---
name: "cproj"
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/cproj"
---

# cproj(3)

project into Riemann Sphere

## NAME

cproj, cprojf, cprojl - project into Riemann Sphere

## LIBRARY

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

## SYNOPSIS

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

**double complex cproj(double complex** *z***);** **float complex cprojf(float complex** *z***);** **long double complex cprojl(long double complex** *z***);**

## DESCRIPTION

These functions project a point in the plane onto the surface of a Riemann Sphere, the one-point compactification of the complex plane. Each finite point *z* projects to *z* itself. Every complex infinite value is projected to a single infinite value, namely to positive infinity on the real axis.

## ATTRIBUTES

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

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **cproj**(), **cprojf**(), **cprojl**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

glibc 2.1. C99, POSIX.1-2001.

In glibc 2.11 and earlier, the implementation does something different (a *stereographic* projection onto a Riemann Sphere).

## SEE ALSO

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