LOG1P(3POSIX)

💖 LOG1P 💖

okay bestie, so this is LOG1P(3POSIX) — it's part of the POSIX Programmer's Manual, and I need you to know up front that the Linux implementation of this interface may be different (so go peek at the Linux manual page for the tea on Linux behavior), or honestly the interface might not even be implemented on Linux at all. 🙈

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🌸 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.

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✨ NAME ✨

log1p, log1pf, log1pl — compute a natural logarithm. that's it, that's the whole job. 🎀

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🦋 SYNOPSIS 🦋

#include <math.h>

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

three flavors, one vibe check: double, float, long double. pick your fighter. 💅

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💜 DESCRIPTION 💜

So the functionality described on this reference page is aligned with the ISO C standard. If there's any conflict between the requirements here and the ISO C standard — that's unintentional, bestie, not a plot twist. This volume of POSIX.1‐2017 defers to the ISO C standard. periodt.

These functions shall compute loge(1.0 + x).

Okay and here's the important little ritual 🫶: an application wishing to check for error situations should set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. Then on return, if errno is non-zero or fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred. that's your vibe check right there. ✨

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🔥 RETURN VALUE 🔥

Upon successful completion, these functions shall return the natural logarithm of 1.0 + x. slay.

• 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, 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 ±0, or +Inf, x shall be returned.
• If x is subnormal, a range error may occur and x should be returned.
• 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.

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💀 ERRORS 💀

These functions shall fail if:

Domain Error 🚫

The finite value of x is less than -1, 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 -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:

Range Error 📉

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.

heads up bestie: the following sections are informative. not the law, just the lore. 🧚

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🌈 EXAMPLES 🌈

None.

not a single one. the void. 💀

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🍓 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. they're doing their own thing but they're both kinda required. iconic honestly. 👑

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🧚 RATIONALE 🧚

None.

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⭐ FUTURE DIRECTIONS ⭐

None.

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🦄 SEE ALSO 🦄

• feclearexcept()
• fetestexcept()
• log()

The Base Definitions volume of POSIX.1‐2017, Section 4.20, Treatment of Error Conditions for Mathematical Functions, <math.h>

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👑 COPYRIGHT 👑

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page, bestie 💅 this one came to you from IEEE/The Open Group, dated 2017, and it's the LOG1P(3POSIX) page in the POSIX Programmer's Manual. go forth and compute that natural log, no cap. 🫶✨