IEEE Std 1003.1, 2004 Edition

Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

log2, log2f, log2l - compute base 2 logarithm functions

#include <math.h>

double log2(doublex);

float log2f(floatx);

long double log2l(long doublex);

^{[CX]}The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to the ISO C standard.These functions shall compute the base 2 logarithm of their argument

x, log_{2}(x).An application wishing to check for error situations should set

errnoto zero and callfeclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, iferrnois non-zero orfetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.

Upon successful completion, these functions shall return the base 2 logarithm of

x.If

xis ±0, a pole error shall occur andlog2(),log2f(), andlog2l() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.For finite values of

xthat are less than 0,^{[MX]}or ifxis -Inf, a domain error shall occur, and^{[MX]}either a NaN (if supported), or an implementation-defined value shall be returned.

^{[MX]}Ifxis NaN, a NaN shall be returned.If

xis 1, +0 shall be returned.If

xis +Inf,xshall be returned.

These functions shall fail if:

- Domain Error
- The finite value of
xis less than zero,^{[MX]}orxis -Inf.If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then

errnoshall be set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall be raised.- Pole Error
- The value of
xis zero.If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then

errnoshall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception shall be raised.

None.

On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.

None.

None.

feclearexcept(),fetestexcept(),log(), the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions,<math.h>

First released in Issue 6. Derived from the ISO/IEC 9899:1999 standard.

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