atanh(3)

✨ atanh(3) ✨

okay bestie, pull up a chair 🪑 because we're talking about atanh, atanhf, and atanhl — the inverse hyperbolic tangent girlies. they live in section 3 of the Library Functions Manual and they are genuinely so good at what they do. no cap. 💅

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🌸 NAME 🌸

atanh, atanhf, atanhl — inverse hyperbolic tangent function ✨

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📚 LIBRARY 📚

Math library (libm, link it with -lm) 🧚

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🎀 SYNOPSIS 🎀

like literally just include the header and you're set:

#include <math.h>

double atanh(double x);
float atanhf(float x);
long double atanhl(long double x);

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

bestie, glibc is picky about when it lets you have these. here's the whole vibe check:

atanh():

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

atanhf(), atanhl():

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

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💖 DESCRIPTION 💖

these functions calculate the inverse hyperbolic tangent of x 💫 — meaning they hand you back the value whose hyperbolic tangent is x. it's giving reverse-engineering, it's giving un-tanhing. iconic.

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🔥 RETURN VALUE 🔥

when everything goes right, these functions return the inverse hyperbolic tangent of x. slay.

• if x is a NaN, a NaN is returned. 💀
• if x is +0 (-0), +0 (-0) is returned. 🫶
• if x is +1 or -1, a pole error occurs, and the functions return HUGE_VAL, HUGE_VALF, or HUGE_VALL, respectively, with the mathematically correct sign. 😭 (that's a real big number bestie, brace yourself)
• if the absolute value of x is greater than 1, a domain error occurs, and a NaN is returned. 🙈

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⚠️ ERRORS ⚠️

go read math_error(7) for the whole lore on how to figure out whether an error even happened when you called these functions. iykyk.

these are the errors that can pop up:

• Domain error: x less than -1 or greater than +1 — errno is set to EDOM. an invalid floating-point exception (FE_INVALID) is raised. 🚨
• Pole error: x is +1 or -1 — errno is set to ERANGE (but see BUGS below, it's a whole thing). a divide-by-zero floating-point exception (FE_DIVBYZERO) is raised. 💥

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👑 ATTRIBUTES 👑

for what all these fancy terms even mean, check attributes(7) — it's required reading fr fr.

• Interface: atanh(), atanhf(), atanhl()
  • Attribute: Thread safety
  • Value: MT-Safe ✅

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🌈 STANDARDS 🌈

C11, POSIX.1-2008. periodt. 💅

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🕰️ HISTORY 🕰️

C99, POSIX.1-2001.

the variant returning double also conforms to SVr4, 4.3BSD. a well-traveled legend honestly. ⭐

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💀 BUGS 💀

in glibc 2.9 and earlier, when a pole error occurs, errno is set to EDOM instead of the POSIX-mandated ERANGE. that's a not it moment. since glibc 2.10, glibc does the right thing. we love growth. 🌱

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🫶 SEE ALSO 🫶

• acosh(3)
• asinh(3)
• catanh(3)
• cosh(3)
• sinh(3)
• tanh(3)

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and that's the whole page bestie!! this one is from Linux man-pages 6.20, dated 2026-02-08, and it belongs to atanh(3). go forth and invert those hyperbolics 💖✨🎀