casin(3)

✨ casin — complex arc sine, but make it a vibe ✨

okay bestie, pull up a chair 🎀 this is casin(3), like, the third section of the manual (yes that's what the (3) means, iykyk), and it's about the complex arc sine. three whole functions, all named almost the same, all serving absolute cunt. let's go 💅

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🌸 LIBRARY 🌸

Math library — that's libm, and you link it with -lm. (yes, lowercase L, not a one, not an I, we've all made that mistake and we've all cried about it 😭)

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✨ SYNOPSIS ✨

#include <complex.h>

double complex casin(double complex z);
float complex casinf(float complex z);
long double complex casinl(long double complex z);

three flavors of the exact same thing, one for each size of float. it's giving matching friend group energy fr fr 🫶

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💖 DESCRIPTION 💖

so here's the tea ☕ these functions compute the complex arc sine of z. that's the whole gig. main character energy, no notes.

if you're the kind of girl who likes it formal: if y = casin(z), then z = csin(y). see how they're doing the whole inverse thing? obsessed. 🔥

and the real part of y gets chosen inside the interval [-pi/2,pi/2]. that's the constraint. the bounds are the bounds, periodt.

also, ngl, this identity is kinda iconic:

casin(z) = -i clog(iz + csqrt(1 - z * z))

memorize it or don't, but it's right there, bestie. 💜

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🦋 ATTRIBUTES 🦋

for an explanation of the terms used in this section, go peek at attributes(7). and yes, the original had a cute little table, and we're doing bullets now because tables are not the moment. same facts though, no cap:

• Interface: casin(), casinf(), casinl()
• Attribute: Thread safety
• Value: MT-Safe

translation: she's safe to call from multiple threads at once. multitasking queen 👑

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🌈 STANDARDS 🌈

C11, POSIX.1-2008. she's standardized, she's official, she's that girl. ⭐

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🧚 HISTORY 🧚

she showed up in glibc 2.1. and before that? C99, POSIX.1-2001. a legend with a long lineage honestly 🌸

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🎀 SEE ALSO 🎀

go read her friends, they're all stunning:

• clog(3)
• csin(3)
• complex(7)

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okay that's the whole page, you ate it, you understood it, go write some complex math 💖

this was Linux man-pages 6.20, dated 2026-02-08 — mwah, love u, bye 🫶✨