LOG(3M) Mathematical Library Functions LOG(3M)

log, logf, logl - natural logarithm function

c99 [ flag... ] file... -lm [ library... ]
#include <math.h>
double log(double x);

float logf(float x);

long double logl(long double x);

These functions compute the natural logarithm of their argument x, log(e)(x).

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

If x is ±0, a pole error occurs and log(), logf(), and logl() return −HUGE_VAL, −HUGE_VALF, and −HUGE_VALL, respectively.

For finite values of x that are less than 0, or if x is −Inf, a domain error occurs and a NaN is returned.

If x is NaN, a NaN is returned.

If x is 1, +0 is returned.

If x is +Inf, x is returned.

For exceptional cases, matherr(3M) tabulates the values to be returned by log() as specified by SVID3 and XPG3.

These functions will fail if:

Domain Error

The finite value of x is negative, or x is -Inf.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the invalid floating-point exception is raised.

The log() function sets errno to EDOM if the value of x is negative.

Pole Error

The value of x is 0.

If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the divide-by-zero floating-point exception is raised.

An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.

An application can also set errno to 0 before calling log(). On return, if errno is non-zero, an error has occurred. The logf() and logl() functions do not set errno.

See attributes(7) for descriptions of the following attributes:

Interface Stability Standard
MT-Level MT-Safe

math.h(3HEAD), exp(3M), feclearexcept(3M), fetestexcept(3M), isnan(3M), log10(3M), log1p(3M), matherr(3M), attributes(7), standards(7)

July 12, 2006 SunOS 5.11