logb, logbf, logbl
| 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) |
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.
Parameters
| arg | - | floating-point value |
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.
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),
- 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.
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.
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 raisedReferences
- C23 standard (ISO/IEC 9899:2024):
- 7.12.6.11 The logb functions (p: TBD)
- 7.25 Type-generic math <tgmath.h> (p: TBD)
- F.10.3.11 The logb functions (p: TBD)
- 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)
See also
| (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
| |