The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group

NAME

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

[CX] [Option Start] 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. [Option End]

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, [MX] [Option Start]  or if x is -Inf, [Option End] a domain error shall occur, and [MX] [Option Start]  either a NaN (if supported), or [Option End]  an implementation-defined value shall be returned.

[MX] [Option Start] 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. [Option End]

ERRORS

These functions shall fail if:

Domain Error
The finite value of x is negative, [MX] [Option Start]  or x is -Inf. [Option End]

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.

EXAMPLES

None.

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.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

exp, feclearexcept, fetestexcept, isnan, log10, log1p

XBD Treatment of Error Conditions for Mathematical Functions, <math.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated to indicate how an application should check for an error. This text was previously published in the APPLICATION USAGE section.

Issue 6

The normative text is updated to avoid use of the term "must" for application requirements.

The logf() and logl() functions are added for alignment with the ISO/IEC 9899:1999 standard.

The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999 standard.

IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 standard are marked.

End of informative text.

 

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