cos(3)

💖 cos(3) — like, the cosine function, bestie 💖

okay so this is page cos(3) from the Library Functions Manual, and we're about to get SO cozy with trigonometry. pull up a chair. 🎀

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✨ NAME ✨

cos, cosf, cosl — cosine function 🌊

📚 LIBRARY 📚

Math library (libm, link it with -lm) — yeah you gotta actually link it, don't @ me when it doesn't compile 💅

🌸 SYNOPSIS 🌸

#include <math.h>

double cos(double x);
float cosf(float x);
long double cosl(long double x);

🦋 Feature Test Macro Requirements for glibc 🦋

(peep feature_test_macros(7) if you wanna know what these are, iykyk)

cosf(), cosl():

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

like, you need ALL of that or she simply will not exist for you. the audacity. 😭

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💫 DESCRIPTION 💫

These functions return the cosine of x, where x is given in radians. that's it. that's the whole vibe. radians only, bestie — if you hand her degrees she's gonna be real confused and it's gonna be wrong and it's gonna be your fault 🙈

🫶 RETURN VALUE 🫶

On success, these functions return the cosine of x. slay. ✨

• If x is a NaN, a NaN is returned. (NaN in, NaN out, iconic behaviour honestly)
• If x is positive infinity or negative infinity, a domain error occurs, and a NaN is returned. infinity is not on the guest list for this function fr fr 💀

🔥 ERRORS 🔥

Go read math_error(7) for the whole scoop on how to figure out whether an error happened when you called these. detective work. 🔍

The following errors can occur:

🚨 Domain error: x is an infinity 🚨

errno is set to EDOM (but see BUGS — there's a lil plot twist down there). An invalid floating-point exception (FE_INVALID) is raised.

lowkey important: yes you read that right, EDOM. write it down.

👑 ATTRIBUTES 👑

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

• cos(), cosf(), cosl() — Thread safety — MT-Safe 💅✅

(translation: she's safe to use from multiple threads, go wild, you're fine)

⭐ STANDARDS ⭐

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

🌈 HISTORY 🌈

C89, POSIX.1-2001. she's been around. she's a legend. she's been doing this since before your parents had phones.

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

💀 BUGS 💀

Before glibc 2.10, the glibc implementation did not set errno to EDOM when a domain error occurred. so if you're on some ancient glibc, don't trust errno, she's lying to you. that's the tea. 🍵

🧚 SEE ALSO 🧚

acos(3), asin(3), atan(3), atan2(3), ccos(3), sin(3), sincos(3), tan(3)

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and that's the whole page bestie!! 💖 this one's from Linux man-pages 6.20, dated 2026-02-08 — go forth and compute your cosines, radians ONLY, and remember: you're the main character of your own trig ✨🦋💅