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FreeBSD 11.0 - man page for log2 (freebsd section 3)

LOG(3)							   BSD Library Functions Manual 						    LOG(3)

log, logf, logl, log10, log10f, log10l, log2, log2f, log2l, log1p, log1pf, log1pl -- logarithm functions
Math Library (libm, -lm)
#include <math.h> double log(double x); float logf(float x); long double logl(long double x); double log10(double x); float log10f(float x); long double log10l(long double x); double log2(double x); float log2f(float x); long double log2l(long double x); double log1p(double x); float log1pf(float x); long double log1pl(long double x);
The log(), logf(), and logl() functions compute the natural logarithm of x. The log10(), log10f(), and log10l() functions compute the logarithm base 10 of x, while log2(), log2f(), and log2l() compute the logarithm base 2 of x. The log1p(), log1pf(), and log1pl() functions compute the natural logarithm of 1 + x. Computing the natural logarithm as log1p(x) is more accurate than computing it as log(1 + x) when x is close to zero.
These functions return the requested logarithm; the logarithm of 1 is +0. An attempt to take the logarithm of +-0 results in a divide-by- zero exception, and -infinity is returned. Otherwise, attempting to take the logarithm of a negative number results in an invalid exception and a return value of NaN.
exp(3), ilogb(3), math(3), pow(3)
The log(), logf(), logl(), log10(), log10f(), log10l(), log2(), log2f(), log2l(), log1p(), log1pf(), and log1pl() functions conform to ISO/IEC 9899:1999 (``ISO C99'').
June 3, 2013 BSD