IEEE Std 1003.1-2001

Copyright © 2001 The IEEE and The Open Group, All Rights reserved.

log1p, log1pf, log1pl - compute a natural logarithm

#include <math.h>

double log1p(doublex);

float log1pf(floatx);

long double log1pl(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 IEEE Std 1003.1-2001 defers to the ISO C standard.These functions shall compute log

_{e}(1.0 +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 1.0 +

x.If

xis -1, a pole error shall occur andlog1p(),log1pf(), andlog1pl() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.For finite values of

xthat are less than -1,^{[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 ±0, or +Inf,xshall be returned.If

xis subnormal, a range error may occur andxshould be returned.

These functions shall fail if:

- Domain Error
- The finite value of
xis less than -1,^{[MX]}orxis -Inf.If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then

errnoshall 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 -1.If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then

errnoshall 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.These functions may fail if:

- Range Error
^{[MX]}The value ofxis subnormal.If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then

errnoshall be set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, then the underflow 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.

feclearexcept() ,fetestexcept() ,log() , the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical Functions,<math.h>

First released in Issue 4, Version 2.

Moved from X/OPEN UNIX extension to BASE.

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

The

log1p() function is no longer marked as an extension.The

log1pf() andlog1pl() 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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