Manual Pages  — LOG

log, logf, logl, log10, log10f, log10l, log2, log2f, log2l, log1p, log1pf, log1pl – logarithm functions

LIBRARY SYNOPSIS DESCRIPTION RETURN VALUES SEE ALSO STANDARDS HISTORY

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 -∞ is returned. Otherwise, attempting to take the logarithm of a negative number results in an invalid exception and a return value of NaN.

The log(), logf(), logl(), log10(), log10f(), log10l(), log2(), log2f(), log2l(), log1p(), log1pf(), and log1pl() functions conform to ISO/IEC 9899:1999 ("ISO C99").

The log() function first appeared in AT&T v1 ; log10() in AT&T v7 ; log1p() in BSD 4.3 .