log2, log2f, log2l

 

C

Compiler support

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Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Strings library

Algorithms

Numerics

Date and time utilities

Input/output support

Localization support

Concurrency support(C11)

Technical Specifications

Symbol index

 

Numerics

Common mathematical functions

Floating-point environment(C99)

Pseudo-random number generation

Complex number arithmetic(C99)

Type-generic math(C99)

Bit manipulation(C23)

Checked integer arithmetic(C23)

 

Common mathematical functions

Functions Basic operations abslabsllabsimaxabs (C99)(C99) fabs divldivlldivimaxdiv (C99)(C99) fmod remainder (C99) remquo (C99) fma (C99) fdim (C99) nannanfnanlnandN (C99)(C99)(C99)(C23) Maximum/minimum operations fmax (C99) fmin (C99) fmaximum (C23) fminimum (C23) fmaximum_mag (C23)  fmaximum_num (C23) fminimum_mag (C23) fminimum_num (C23) fmaximum_mag_num (C23) fminimum_mag_num (C23)   Exponential functions exp exp10 (C23) exp2 (C99) expm1 (C99) exp10m1 (C23) exp2m1 (C23) log log10 log2 (C99) log1plogp1 (C99)(C23) log10p1 (C23) log2p1 (C23) Power functions sqrt cbrt (C99) rootn (C23) rsqrt (C23) hypot (C99) compound (C23) pow pown (C23) powr (C23) Trigonometric and hyperbolic functions sin cos tan asin acos atan atan2 sinpi (C23) cospi (C23) tanpi (C23) asinpi (C23) acospi (C23) atanpi (C23) atan2pi (C23) sinh cosh tanh asinh (C99) acosh (C99) atanh (C99)

Nearest integer floating-point ceil floor roundlroundllround (C99)(C99)(C99) roundeven (C23) trunc (C99) nearbyint (C99) rintlrintllrint (C99)(C99)(C99) fromfpfromfpxufromfpufromfpx (C23)(C23)(C23)(C23) Floating-point manipulation ldexp frexp scalbnscalbln (C99)(C99) ilogbllogb (C99)(C23) logb (C99) modf nextafternexttoward (C99)(C99) nextupnextdown (C23)(C23) copysign (C99) canonicalize (C23) Narrowing operations fadd (C23) fsub (C23) fmul (C23) fdiv (C23) ffma (C23) fsqrt (C23) Quantum and quantum exponent quantizedN (C23) quantumdN (C23) samequantumdN (C23) llquantexpdN (C23) Decimal re-encoding functions encodedecdN (C23) decodedecdN (C23) encodebindN (C23) decodebindN (C23) Total order and payload functions totalorder (C23) getpayload (C23) setpayload (C23) setpayloadsig (C23) Classification fpclassify (C99) iscanonical (C23)  isfinite (C99) isinf (C99) isnan (C99) isnormal (C99) signbit (C99) issubnormal (C23) iszero (C23) isgreater (C99) isgreaterequal (C99)   isless (C99) islessequal (C99) islessgreater (C99) isunordered (C99) issignaling (C23) iseqsig (C23)

Error and gamma functions erf (C99) erfc (C99) lgamma (C99) tgamma (C99) Types div_tldiv_tlldiv_timaxdiv_t (C99)(C99) float_tdouble_t (C99)(C99) _Decimal32_t_Decimal64_t (C23)(C23) Macro constants Special floating-point values HUGE_VALHUGE_VALFHUGE_VALLHUGE_VALDN (C99)(C99)(C23) INFINITYDEC_INFINITY (C99)(C23) NANDEC_NAN (C99)(C23) Arguments and return values FP_ILOGB0FP_ILOGBNAN (C99)(C99) FP_NORMALFP_SUBNORMALFP_ZEROFP_INFINITEFP_NAN (C99)(C99)(C99)(C99)(C99) FP_LLOGB0FP_LLOGBNAN (C23)(C23) FP_INT_UPWARDFP_INT_DOWNWARDFP_INT_TOWARDZEROFP_INT_TONEARESTFROMZEROFP_INT_TONEAREST (C23)(C23)(C23)(C23)(C23) Error handling MATH_ERRNOMATH_ERRNOEXCEPT (C99)(C99)  math_errhandling (C99) Fast operation indicators FP_FAST_FMAFFP_FAST_FMA (C99)(C99) FP_FAST_FADDFP_FAST_FADDLFP_FAST_DADDLFP_FAST_DMADDDN (C23)(C23)(C23)(C23) FP_FAST_FMULFP_FAST_FMULLFP_FAST_DMULLFP_FAST_DMMULDN (C23)(C23)(C23)(C23) FP_FAST_FFMAFP_FAST_FFMALFP_FAST_DFMALFP_FAST_DMFMADN (C23)(C23)(C23)(C23) FP_FAST_FMALFP_FAST_FMADN (C99)(C23) FP_FAST_FSUBFP_FAST_FSUBLFP_FAST_DSUBLFP_FAST_DMSUBDN (C23)(C23)(C23)(C23) FP_FAST_FDIVFP_FAST_FDIVLFP_FAST_DDIVLFP_FAST_DMDIVDN (C23)(C23)(C23)(C23) FP_FAST_FSQRTFP_FAST_FSQRTLFP_FAST_DSQRTLFP_FAST_DMSQRTDN (C23)(C23)(C23)(C23)

 

Defined in header <math.h>
float       log2f(float arg );	(1)	(since C99)
double      log2(double arg );	(2)	(since C99)
longdouble log2l(longdouble 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 longdouble, 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 (log2(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 ONint 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 valuesprintf("log2(1) = %f\n", log2(1));printf("log2(+Inf) = %f\n", log2(INFINITY));//error handlingerrno=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

• C17 standard (ISO/IEC 9899:2018):

• 7.12.6.10 The log2 functions (p: 179)

• 7.25 Type-generic math <tgmath.h> (p: 272-273)

• F.10.3.10 The log2 functions (p: 381)

• 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 (\({\small \ln{x} }\)ln(x)) (function)
log10log10flog10l (C99)(C99)	computes common (base-10) logarithm (\({\small \log_{10}{x} }\)log10(x)) (function)
log1plog1pflog1pl (C99)(C99)(C99)	computes natural (base-e) logarithm of 1 plus the given number (\({\small \ln{(1+x)} }\)ln(1+x)) (function)
exp2exp2fexp2l (C99)(C99)(C99)	computes 2 raised to the given power (\({\small 2^x}\)2x) (function)
C++ documentation for log2