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c99 [ flag... ] file... -lm [ library... ] #include <math.h> double log2(double x);
float log2f(float x);
long double log2l(long double x);
These functions compute the base 2 logarithm of their argument x, log2(x).
Upon successful completion, these functions return the base 2 logarithm of x.
If x is \(+-0, a pole error occurs and log2(), log2f(), and log2l() return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.
For finite values of x that are less than 0, or if x is -Inf a domain error occurs and a NaN is returned.
If x is NaN, a NaN is returned.
If x is 1, +0 is returned.
If x is +Inf, x is returned.
These functions will fail if:
Domain Error
The finite value of x is less than 0, or x is -Inf. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception is raised.
Pole Error
The value of x is 0. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception is raised.
An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Standard |
| MT-Level | MT-Safe |
feclearexcept(3M), fetestexcept(3M), log(3M), math.h(3HEAD), attributes(5), standards(5)