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.
4) 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 log

r|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.

arg | - | floating point 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.

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),

- If
`arg`

is ±0, -∞ is returned and`FE_DIVBYZERO`

is raised. - If
`arg`

is ±∞, +∞ is returned - If
`arg`

is NaN, NaN is returned. - In all other cases, the result is exact (
`FE_INEXACT`

is never raised) and the current rounding mode is ignored

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 retuned 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`

.

Compares different floating-point decomposition functions.

#include <stdio.h> #include <math.h> #include <float.h> #include <fenv.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

- C17 standard (ISO/IEC 9899:2018):
- 7.12.6.11 The logb functions (p: 179-180)
- 7.25 Type-generic math <tgmath.h> (p: 373-375)
- F.10.3.11 The logb functions (p: 381)
- C11 standard (ISO/IEC 9899:2011):
- 7.12.6.11 The logb functions (p: 246)
- 7.25 Type-generic math <tgmath.h> (p: 373-375)
- F.10.3.11 The logb functions (p: 522)
- C99 standard (ISO/IEC 9899:1999):
- 7.12.6.11 The logb functions (p: 227)
- 7.22 Type-generic math <tgmath.h> (p: 335-337)
- F.9.3.11 The logb functions (p: 459)

(C99)(C99) | breaks a number into significand and a power of `2` (function) |

(C99)(C99)(C99) | extracts exponent of the given number (function) |

(C99)(C99)(C99)(C99)(C99)(C99) | computes efficiently a number times `FLT_RADIX` raised to a power (function) |

C++ documentation for `logb` |

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