log man page

Prolog

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

log, logf, logl — natural logarithm function

Synopsis

#include <math.h>

double log(double x);
float logf(float x);
long double logl(long double x);

Description

The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1‐2008 defers to the ISO C standard.

These functions shall compute the natural logarithm of their argument x, loge(x).

An application wishing to check for error situations should set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.

Return Value

Upon successful completion, these functions shall return the natural logarithm of x.

If x is ±0, a pole error shall occur and log(), logf(), and logl() shall 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 shall occur, and either a NaN (if supported), or an implementation-defined value shall be returned.

If x is NaN, a NaN shall be returned.

If x is 1, +0 shall be returned.

If x is +Inf, x shall be returned.

Errors

These functions shall fail if:

Domain Error

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

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the invalid floating-point exception shall be raised.

Pole Error

The value of x is zero.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the divide-by-zero floating-point exception shall be raised.

The following sections are informative.

Application Usage

On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.

See Also

exp(), feclearexcept(), fetestexcept(), isnan(), log10(), log1p()

The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of Error Conditions for Mathematical Functions, <math.h>

Info

2013 IEEE/The Open Group POSIX Programmer's Manual