LOG(3POSIX)

💖 LOG(3POSIX) — the natural logarithm glow-up 💖

okay bestie, so this is a page from the POSIX Programmer's Manual, and we're doing log, logf and logl today. 🧚✨

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🌸 PROLOG 🌸

So like, heads up first: 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 at all. 🙈

Not me giving you the tea before the tea, but you needed to know.

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👑 NAME 👑

log, logf, logl — natural logarithm function. that's it, that's the whole vibe, the icon, the moment. 💅

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✨ SYNOPSIS ✨

#include <math.h>

double log(double x);
float logf(float x);
long double logl(long double x);

three sisters, three sizes, same energy fr fr. 🎀

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🦋 DESCRIPTION 🦋

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 — no drama intended, bestie. This volume of POSIX.1‐2017 defers to the ISO C standard. periodt.

These functions shall compute the natural logarithm of their argument x, log_e(x). 🍓

Now, if your application wants to check for error situations, it should set errno to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. Then on return:

• if errno is non-zero, or
• if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero,

…then an error has occurred. lowkey a vibe check for your math, honestly. 🔥

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💜 RETURN VALUE 💜

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

• If x is ±0, a pole error shall occur and log(), logf(), and logl() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively. 💀 that's a whole lot of negative.
• For finite values of x that are less than 0, or if x is -Inf, a domain error shall occur, and either a NaN (if supported), or an implementation-defined value shall be returned. 🙈 it's giving "undefined behavior but make it official".
• If x is NaN, a NaN shall be returned. (NaN in, NaN out, the circle of life. 🦋)
• If x is 1, +0 shall be returned. (math's little "no thoughts, head empty" moment 🎀)
• If x is +Inf, x shall be returned. infinity stays infinity, iconic. 👑

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🔥 ERRORS 🔥

These functions shall fail if:

💅 Domain Error

The finite value of x is negative, 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 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.

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The following sections are informative. (translation: the rest is bonus content, bestie, not requirements 🫶)

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🌈 EXAMPLES 🌈

None.

no examples, no crumbs, we move. 😭

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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 not codependent, but they can't both ghost you. 💜

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⭐ RATIONALE ⭐

None.

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🧚 FUTURE DIRECTIONS 🧚

None.

the future is bright and also empty. ✨

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🔍 SEE ALSO 🔍

• exp()
• feclearexcept()
• fetestexcept()
• isnan()
• log10()
• log1p()

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

iykyk 💖

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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 .

not me telling you to read the fine print but ALSO telling you to read the fine print. both. 💅

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and that's the whole page, bestie!! brought to you by IEEE/The Open Group, 2017 — LOG(3POSIX), signing off. 🫶✨ no cap, go compute some natural logs. 🦋