log2l

log2, log2f, log2l

Defined in header
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.

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

#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):

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)