log(3)

💖 log(3) — the natural logarithmic function, bestie 💖

okay so this is log(3) from the Library Functions Manual, and honestly? she's iconic. she's literally the natural log. the way that a logarithm just... undoes an exponent? obsessed. 🎀

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✨ NAME ✨

log, logf, logl — natural logarithmic function

📚 LIBRARY 📚

Math library (libm, -lm)

not me reminding you to link -lm because if you don't you'll get undefined reference and you'll be sobbing. it's giving character development. 😭

🦋 SYNOPSIS 🦋

#include <math.h>

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

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

logf(), logl():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
    || /* Since glibc 2.19: */ _DEFAULT_SOURCE
    || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

yeah those macros are a whole vibe check. copy them exactly, don't freestyle. 💅

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🌸 DESCRIPTION 🌸

These functions return the natural logarithm of x.

that's it. that's the whole job. she logs, naturally. ✨

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💜 RETURN VALUE 💜

On success, these functions return the natural logarithm of x.

If x is a NaN, a NaN is returned. (NaN in, NaN out, no thoughts head empty. 🙈)

If x is 1, the result is +0.

If x is positive infinity, positive infinity is returned. (+inf in, +inf out, they're matching, it's cute. 👑)

If x is zero, then a pole error occurs, and the functions return -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.

If x is negative (including negative infinity), then a domain error occurs, and a NaN (not a number) is returned.

bestie. negative inputs are NOT it. that's a domain error, that's undefined behaviour territory, that's the math equivalent of texting your ex. 🚫

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🔥 ERRORS 🔥

See math_error(7) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:

• Domain error: x is negative — errno is set to EDOM. An invalid floating-point exception (FE_INVALID) is raised.
• Pole error: x is zero — errno is set to ERANGE. A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised.

so like. if x is negative you get EDOM + FE_INVALID. if x is zero you get ERANGE + FE_DIVBYZERO. write it down, there will be a quiz. 📝

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💅 ATTRIBUTES 💅

For an explanation of the terms used in this section, see attributes(7).

• Interface: log(), logf(), logl()
  • Attribute: Thread safety
  • Value: MT-Safe

MT-Safe?? we LOVE a thread-safe queen. take her everywhere, she will not betray you. 🫶

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⭐ STANDARDS ⭐

C11, POSIX.1-2008.

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🌈 HISTORY 🌈

C99, POSIX.1-2001.

The variant returning double also conforms to SVr4, 4.3BSD, C89.

she's been around. she's got lore. respect the elders. 🙏

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💀 BUGS 💀

In glibc 2.5 and earlier, taking the log() of a NaN produces a bogus invalid floating-point (FE_INVALID) exception.

not me finding out the old glibc was just out here raising FE_INVALID for no reason 😭 it's giving glitchy. if you're on glibc 2.5 or earlier... bestie why. 🕰️

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👀 SEE ALSO 👀

cbrt(3), clog(3), log10(3), log1p(3), log2(3), sqrt(3)

the whole log-iverse. go read them all, they're all slaying in their own way. 💖

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and that's the page!! from Linux man-pages 6.20, dated 2026-02-08, log(3) — she's natural, she's logarithmic, she's that girl. stan. 💅✨🎀