include <stdio.h> // #pragma STDC FENV_ACCESS ON   int main(void) { double f = 123.45; printf("Given the number %.2f or %a in hex,\n", f, f);   double f3; double f2 = modf(f, &f3); printf("modf() makes %.0f + %.2f\n", f3, f2);   int i; f2 = frexp(f, &i); printf("frexp() makes %f * 2^%d\n", f2, i);   i = logb(f); printf("logb()/logb() make %f * %d^%d\n", f/scalbn(1.0, i), FLT_RADIX, i);   // error handling feclearexcept(FE_ALL_EXCEPT); printf("logb(0) = %f\n", logb(0)); if (fetestexcept(FE_DIVBYZERO)) puts(" FE_DIVBYZERO raised"); }

Possible output:

Given the number 123.45 or 0x1.edccccccccccdp+6 in hex, modf() makes 123 + 0.45 frexp() makes 0.964453 * 2^7 logb()/logb() make 1.928906 * 2^6 logb(0) = -Inf FE_DIVBYZERO raised

[edit] References

[edit] See also