IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)

Copyright © 2001-2018 IEEE and The Open Group

log, logf, logl - natural logarithm function

#include <math.h>

double log(doublex);

float logf(floatx);

long double logl(long doublex);

^{[CX]}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.1-2017 defers to the ISO C standard.These functions shall compute the natural logarithm of their argument

x, log_{e}(x).An application wishing to check for error situations should set

errnoto zero and callfeclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, iferrnois non-zero orfetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.

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

x.If

xis ±0, a pole error shall occur andlog(),logf(), andlogl() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.For finite values of

xthat are less than 0,^{[MX]}or ifxis -Inf, a domain error shall occur, and^{[MX]}either a NaN (if supported), or an implementation-defined value shall be returned.

^{[MX]}Ifxis NaN, a NaN shall be returned.If

xis 1, +0 shall be returned.If

xis +Inf,xshall be returned.

These functions shall fail if:

- Domain Error
- The finite value of
xis negative,^{[MX]}orxis -Inf.If the integer expression (

math_errhandling& MATH_ERRNO) is non-zero, thenerrnoshall 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
xis zero.If the integer expression (

math_errhandling& MATH_ERRNO) is non-zero, thenerrnoshall 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.

None.

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.

None.

None.

exp,feclearexcept,fetestexcept,isnan,log10,log1pXBD

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

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

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

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

The

logf() andlogl() 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.

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