Legacy Document

log

You can use the log function to compute the natural logarithm of a real number.

double_t log (double_t x);

x

Any positive floating-point number.

DESCRIPTION

The log function returns the natural (base e) logarithm of its argument.

$log(x)\; =\; log$_ex = lnx = y such that $x\; =\; ey$

The exp function performs the inverse (exponential) operation.

EXCEPTIONS

When x is finite and nonzero, the result of $log(x)$ might raise one of the following exceptions:

• inexact (for all finite, nonzero values of x other than +1)
• invalid (if x is negative)

SPECIAL CASES

Table 10-14 shows the results when the argument to the log function is a zero, a NaN, or an Infinity, plus other special cases for the log function.

Special cases for the log function

Operation	Result	Exceptions raised
$log(x)$ for x < 0	NaN	Invalid
$log(+1)$	+0	None
$log(+0)$	$-$	Divide-by-zero
$log(-0)$	$-$	Divide-by-zero
$log(NaN)$	NaN	None[35]
$log(+$ )	+	None
$log(-$ )	NaN	Invalid

EXAMPLES

z = log(+1.0);    /* z = +0.0 because e0 = 1 */
z = log(-1.0);    /* z = NAN because negative arguments are not 
                     allowed. The invalid exception is raised. */

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[35] If the NaN is a signaling NaN, the invalid exception is raised.