log2

float       log2f( float arg );

double      log2( double arg );

long double log2l( long double arg );

#define log2( arg )

Parameters

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

#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");
}

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