LOG2(3P)

💖 LOG2(3P) — the POSIX Programmer's Manual, but make it make sense 💖

okay bestie, welcome to the POSIX Programmer's Manual, page for log2. This is the manual page LOG2(3P) — 3P means it's the POSIX flavor of a library call, iykyk. 🫶

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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.

ngl that's the standard "POSIX says one thing, your distro might say another" disclaimer. It's giving "read the fine print." 🙈

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✨ NAME ✨

log2, log2f, log2l — compute base 2 logarithm functions. that's it, that's the whole vibe. log base two. iconic. 👑

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🎀 SYNOPSIS 🎀

#include <math.h>

double log2(double x);
float log2f(float x);
long double log2l(long double x);

three of them, one for each level of float drama: double, float, long double. choose your fighter. 💅

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💜 DESCRIPTION 💜

so, 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. And this volume of POSIX.1‐2017 defers to the ISO C standard. no cap.

These functions shall compute the base 2 logarithm of their argument x, i.e. log2(x). it's math. it's the inverse of 2^x. slay. ✨

okay now the important part, bestie — if you're an application wishing to check for error situations, you 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,

…an error has occurred. that's your vibe check. 🕵️♀️

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🌈 RETURN VALUE 🌈

Upon successful completion, these functions shall return the base 2 logarithm of x. simple. clean. main character energy.

but babes, the edge cases are where it gets juicy:

• If x is ±0, a pole error shall occur and log2(), log2f(), and log2l() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively. 😭
• 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. 🚩
• If x is NaN, a NaN shall be returned. (garbage in, garbage out, but make it float.)
• If x is 1, +0 shall be returned. (because log2(1) = 0, math ate that. 🔥)
• If x is +Inf, x shall be returned.

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💀 ERRORS 💀

These functions shall fail if:

• Domain Error — the finite value of x is less than zero, 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.

the following sections are informative. aka: not the law, just lore. 🧚

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🍓 EXAMPLES 🍓

None.

…yeah. no examples. the manual said "figure it out, bestie." 💀

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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. so like, they don't talk to each other, but somebody has to show up to the function. periodt. 💅

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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!! 🥳 brought to you by IEEE/The Open Group, dated 2017, and yeah this is LOG2(3P) out. go compute some base-2 logs responsibly. mwah. 💋✨🫶