NAME

log, logf, log10, log10f, log1p, log1pf log2, log2f, — logarithm functions

LIBRARY

Math Library (libm, -lm)

SYNOPSIS

#include <math.h>

double
log(double x);

float
logf(float x);

double
log10(double x);

float
log10f(float x);

double
log1p(double x);

float
log1pf(float x);

double
log2(double x);

float
log2f(float x);

DESCRIPTION

The following functions compute logarithms:

• The log() and logf() functions return the natural logarithm.
• The log10() and log10f() functions return the base 10 logarithm.
• The log1p() and log1pf() functions return the natural logarithm of (1.0 + x) accurately even for very small values of x.
• The log2() and log2f() functions return the base 2 logarithm.

RETURN VALUES

Upon successful completion, the functions return the logarithm of x as descibed above. Otherwise the following may occur:

1. If x is NaN, all functions return NaN.
2. If x is positive infinity, all functions return x. If x is negative infinity, all functions return NaN.
3. If x is +0.0 or -0.0, the log(), log10(), and log2() families return either -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, whereas the log1p() family returns x.
4. If x is +1.0, the log(), log10(), and log2() families return +0.0. If x is -1.0, the log1p() family returns -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL.

In addition, on a VAX, errno is set to EDOM and the reserved operand is returned by log() unless x > 0, by log1p() unless x > -1.

SEE ALSO

exp(3), ilogb(3), math(3)

STANDARDS

The described functions conform to ISO/IEC 9899:1999 (“ISO C99”).

HISTORY

The history of the logarithm functions dates back to Version 6 AT&T UNIX.