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 described above. Otherwise the following may occur:
- If x is NaN, all functions return
NaN.
- If x is positive infinity, all
functions return x. If x is
negative infinity, all functions return NaN.
- 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.
- 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.