LOG(3)

NAME
log, logf, log10, log10f, log1p, log1pf log2, log2f - logarithm functions

LIBRARY
libm

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 described above. Otherwise the following may occur:

1.  If x is , all functions return .

2.  If x is positive infinity, all functions return x. If x is negative infinity, all functions return .

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 isoC-99.

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