log2, log2f, log2l

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Atomic operations (C11)

Thread support (C11)

Technical Specifications

Numerics

Common mathematical functions

Floating-point environment

Complex number arithmetics

Random number generation

Type-generic math

Common mathematical functions

Functions
Basic operations
abslabsllabsimaxabs (C99) fabs divldivlldivimaxdiv (C99) fmod	remainder (C99) remquo (C99) fma (C99) fmax (C99) fmin (C99) fdim (C99) nannanfnanl (C99)(C99)(C99)
Exponential functions
exp exp2 (C99) expm1 (C99)	log log10 log1p (C99) log2 (C99)
Power functions
sqrt cbrt (C99)	hypot (C99) pow
Trigonometric and hyperbolic functions
sin cos tan asin acos atan atan2	sinh cosh tanh asinh (C99) acosh (C99) atanh (C99)
Error and gamma functions
erf (C99) erfc (C99)	lgamma (C99) tgamma (C99)
Nearest integer floating point operations
ceil floor roundlroundllround (C99)(C99)(C99)	trunc (C99) nearbyint (C99) rintlrintllrint (C99)(C99)(C99)
Floating point manipulation functions
ldexp scalbnscalbln (C99)(C99) ilogb (C99) logb (C99)	frexp modf nextafternexttoward (C99)(C99) copysign (C99)
Classification
fpclassify (C99) isfinite (C99) isinf (C99) isnan (C99) isnormal (C99) signbit (C99)	isgreater (C99) isgreaterequal (C99) isless (C99) islessequal (C99) islessgreater (C99) isunordered (C99)
Types
div_tldiv_tlldiv_timaxdiv_t (C99)(C99)	float_tdouble_t (C99)(C99)
Macro constants
HUGE_VALFHUGE_VALHUGE_VALL (C99)(C99) FP_FAST_FMAFFP_FAST_FMAFP_FAST_FMAL (C99)(C99)(C99) math_errhandlingMATH_ERRNOMATH_ERRNOEXCEPT (C99)(C99)(C99)	INFINITY (C99) NAN (C99) FP_ILOGB0FP_ILOGBNAN (C99)(C99) FP_NORMALFP_SUBNORMALFP_ZEROFP_INFINITEFP_NAN (C99)(C99)(C99)(C99)(C99)

Defined in header <math.h>
float log2f( float arg );	(1)	(since C99)
double log2( double arg );	(2)	(since C99)
long double log2l( long double arg );	(3)	(since C99)
Defined in header <tgmath.h>
#define log2( arg )	(4)	(since C99)

1-3) Computes the base 2 logarithm of arg.

4) Type-generic macro: If arg has type long double, log2l is called. Otherwise, if arg has integer type or the type double, log2 is called. Otherwise, log2f is called.

Parameters

arg

-

floating point value

Return value

If no errors occur, the base-2 logarithm of arg (log
2(arg) or lb(arg)) is returned.

If a domain error occurs, an implementation-defined value is returned (NaN where supported).

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 error occurs if arg is less than zero.

Pole error may occur if arg is zero.

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

• If the argument is 0, - is returned and FE_DIVBYZERO is raised.
• If the argument is 1, +0 is returned
• If the argument is negative, NaN is returned and FE_INVALID is raised.
• If the argument is +, + is returned
• If the argument is NaN, NaN is returned

Notes

For integer arg, the binary logarithm can be interpreted as the zero-based index of the most significant 1 bit in the input.

Example

Run this code

#include <stdio.h>
#include <math.h>
#include <float.h>
#include <errno.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
int main(void)
{
    printf("log2(65536) =%f\n", log2(65536));
    printf("log2(0.125) =%f\n", log2(0.125));
    printf("log2(0x020f) =%f (highest set bit is in position 9)\n", log2(0x020f));
    printf("base-5 logarithm of 125 =%f\n", log2(125)/log2(5));
    // special values
    printf("log2(1) =%f\n", log2(1));
    printf("log2(+Inf) =%f\n", log2(INFINITY));
    //error handling
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("log2(0) =%f\n", log2(0));
    if(errno == ERANGE) perror("    errno == ERANGE");
    if(fetestexcept(FE_DIVBYZERO)) puts("    FE_DIVBYZERO raised");
}

Possible output:

log2(65536) = 16.000000
log2(0.125) = -3.000000
log2(0x020f) = 9.041659 (highest set bit is in position 9)
base-5 logarithm of 125 = 3.000000
log2(1) = 0.000000
log2(+Inf) = inf
log2(0) = -inf
    errno == ERANGE: Numerical result out of range
    FE_DIVBYZERO raised

References

• C11 standard (ISO/IEC 9899:2011):

• 7.12.6.10 The log2 functions (p: 246)

• 7.25 Type-generic math <tgmath.h> (p: 373-375)

• F.10.3.10 The log2 functions (p: 522)

• C99 standard (ISO/IEC 9899:1999):

• 7.12.6.10 The log2 functions (p: 226)

• 7.22 Type-generic math <tgmath.h> (p: 335-337)

• F.9.3.10 The log2 functions (p: 459)

See also

loglogflogl (C99)(C99)	computes natural (base-e) logarithm (ln(x)) (function)
log10log10flog10l (C99)(C99)	computes common (base-10) logarithm (log10(x)) (function)
log1plog1pflog1pl (C99)(C99)(C99)	computes natural (base-e) logarithm of 1 plus the given number (ln(1+x)) (function)
exp2exp2fexp2l (C99)(C99)(C99)	computes 2 raised to the given power (2x) (function)