atan(3)

✨ atan — arc tangent function ✨

okay bestie so you know how tangents are everywhere and sometimes you have the answer and you need to work backwards to the angle? yeah. that's this. this is the atan(3) page from the Library Functions Manual, and it's lowkey one of the most useful little guys in libm 💖

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🦋 LIBRARY

Math library (libm, link it with -lm) — do not forget the -lm, I am literally begging you 🙈

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🌸 SYNOPSIS

#include <math.h>

double atan(double x);
float atanf(float x);
long double atanl(long double x);

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

atanf(), atanl():

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

so if you're on an old glibc and atanf is ghosting you, it's a macro thing, not a you thing 🫶

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🎀 DESCRIPTION

These functions calculate the principal value of the arc tangent of x — which is just a fancy way of saying: the value whose tangent is x. you hand them a tangent, they hand you back the angle. that's the whole job. icon behavior honestly 👑

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💅 RETURN VALUE

• On success, these functions return the principal value of the arc tangent of x, in radians 🌈 — and the return value lives in the range [-pi/2, pi/2].
• If x is a NaN, a NaN is returned. (garbage in, garbage out, but make it polite 💀)
• If x is +0 (-0), +0 (-0) is returned — yes it preserves the sign, the little overachiever ✨
• If x is positive infinity (negative infinity), +pi/2 (-pi/2) is returned 🌸

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😭 ERRORS

No errors occur. literally none. not a single one. this function has zero drama, we love a stable queen 💜

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🔥 ATTRIBUTES

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

• atan(), atanf(), atanl() — Thread safety — MT-Safe 🧚

(translation: it's thread safe, go wild, run it in every thread you want, it will not start anything 👑)

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⭐ STANDARDS

C11, POSIX.1-2008. certified, stamped, approved 💅

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🦋 HISTORY

• C99, POSIX.1-2001.
• The variant returning double also conforms to SVr4, 4.3BSD, C89 — it's been doing this since basically forever, an actual elder icon 🌈

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👑 SEE ALSO

acos(3), asin(3), atan2(3), carg(3), catan(3), cos(3), sin(3), tan(3)

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no cap: if you catch yourself needing the arc tangent of two arguments, that's atan2(3)'s whole thing. go say hi to it 🫶

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and that's the page, bestie!! this was brought to you by Linux man-pages 6.20, dated 2026-02-08, and it's giving atan(3) realness ✨💖 slay responsibly, and remember: radians, not degrees, or the math girlies will find you 🎀