The log() function computes the natural logarithm of x, log_e(x). The value of x must be positive.

Upon successful completion, log() returns the natural logarithm of x.

If x is NaN, NaN is returned.

If x is less than 0, -HUGE_VAL or NaN is returned and errno is set to EDOM.

If x is 0, -HUGE_VAL is returned and errno may be set to ERANGE.

In IEEE754 mode (the -Xlibmieee cc compilation option), if x is Inf or a quiet NaN, x is returned; if x is a signaling NaN, a quiet NaN is returned and the invalid operation exception is raised; if x is 1, 0 is returned; for all other positive x, a normalized number is returned and the inexact exception is raised.

For exceptional cases, matherr(3M) tabulates the values to be returned as dictated by Standards other than XPG4.

The log() function will fail if:

EDOM

The value of x is negative.

The log() function may fail if:

ERANGE

The value of x is 0.

No other errors will occur.

An application wishing to check for error situations should set errno to 0 before calling log(). If errno is non-zero on return, or the return value is NaN, an error has occurred.

See attributes(5) for descriptions of the following attributes:

exp(3M), isnan(3M), log10(3M), log1p(3M), matherr(3M), attributes(5), standards(5)