LOG2(3POSIX)

💖 LOG2(3POSIX) — the POSIX Programmer's Manual, bestie edition 💖

okay so this page lives in the POSIX Programmer's Manual and we're doing LOG2(3POSIX) today, and I am so glad you're here 🫶

🌸 PROLOG 🌸

real talk before we even start: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ — so if you're on Linux, pls go consult the corresponding Linux manual page for the deets on Linux behavior. OR the interface might not even be implemented on Linux at all. it's giving "check your local listings" 💀

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✨ NAME ✨

log2, log2f, log2l — compute base 2 logarithm functions 👑

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🎀 SYNOPSIS 🎀

#include <math.h>

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

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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, ok? no drama intended. This volume of POSIX.1‐2017 defers to the ISO C standard.

These functions shall compute the base 2 logarithm of their argument x, like log2(x). it's literally just "how many times do I multiply 2 to get here" energy ⭐

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. so like, clear the vibes first, then vibe check after 💅

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🔥 RETURN VALUE 🔥

Upon successful completion, these functions shall return the base 2 logarithm of x. slay.

• If x is ±0, a pole error shall occur and log2(), log2f(), and log2l() shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively. 😭 negative infinity jump scare
• 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.
• If x is 1, +0 shall be returned. (makes sense bestie, 2^0 = 1, we been knew 🧚)
• 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. yes, that means what you think it means — they're the commentary track, not the rules 📝

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🍓 EXAMPLES 🍓

None. (iconic of them honestly, zero examples, no notes)

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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 they don't talk to each other but they do have to both show up. group project energy fr fr 🙈

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💀 RATIONALE 💀

None. (they said "no thoughts, head empty" and honestly? respected)

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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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okay that's the whole page bestie, you ate it 💅✨ this edition came to you from IEEE/The Open Group, 2017 — mwah, take care of your logarithms, drink water, and remember: log base 2 of 1 is always 0 and so is my tolerance for undefined behavior 🎀💖🦋