Defined in header `<math.h>` | ||
---|---|---|

int ilogbf( float arg ); | (1) | (since C99) |

int ilogb( double arg ); | (2) | (since C99) |

int ilogbl( long double arg ); | (3) | (since C99) |

Defined in header `<tgmath.h>` | ||

#define ilogb( arg ) | (4) | (since C99) |

Defined in header `<math.h>` | ||

#define FP_ILOGB0 /*implementation-defined*/ | (5) | (since C99) |

#define FP_ILOGBNAN /*implementation-defined*/ | (6) | (since C99) |

1-3) Extracts the value of the unbiased exponent from the floating-point argument

`arg`

, and returns it as a signed integer value.
4) Type-generic macros: If

`arg`

has type `long double`

, `ilogbl`

is called. Otherwise, if `arg`

has integer type or the type `double`

, `ilogb`

is called. Otherwise, `ilogbf`

is called.Formally, the unbiased exponent is the integral part of log

r|arg| as a signed integral value, for non-zero arg, where `r`

is `FLT_RADIX`

.

arg | - | floating point value |

If no errors occur, the unbiased exponent of `arg`

is returned as a signed int value.

If `arg`

is zero, `FP_ILOGB0`

is returned.

If `arg`

is infinite, `INT_MAX`

is returned.

If `arg`

is a NaN, `FP_ILOGBNAN`

is returned.

If the correct result is greater than `INT_MAX`

or smaller than `INT_MIN`

, the return value is unspecified and a domain error or range error may occur.

Errors are reported as specified in math_errhandling.

A domain error or range error may occur if `arg`

is zero, infinite, or NaN.

If the correct result is greater than `INT_MAX`

or smaller than `INT_MIN`

, a domain error or a range error may occur.

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

- If the correct result is greater than
`INT_MAX`

or smaller than`INT_MIN`

,`FE_INVALID`

is raised. - If
`arg`

is ±0, ±∞, or NaN,`FE_INVALID`

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

is never raised) and the current rounding mode is ignored

If `arg`

is not zero, infinite, or NaN, the value returned is exactly equivalent to `(int)logb(arg)`

.

POSIX requires that a domain error occurs if `arg`

is zero, infinite, NaN, or if the correct result is outside of the range of `int`

.

POSIX also requires that, on XSI-conformant systems, the value returned when the correct result is greater than `INT_MAX`

is `INT_MAX`

and the value returned when the correct result is less than `INT_MIN`

is `INT_MIN`

.

The correct result can be represented as `int`

on all known implementations. For overflow to occur, `INT_MAX`

must be less than `LDBL_MAX_EXP*log2(FLT_RADIX)`

or `INT_MIN`

must be greater than `LDBL_MIN_EXP-LDBL_MANT_DIG)*log2(FLT_RADIX)`

.

The value of the exponent returned by `std::ilogb`

is always 1 less than the exponent retuned by `frexp`

because of the different normalization requirements: for the exponent `e`

returned by `ilogb`

, |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`

.

#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 = ilogb(f); printf("logb()/ilogb() make %f * %d^%d\n", f/scalbn(1.0, i), FLT_RADIX, i); // error handling feclearexcept(FE_ALL_EXCEPT); printf("ilogb(0) = %d\n", ilogb(0)); if(fetestexcept(FE_INVALID)) puts(" FE_INVALID 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()/ilogb() make 1.92891 * 2^6 ilogb(0) = -2147483648 FE_INVALID raised

- C11 standard (ISO/IEC 9899:2011):
- 7.12/8 Mathematics <math.h> (p: 232)
- 7.12.6.5 The ilogb functions (p: 244)
- 7.25 Type-generic math <tgmath.h> (p: 373-375)
- F.10.3.5 The ilogb functions (p: 521)
- C99 standard (ISO/IEC 9899:1999):
- 7.12/8 Mathematics <math.h> (p: 213)
- 7.12.6.5 The ilogb functions (p: 224-225)
- 7.22 Type-generic math <tgmath.h> (p: 335-337)
- F.9.3.5 The ilogb functions (p: 458)

(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 `ilogb` |

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