acos(3)

✨ acos — arc cosine function 💜

okay bestie, so like, this is the manual page for acos, acosf, and acosl from the Linux man-pages collection (that's the manual section 3, library functions, iykyk). Sit down, grab a lil snack 🍓, and let's get into it.

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🎀 NAME

acos, acosf, acosl — the arc cosine function, aka the math girlies who go backwards from a cosine value and hand you the angle. iconic behaviour fr.

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📚 LIBRARY

Math library, aka libm, which you link with -lm — and no cap, if you forget the -lm you're gonna have a bad time. 🫶

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🌸 SYNOPSIS

#include <math.h>

double acos(double x);
float acosf(float x);
long double acosl(long double x);

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

acosf(), acosl():

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

💀 yes bestie, the feature test macros are giving "final boss of build system errors". Copy them exactly, don't freestyle.

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🌈 DESCRIPTION

These functions calculate the arc cosine of x. Which is a fancy way of saying: they hand you back the value whose cosine is x. It's the reverse-girlie of cos(). 💅

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⭐ RETURN VALUE

On success — the glow-up moment — these functions return the arc cosine of x in radians, and that return value lives in the range [0, pi]. So it's never gonna go rogue on you. 🌸

• If x is a NaN, a NaN is returned.
• If x is +1, +0 is returned.
• If x is positive infinity or negative infinity, a domain error occurs, and a NaN is returned. 💀
• If x is outside the range [-1, 1], a domain error occurs, and a NaN is returned.

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🔥 ERRORS

Peep math_error(7) if you wanna know how to figure out whether an error went down while you were calling these functions. Detective work, but make it C. 🧚

The following errors can occur:

• Domain error: x is outside the range [-1, 1] — errno is set to EDOM, and an invalid floating-point exception (FE_INVALID) is raised.

😭 literally the second you step outside [-1, 1] the function says "not me, not today". Respect the domain bestie.

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👑 ATTRIBUTES

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

• Interface: acos(), acosf(), acosl()
• Attribute: Thread safety
• Value: MT-Safe

so yeah, all three of them are MT-Safe, which is the good one. go off. ✨

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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 C89, SVr4, 4.3BSD. She's been around. She's a legend. She's been serving since before your parents had a landline. 👑

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🌟 SEE ALSO

asin(3), atan(3), atan2(3), cacos(3), cos(3), sin(3), tan(3)

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and that's the whole vibe check bestie 🎀 this page is from Linux man-pages 6.20, dated 2026-02-08. slay, go compute some angles, and don't forget -lm 💅✨