---
name: "casin"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/casin"
---

# casin(3)

complex arc sine

## NAME

casin, casinf, casinl - complex arc sine

## LIBRARY

Math library (*libm*, *-lm*)

## SYNOPSIS

```
#include <complex.h>
```

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

## DESCRIPTION

These functions calculate the complex arc sine of *z*. If *y = casin(z)*, then *z = csin(y)*. The real part of *y* is chosen in the interval \[-pi/2,pi/2\].

One has:

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

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **casin**(), **casinf**(), **casinl**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

glibc 2.1. C99, POSIX.1-2001.

## SEE ALSO

[**clog**(3)](/clog/3.md), [**csin**(3)](/csin/3.md), [**complex**(7)](/complex/7.md)