log1p(3)

💖 log1p(3) — bestie's guide to the log of 1+x 💖

okay so this is the manual page for log1p, from the Library Functions Manual, and honestly? she's a baddie. let me put you on. 🫶

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✨ NAME ✨

log1p, log1pf, log1pl — logarithm of 1 plus argument. that's it, that's the girl. 💅

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📚 LIBRARY 📚

Math library (libm, -lm). yes bestie you have to link her in, she does not come free. 🎀

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🌸 SYNOPSIS 🌸

#include <math.h>

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

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

log1p():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
    || _XOPEN_SOURCE >= 500
    || /* Since glibc 2.19: */ _DEFAULT_SOURCE
    || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

log1pf(), log1pl():

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

like, define the right macro or she will simply not speak to you, no cap. 🙈

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💜 DESCRIPTION 💜

these functions return a value equivalent to

log(1 + x)

and the way they do it is accurate even when x is super close to zero. that's the whole point of her existence fr fr — naive log(1 + x) loses precision down there and it's giving cancellation, but log1p ate. 😌✨

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🔥 RETURN VALUE 🔥

• On success, these functions return the natural logarithm of (1 + x). slay.
• If x is a NaN, a NaN is returned. she matches your energy.
• If x is positive infinity, positive infinity is returned. iconic behavior, honestly.
• If x is -1, a pole error occurs, and the functions return -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively. 💀
• If x is less than -1 (including negative infinity), a domain error occurs, and a NaN (not a number) is returned. she said "not today."

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😭 ERRORS 😭

see math_error(7) for info on how to figure out whether an error has even happened when you call these functions. iykyk.

the following errors can occur:

• Domain error: x is less than -1. errno is set to EDOM (but see BUGS, bestie, it's messy down there). An invalid floating-point exception (FE_INVALID) is raised.
• Pole error: x is -1. errno is set to ERANGE (but see BUGS again, I'm so serious). A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised.

⚠️ when the manpage says "but see BUGS" twice, you go read BUGS. that's the vibe check. ⚠️

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👑 ATTRIBUTES 👑 (for an explanation of the terms used in this section, see attributes(7))

• Interface: log1p(), log1pf(), log1pl() — Attribute: Thread safety — Value: MT-Safe

translation: all three of them are MT-Safe, so she's not gonna start drama across your threads. periodt. 💅

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🌈 STANDARDS 🌈

C11, POSIX.1-2008. certified, validated, official. 👑

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🕰️ HISTORY 🕰️

C99, POSIX.1-2001. she's been around since the turn of the millennium and she's still that girl. ⭐

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💀 BUGS 💀

• Before glibc 2.22, the glibc implementation did not set errno to EDOM when a domain error occurred.
• Before glibc 2.22, the glibc implementation did not set errno to ERANGE when a range error occurred.

🚨 so if you're on old glibc and you're checking errno, bestie, she ghosted it. don't be surprised. 🚨

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🎀 SEE ALSO 🎀

exp(3), expm1(3), log(3) — go read her friends too, they're all part of the same clique. 💖

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and that's the whole page, bestie — this was the log1p(3) entry from Linux man-pages 6.20, dated 2026-02-08, and you absolutely ate by reading the whole thing. go compute something tiny and near zero responsibly. mwah. 💅✨💖