LOG1P(3) Library Functions Manual LOG1P(3)

log1p log1pf log1pl — compute a natural logarithm

#include <math.h>

double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);

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.1-2024 defers to the ISO C standard.

These functions shall compute loge(1.0 + x).

An application wishing to check for error situations should set errno to zero and call (FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or (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 x is -1, a pole error shall occur and log1p (), log1pf (), and log1pl () shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.

For finite values of x that are less than -1, [MX] or if x is -Inf, a domain error shall occur, and [MX] either a NaN (if supported), or an implementation-defined value shall be returned.

[MX] If x is NaN, a NaN shall be returned.

If x is ±0, or +Inf, x shall be returned.

If x is subnormal, x should be returned. [MX] If x is subnormal, a range error may occur and, if x is not returned, log1p (), log1pf (), and log1pl () shall return an implementation-defined value no greater in magnitude than DBL_MIN, FLT_MIN, and LDBL_MIN, respectively.

These functions shall fail if:

The finite value of x is less than -1, [MX] 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.

The value of x is -1.

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.

These functions may fail if:

[MX] The value of x is subnormal.

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 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(3), fetestexcept(3), log(3)

XBD V1_chap04(7), <math.h>

First released in Issue 4, Version 2.

Moved from X/OPEN UNIX extension to BASE.

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

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

The log1pf () and log1pl () functions are added for alignment with the ISO/IEC 9899:1999 (“ISO C99”) standard.

The DESCRIPTION, RETURN VALUE, ERRORS, and APPLICATION USAGE sections are revised to align with the ISO/IEC 9899:1999 (“ISO C99”) standard.

IEC 60559:1989 standard floating-point extensions over the ISO/IEC 9899:1999 (“ISO C99”) standard are marked.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0364 [68] is applied.

Austin Group Defect 1382 is applied, rearranging the text describing the behavior when x is subnormal to avoid the need for two shading changes.

June 14, 2024 posix.fail