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#include <math.h> double log2(double x);
This function computes the base-2 logarithm of x.
The base-2 logarithm of x. If x is zero, a negative
infinity is returned and errno
is set to ERANGE
. If
x is negative or +Inf
or a NaN
, the return value is
NaN
and errno
is set to EDOM
.
not ANSI, not POSIX
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